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POPLAR

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, in Europe

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Significance and Application

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AGRICULTURE HANDBOOK NO. 150

U. S. DEPARTMENT OF AGRICULTURE • FOREST SERVICE
yUL 1 8 199T

POPLAR TIMBER

in the United States

hy

ERNST J» SCHREINER

in charge, Forest Genetics Research

Noriheasieni Forest Expermient SfMion

U. S. DEPARTMENT OF AGRICULTURE FOREST SERVICE

Washington, D. C. February 1959
Acknowledgments

Acknowledgment is made particularly of the wholehearted cooperation of the late Prof. Dr. G. Houtzagers of the Instituut voor

Bosbouwkundig Onderzoek, Afd. Houtteelt, in Wageningen, The

Netherlands, who made all arrangements for the author's travels

through the poplar regions of Europe. His comprehensive firsthand knowledge of European poplar culture and his personal acquaintance with the poplar specialists of Europe made the highly

successful study tours possible.

The author is also heavily indebted to Prof. Dr. G. Piccarolo,

Director, Instituto de Sperimentazione per la Pioppicoltura, Cásale Monferrato, Italy, who spent 3 weeks showing him the intensive poplar culture in the Po Valley.

Appreciation and thanks are also expressed to the following individuals who conducted the author on poplar study tours in their

respective countries :

Prof. Dr. F. Bauer, Waldbau Institut der Universität Freiburg,

Freiburg i. Br., Germany.

Prof. Ola B0rset, Norges Landbruksh0gskole, Instituut for

Skogskj0tsel, Vollebekk, Norway.

Dr. B. Bouvarel, Station de Recherches et Expériences Forestières, Nancy, France.

Mr. J. Chardenon, Service de Plantations de Peupliers du

SEITA, Saintines (Oise), France.

Oberforstmeister F. Eichbaum, Deutche Pappelverein, Bonn,

Germany.

Mr. H. Erhelens, Director, Nederlandsche Algemeene Keuringsdienst voor Boomkweekerijgewassen (NAKB), The Hague, The

Netherlands.

Dr. Fritz Fischer, Eidgenössische Anstalt f.d. Forstliche Versuchswesen, Zurich, Switzerland.

Mr. F. Flon, Forester, Belgium Match Co., Grammont, Belgium.

Mr. H. J. Gerritsen, Nederlandsche Heidemaatschappij, Arnhem, The Netherlands.

Dr. A. Herbignat, Directeur Général de Eaux et Forêts, Brussels, Belgium.

Prof. Dr. H. H. Hilf, Director, Forschungsstelle f. Flurholzanbau d. Lignikultur, Reinbek b. Hamburg, Germany.

Mr. F. Jaime, Ingeniero de Montes, Jefe de la Brigada de Teruel, Zaragoza, Spain.

Dr. Helge Johnsson, Director, Föreningen for Växtförädling av

Skogsträd, Ekebo, Källstorp, Sweden.

Dr. W. Langner, Director, Institut f. Forstgenetik u. Forstpflanzenzüchtung, Schmalenbek ü. Ahrensburg, Germany.

Dr. C. Mühle Larsen, Directeur Scientifique, Institut de Populiculture, Grammont, Belgium.
Dr. C. Syrach Larsen, Director, Arboretum, Royal Veterinary

and Agricultural College, Hörsholm, Denmark.

Dr. R. Müller, Director, Wissenschaftliches Institut des Deutchen Pappelvereins, Brühl, Germany.

Mr. M. Navarro, Ingeniero de Montes, Jefe de la Brigada de

Aragon, Zaragoza, Spain.

Prof. A. Pavari, Director, Stazione Sperimentale di Selvicoltura, Florence, Italy.

Mr. T. R. Peace, Forest Pathologist, Forestry Commission, Forest Research Station, Alice Holt Lodge, Wrecclesham, Farnham,

Surrey, England.

Dr. J. Pourtet, Ingenier Principal de Eaux et Forêts, Station

de Recherches et Expériences Forestières, Nancy, France.

Dr. Ernst Rohmeder, Director, Institut f. Forstsamenkunde u.

Pflanzenzüchtung, Munich, Germany.

Dr. René Roi, Le Sous-Directeur de TÉcole Nationale de Eaux

et Forêts, Nancy, France.

Mr. J. M. Sanz-Pastor, Ingeniero del Patrimonio Forestal del

Estado, Jefe de la División Hidrológico-Forestal del Tajo, Madrid,

Spain.

Dr. H. van Vloten, Director, Bosbouwproefstation T.N.O.,

Wageningen, The Netherlands.

Dr. W. Wettstein, Leiter Doz., Abteilung Forstpflanzenzüchtung, Forstlichen Bundesversuchsanstalt Mariabrunn, Vienna,

Austria.

In addition to the help and cooperation acknowledged above,

the author ov^es a debt of gratitude to several hundred European

foresters, poplar specialists, nurserymen, and poplar growers. The

success of the study reported here stems primarily from the information they so kindly provided.

Schreiner, Ernst Jefferson, 1902-

Production of poplar timber in Europe and its significance

and application in the united States. Washington, U. S.

Dept. of Agricultur^e, Forest Service, 1959.

iv, 124 p. Ulus., map. 24 cm. (U. S. Dept. of Agriculture. Agriculture handbook no. 150)

Bibliographical footnotes.

1. Poplar. (Series)

SD397.P85S35 634.9723 Agr 59-29

U. S. Dept. of Agr. Libr. lAg84Ah no. 150

for Library of Congress t
Contents

Pago

Introduction 1

History and importance of poplar

in Europe. 3

Expansion in poplar culture _ _ 4

Significance for the United

States 5

Poplars grown commercially in

Europe.--, 6

Northern Europe 6

Central Europe 9

Southern Europe 12

Significance for the United

States 14

Selection and breeding 14

Clonal selection 14

Controlled breeding 23

Significance for the United

States 28

Nursery practice 29

Aigeiros poplars 30

Leuce poplars 34

Significance for the United

States 36

Site and soil requirements 37

Aigeiros poplars 37

Leuce poplars 42

Significance for the United

States 43

Outplanting 44

Site preparation 44

Planting stock_ 44

Spacing 47

Planting 49

First-year plantation care 50

Significance for the United

States 52

Management ^nd silviculture 54

Leuce poplars 54

Aigeiros poplars 56

Significance for the United

States 66

Diseases 68

Trunk, branch, and twig diseases 68

Page

Diseases—Continued

Leaf diseases 76

Root diseases 78

Unidentified diseases 78

Wood decay and stains 80

Significance for the United

States- _ 80

Insects 83

Boring insects 83

Insects on the leaves 85

Insects on the bark i 87

Significance for the United

States 87

Damage by other agents 89

Mistletoe 89

Environmental and climatic

damage 89

Damage by animals 90

Significance for the United

States 90

Growth and yield 91

Aigeiros poplars in plantations 91

Aigeiros poplars in row plantings 98

Native aspen in Scandinavia. _ 99

Significance for the United

States 101

Characteristics of the wood 105

Utilization in Europe 106

Utilization in the United

States 109

Appendix 112

Identification and naming of

the poplars 112

Poplar names used in this report 114

Insect names used in this report 118

Description of poplar stands

listed in table 2 119

Description of row plantings

listed in table 5 123

For sale by the Superintendent of Documents, U. S. Government Printing Office

Washington 25, D. C. - Price 45 cents
Production of Poplar Timber In Europe

and its Significance and Application in

the United States

INTRODUCTION

Between October 1951 and October 1952 the author studied poplar culture and research in western Europe, partly under a Fulbright Research Grant and partly under the auspices of the U.S.

Forest Service. He attended the International Poplar Commission

meetings and study tours in Spain in 1955, and in France in 1957,

as official representative for the United States.

During these visits, which involved more than 30,000 miles of

automobile travel (fig. 1), and through personal correspondence

with poplar experts and growers in all the western European

countries, he has had the opportunity to observe and collect information about European experience in poplar culture and research.

This is his report on these studies, with special reference to the

significance of European poplar work for the United States.

Since hybrid poplar culture started in Europe, there is much to

be learned from the practices that have developed during the past

200 years. European poplar culture is impressively extensive and

successful; but practice, and to some extent research, still lean

heavily on traditional concepts and methods. The author has attempted to differentiate among the traditional, the biological, and

the economic aspects of poplar production. Research will be necessary to test the biological and economic validity of some of the

traditional practices.

There is now a worldwide interest in the use of hybrid poplars

for timber production. Within recent years the interest in hybrid

poplars has grown rapidly in the United States. For example,

during the past 3 years the Northeastern Forest Experiment Station has received about 8,000 letters from farmers, landowners,

and forest industries that wanted information about planting

stock, planting methods, culture, and markets for poplar wood.

This publication will, it is hoped, answer many of these questions.

The question of probable markets for hybrid poplar timber can

be answered only for localities within the commercial range of

our native poplars (cottonwoods and aspens) ; that is, where there

is a present market for poplar timber. It will seldom be possible

to profitably market the limited poplar production of one or even

several small landowners where there is no industry that utilizes

poplar within reasonable transportation distance.
HANDBOOK 150, U. S. DEPARTMENT OF AGRICULTURE

POPLAR STUDY TOURS

BY ERNST J. SCHREINER, 1951-52. 1955, AND 1957

FIGURE 1.—The author traveled more than 30,000 miles by automobile in

his studies of poplar culture and research, forest genetics, and general

forestry, during visits to Europe in 1951-52, 1955, and 1957.

Where there is no present market, hybrid poplars can be expected to produce their potential share of the local timber wealth

only if they are grown in sufficiently large quantities to attract

industry. Existing local industries are not essential; with a sufficient and continuing supply of gQod poplar timber in sight, industry can be expected to compete for the product and even for the

establishment of local conversion plants.

In localities where climate and soils are apparently suitable for

poplar culture, but where there now are no available markets, it
PRODUCTION OF POPLAR TIMBER IN EUROPE 6

would require the cooperation of many farmers and landowners to

foster a production program on a scale that would create a profitable market. An essential first step in such new territory is the

establishment of test and demonstration plantings. It would also

be necessary to determine—

1. The economic limits of the proposed production area.

2. How much suitable poplar land would be available within

the production area.

3. How many landowners would agree to plant poplar, and the

acreage of suitable land the prospective growers would use for

poplar production.

4. The possibility of agreement among prospective growers on

a plan for continuous annual production (sustained yield). This

would require agreement on a minimum acreage to be planted

annually.

5. Whether the estimated annual production would create a

reliable market.

6. The need and desirability for a local poplar-growers organization for mutual protection, cooperation, and exchange of ideas

and information.

Demonstration and test plantings and a survey of local possibilities could be made by interested individuals, independently or

under the sponsorship of local organizations. Successful demonstration plantings, established a few years before an action program on large-scale planting is contemplated, would provide the

best argument for concerted action.

Predictions of costs and returns from hybrid poplar in the

United States have not been included in this report for the following reasons :

1. European figures, based on the use of relatively cheap handlabor and proportionately high prices for timber products, Cannot

be converted for American conditions with sufficient accuracy to

justify their use.

2. Costs and returns in the United States will depend in large

measure on the extent to which poplar production can be successfully mechanized. There is a lack of experience and cost information on this problem both in Europe and in this country.

HISTORY AND IMPORTANCE OF POPLAR

IN EUROPE

It is usually convenient to have a more or less definite starting

date for any historical sketch, but this is not possible for poplar

culture in Europe. There is evidence that the native poplars of the

Po Valley, which stretches across northern Italy from the French

border to the Adriatic, were heavily exploited in Roman times.

Here, as well as in southern Italy, the native poplars have been

important for hundreds of years, to some extent planted but more

often protected or at least favored where they occurred naturally.
4 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

In the Caudina Valley, a region of small land ownerships northeast of Naples, the poplar is used in a custom that has been handed

down from generation to generation : at the birth of a daughter,

it is customary for the father to plant a small number of poplars,

usually about 100 ; and when the daughter marries, these trees are

her dowry.

The native black poplar has also been used for centuries in parts

of France, in the Low Countries, and in Germany along the lower

Rhine. Poplar lumber has been found in German farm buildings

old enough to indicate that the European poplar was used long

before the American cottonwood brought hybrid vigor into European poplar culture.

Modern European poplar culture stems from the introduction

of our American cottonwood about 250 years ago. During these

two and a half centuries countless numbers of hybrids have been

produced by natural hybridization between the American species

and the European black poplar, followed by crossing between the

hybrids and backcrossing between hybrids and the European parent species. Many of these hybrids were superior to the parent

species in such characters as rate of growth, disease resistance,

stem form, or other desirable characteristics. Since such natural

hybrids could be propagated easily from cuttings, superior and

especially vigorous Wildlings were frequently used in local plantations.

The nurserymen and growers of the Parisian Basin were the

first to select and exploit outstanding natural hybrids.^ Although

France leads Europe in poplar acreage, in recent years it has been

surpassed by several other countries in the intensity and efficiency

of its poplar culture. A recent estimate indicates that there are,

in round figures, 247,000 acres of poplar in France ; and that approximately 40 percent (98,800 acres) of these plantations are in

the Parisian Basin.

An annual increment between 150 and 160 cubic feet per acre

is a reasonable estimate of the average productivity of all poplar

plantations in France. On the basis of 155 cubic feet per acre per

year, the total annual growth is approximately 38.3 million cubic

feet (roughly 426,000 cords or 192 million board-feet).

The American cottonwood is reported to have been introduced

into the Po Valley in northern Italy in 1770. Free and easy natural hybridization with the native black poplar followed the same

pattern here as it did north of the Alps, but the earliest industrial

use of hybrid poplars in this region was for pulpwood about 1890.

Expansion in Poplar Culture

Although poplar has long been an important bottom-land timber tree in many localities, recognition of its importance to the

forest economy of Europe has increased astoundingly since the

end of World War II. On the initiative of French, Belgian, and

Netherlands poplar experts, the International Poplar Commission

was organized in 1947 under sponsorship of the Food and Agriculture Organization of the United Nations.
PRODUCTION OF POPLAR TIMBER IN EUROPE 5

The aim of the International Commission is to promote international collaboration in the study of scientific, technical, and economic questions relating to poplar. Most of the twenty-one member countries (France, Belgium, The Netherlands, Italy, the

United Kingdom, Sweden, Switzerland, Austria, Hungary, Western Germany, Spain, Turkey, Argentina, Egypt, Iraq, Iran, Syria,

Greece, Lebanon, Pakistan, and Yugoslavia) have national poplar

commissions to promote the study and culture of poplars.

There are two major reasons for the great importance of poplar in Europe today: (1) the drastic shortage of wood in all countries, and (2) the rapid growth of poplar under proper culture on

suitable sites. Farmers and small and large owners of land suitable for the growth of poplar can harvest these trees profitably in

12 to 25 years. In especially favorable localities in France, Germany, The Netherlands, and particularly Italy, poplar is the farmer's or landowner's most profitable crop.

Netherlands figures, based on all costs over a 25-year rotation

(including soil rent, taxes, and interest at 3 percent on the invested capital) indicate a net profit of $9.50 to $12.50 (36 to 48

guilders) per tree. On the basis of 64 trees per acre (based on an

average loss of 8 percent of the 70 trees originally planted) this

is a net profit of $608 to $800 per acre.

The German Poplar Society has reported that the normal yield

of widely spaced poplars on the'bottom lands of the lower Rhine

is 285.8 cubic feet per acre per year—a gross value of $96 per

acre per year.

In the Po Valley of northern Italy, poplars grown on fertile

agricultural land on 12- to 25-year rotations are said to produce

a greater net return than any other agricultural crop.

Significance for the United States

The diverse and extensive timber supplies in the United States

and our nationwide effort to conserve our forest heritage should

prevent the drastic timber shortages that have made the role of

hybrid poplar so important in Europe. Nevertheless, the rapid

growth of hybrid poplars and their high productivity on sites

suitable for their growth oifers a profitable addition to our forest

wealth. There is some land on most farms, and extensive areas in

some of our major river valleys, on which hybrid poplars would

be more productive than any other timber tree.

There is a demand for poplar in many parts of the United

States at the present time. And hybrid poplar timber can meet

the requirements of industries that are now using other and less

rapid-growing native species. Since the International Poplar

Commission has become the worldwide clearing house for the exchange of information on the production and utilization of poplar

timber, poplar culture in the United States can reap rich benefits

from the work of the Commission.
6 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

POPLARS GROWN COMMERCIALLY IN EUROPE

Northern Europe

Denmark, Sweden, Norway,—The match industries in these

countries are strongly prejudiced in favor of aspen for matchwood and are reluctant to use the Aigeiros hybrids. In some respects aspen wood does make a somewhat better match, but the

difference seems rather insignificant for purposes of usability and

salability. This observation is based on the almost exclusive use of

Aigeiros hybrids for matchwood on the Continent.

There are sound biological reasons for the importance of aspen

in Sweden and Norway. Most important are the naturally regenerated aspen stands (Populus trémula L.). Furthermore, river

bottom-land sites, generally considered essential for the most

profitable culture of Aigeiros hybrids, are much more limited than

on the Continent.

The strongest demand at present for poplar planting stock in

Denmark, Sweden, and Norway is for first generation (Fi hybrids

between European and American aspens (P. trémula X tremuloides Michx.). This demand is so large that it cannot be met by

the nurseries that are engaged exclusively in mass production of

these Fi hybrids.

Aigeiros poplars and hybrids have been planted to some extent

in Denmark for windbreaks but many of these are branchy types

that are not used by the match industry. Tacamahaca hybrids of

the type of P. xberolinensis Dipp.^ have grown well but have not

been generally disease resistant. Specimens of both Aigeiros and

Tacamahaca hybrids in gardens, hedgerows, and along roadsides

indicate that carefully selected clones of these poplars might fill

an important niche in the forest economy of Denmark.

The Netherlands.—Occasional natural hybrids were undoubtedly selected in the past for clonal propagation by farmers and

nurserymen, but the number of clones cultivated in The Netherlands was never so large as in some other countries of Europe.

One probable reason is that natural regeneration from seed, which

under European cultural conditions almost invariably produces

new hybrids, was limited in The Netherlands by the intensive

utilization of land along the rivers and canals for agriculture—

particularly for forage. Hay and grazing in the poplar plantations

and on the overflow lands along the riverbanks is extremely unfavorable to survival of poplar seedlings.

The excellent studies of the late Prof. G. Houtzagers of the

Forest School in Wageningen, published under the title ''Het Geslacht Populus in Verband met Zijn Beteekanis voor de Houtteelt"

^ The poplar names used in this report conform to the International Code.

The application of these rules to the cultivated poplars, and synonymous

names in European poplar literature or in local use, are discussed in the

appendix.
PRODUCTION OF POPLAR TIMBER IN EUROPE 7

(The Genus Populus and its Significance in Silviculture) laid the

groundwork for clonal certification in The Netherlands. The following poplars are now in commercial use : P. 'Heidemij,' '1-214,'

'Robusta,' 'Robusta Zealand,' 'Robusta Vernirubens,' 'Robusta

Bachelieri,' 'Marilandica,' 'Gelrica' (fig. 2), 'Manitobensis,' 'Serotina,' and 'Serotina Keppelse Groene.' Most of these clones have

been studied and tested for more than 20 years. They have been

subjected to pathological tests intensive enough to justify their

certification as resistant to the type of bacterial canker that is

present in The Netherlands.

VhoLo by G. HOUTZAGEIíS

FIGURE 2.—Twelve-year-old poplar clonal test in Best, The Netherlands.

At left is Populus 'Gelrica,' at right 'Marilandica.' Note the difference

in size and stem form.

Clonal identity is controlled under strict government regulation. Every poplar sold must carry a tag of the Netherlands Inspection Service for Nursery Stock certifying the clonal identity

and plant quality. Certified cuttings are available only from a central nursery, originally established by the Netherlands Land Reclamation Society.-

Commercial nurserymen are permitted to take cuttings from

each purchase of certified cutting stock only as long as the State

inspector is certain that there is no danger of mixing clones. State

inspectors go through every commercial nursery twice during the

first year and once in each succeeding year until the stock is sold.

Before the trees are dug for sale, an inspector labels each tree

with a certification tag (fig. 3). This is the most intensive federal

certification system in Europe.

" Early in 1956 this nursery was transferred to the Foundation for Improvement of Breeding Basic Materials for Forest Stands (Stichting ter

Verbetering van Voortkwekingsmaterial van Houtopstanden).
8 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

N. A. K. B. photo

FIGURE 3.—Certification of poplars in a Netherlands nursery by an inspector of the Netherlands Inspection Service for Nursery Stock. This

nursery stock is ready for sale. Each tree is individually tagged.
PRODUCTION OF POPLAR TIMBER IN EUROPE 9

Central Europe

Belgium,—The poplars most generally planted in the past were

the same mixtures of clones used in France, but in recent years

the certified Netherlands clones have come into increasing use in

Belgium. Between 1940 and 1950 the Belgian Match Company^

used Populus 'Robusta' almost exclusively in its plantations. About

1950, 50 to 60 percent of their plantings were 'Serotina Erecta'

and the remainder 'Robusta.' Select clones from many countries

in Europe are being tested at the Company's Poplar Institute in

Grammont.

Germany.—The poplar types commonly planted in the past in

the principal poplar regions of Germany were P. 'Robusta,' 'Serotina,' 'Regenerata,' and 'Marilandica.' Types of P. xberolinensis

were widely planted in the northwest. Although their susceptibility to disease has brought them into disrepute, they are still used

rather extensively for roadside and street trees. In SchleswigHolstein and the coastal region along the North Sea, P. x canescens (Ait.) Sm. cultivars are still favored for windbreaks because

of their windfirmness and their adaptability to coastal conditions.

The extensive poplar plantings in the Rhine bottom lands in

Baden exhibit clonal mixtures that have probably been derived

through local selection and clonal propagation of natural hybrids.

Such natural crossing could have been between hybrids (apparently the older French hybrids) and backcrosses to the native P.

nigra L., which is still surprisingly common on the flood lands of

the Rhine.

Since 1950 the German Poplar Society has certified poplar

nursery stock, on the basis of nursery inspections, as to cultivar

name, condition, and health. Certified stock bears a protected

trademark label, which is attached to individual trees or to bundles of not more than 10 trees. Participation by commercial nurseries is voluntary. Following are some of the cultivars certified

for sale during the winter of 1956-57: P. alba L., P. Xccinescens,

and P. trichocarpa Torr. & Grey, all with indication of clone; P.

'Heidemij' and 'Gelrica,' both from The Netherlands; and 'Regenerata,' 'Robusta,' 'Serotina,' 'Eugenei Feminine,' 'Robusta Vernirubens,' 'Marilandica,' 'Eukalyptus,' 'Allenstein,' 'Bietigheim,'

'Drömling,' 'Eckhof,' 'Grandis,' 'Isar,' 'Leipsig,' 'Löns,' 'Neupotz,'

and 'Flachslanden.'

Great Britain,—Poplar culture has not been extensive in Great

Britain. A few commercial plantations have been established from

time to time ; but in general, poplars have been produced in only

small quantities by commercial nurseries for ornamental and

windbreak planting.

As on the Continent, the poplar types that have been planted

in Great Britain represent a mixture of clones. Even the artificial

hybrid P. Xffonerosa Henry is a clonal mixture because Henry

^ The Union Allumettière S.A., which does not lend itself to exact translation, is referred to in this report as "The Belgian Match Company." In

the same way, the Service d'Exploitation Industrielle des Tabacs et des

Allumettes (SEITA) is referred to as "The French Match Company."
10 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

distributed cuttings of a number of taxonomically similar Fi male

and female seedlings under this name.

In addition to the natural species (P. alba, P, trémula L., P.

nigra, P. deltoides Bartr., P. trichocarpa, and P. balsamifera L.),

one or more clones of the following hybrids have been grown in

Great Britain: P. 'Serotina/ *Regenerata/ 'Eugenei/ 'Robusta/

*Marilandica/ *Lloydii/ Xffenerosa and Xberolinensis.

On the basis of climatic adaptability, growth and development,

and particularly disease and insect resistance, the Forestry Commission has for the present limited its recommendation for commercial planting to the following clones: P. 'Serotina,' the socalled 'Serótina Erecta,' 'Robusta' (fig. 4), 'Gelrica,' and 'Eugenei.' P. Xberolinensis is recommended for shade and ornamental planting.

France.—The poplars that have been most commonly planted

in France are P. 'Itálica,' 'Blanc de Garonne,' 'Vert de Garonne,'

'Carolin,' 'Angulata,' 'Virginiana,' 'Monihfera,' 'Serotina,' 'Serotina de Champagne,' 'Serotina du Poitou' 'Regenerata,' 'Robusta,'

'Eugenei,' Marilandica,' and 'Regenerata d'Hautervive.' These

names have not in all cases characterized or assured pure (single)

clones as in The Netherlands.

In the region of Parthois, in the Marne Valley, there are reported to be approximately 49,000 acres of poplar, 80 percent of

which is called P. 'Virginiana,' the remaining 20 percent 'Robusta'

and 'Regenerata.' The 'Virginiana,' although rather crooked in

stem form, grows about as well as 'Robusta' on the good sites and

somewhat better than 'Robusta' on the poorer sites. The occurrence of both sexes in plantations believed by the owners to be

'Virginiana' is irrefutable evidence of a clonal mixture. Considerable variation in stem form (crookedness) is collaborative evidence, because on any particular site uniformly spaced ramets of

a single clone would exhibit only slight variation in growth habit.

The new terms "ramet" and "ortet," which are used in this report,

are defined in the appendix, p. 112.

In the valley of Garonne, a poplar locally called Carolinensis

was most common until about 30 years ago. It is said to be the

best type for this region but does not root so readily from cuttings

as other clones. P. 'Robusta,' 'Regenerata,' 'Vert de Garrone,'

and 'Blanc de Garonne' are now most commonly planted here.

P. 'Itálica' has been widely planted in southern France, where

the wood is preferred for construction lumber. It is obvious from

the wide variation in growth habit (from fastigiate to columnar)

and the occurrence of both sexes that this name has covered a

mixture of clones. The cultivars 'Robusta,' 'Regenerata,' and

'Serotina' are now most extensively planted.

Excellent naturally regenerating stands of P. alba occur in

some of the river valleys of southern France. P. Xcanescens

cultivars (hybrids between P. alba and P. trémula) are generally

considered to grow better than Aigeiros poplars on dry and on

excessively wet sites ; and like P. alba they are used frequently as

ornamental and roadside trees. Cultivars of P. Xcanescens are occasionally planted for lumber production but not for matchwood

because the wood is said to be too rough.
PRODUCTION OF POPLAR TIMBER IN EUROPE 11

iâ«j

GREAT BRITAIN FORESTRY COMMISSION photo

FIGURE 4.—Populus 'Robusta' about 18 years old, at Kett's Wood, Ryston Hall,

England.
12 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

The following poplars are recommended by the French Forest

Service for planting in the several poplar regions of France :

Western Parisian Basin : P. ^Serótina de Champagne/ *Serotina du Poitou/

Eastern Parisian Basin: P. 'Serotina de Champagne/ Robusta/ 'Virginiana/

Western Region (Bretagne, Secteur Ligerien, Charentes) :

P. 'Serotina du Poitou/ 'Robusta/ 'Regenerata/ 'Virginiana/

Southwestern Region: P. 'Blanc de Garonne/ 'Regenerata/

'Virginiana Carolin/

Southeastern Region (Secteur Rhodanien) : P. 'Regenerata,

'Angulata,' 'Robusta,' 'Virginiana/

Switzerland.—Poplar culture in Switzerland has been limited

by the scarcity of available bottom-land sites. P. 'Robusta,'

'Regenerata,' and 'Serotina,' are now most commonly planted.

The types grown during the past 50 years in the vicinity of

Yvonand on Lake Neuchatel and Novillo at the head of Lake

Geneva are believed to have been imported originally from French

nurseries. In Tessin, in the valley below Bellinzona, there are

some small poplar plantations for which the planting stock may

have come from Italy. There is some evidence that a few clonal derivatives of local natural hybridization have come into cultivation

in Switzerland within the present century.

Southern Europe

Italy.—The introduction of the American cottonwood into the

Po Valley in 1790, and of the older European hybrids somewhat

later, set the stage for extensive natural hybridization with the

native black poplar (P. nigra). The natural hybrid population now

represents many kinds and degrees of intercrosses and backcrosses

to the originally more abundant P. nigra.

The almost phenomenal economic importance of hybrid poplar

in Italy today is attributable in large measure to the excellent

work and enthusiasm of Prof. Giacomo Piccarolo and his associates at the Institute for Poplar Research at Cásale Monferrato.

By 1928, when epidemic spring dieback (Defogliazone primaverile), and to a lesser degree leaf rust, had become a serious threat

to profitable poplar culture, the plantations in northern Italy

already contained innumerable natural hybrids. ^ As a result of selection and tests of thousands of natural hybrids

during the past 20 years by the Poplar Institute and several

other organizations, the following clones are recommended for

planting in Italy: P. '1-37,' '1-137,' '1-154,' '1-214,' '1-262/

'1-455,' '1-477,' and 'I-Caroliniano Bianco de Cercenasco. With

the exception of the last, these clones have not yet (1957) been

given clonal names. They are grown and sold under clonal numbers, the capital "I" indicating Italian origin. The greatest demand is for clone '1-214' because of its vigorous growth and

disease resistance

Spain.—European black poplars, especially columnar and fastigiate types like the Lombardy poplar, have been planted along
PRODUCTION OF POPLAR TIMBER IN EUROPE 13

many Spanish highways and river bottom lands. For centuries

they have characterized the Castilian roadsides under conditions

that are too dry for many trees. Poplars are often the principal

source of lumber and of fagots for fuel; in some localities even

the fallen leaves have been used for fuel.

Poplar culture has been locally important for centuries, particularly in the Mediterranean region and in the Duero and Tajo

Valleys of Spain. In addition to numerous cultivars of the European black poplar, there are also many Aigeiros hybrids in cultivation (fig. 5). The original source of most of these hybrids is

obscure, but the most probable guess vi^ould be imported cultivars

and local selections of natural hybrids.

FIGURE 5.—A 6-year-old poplar plantation in the Genii River Valley near

Granada, Spain. The first thinning had been made in this plantation

during the previous winter.

In the region between Barcelona and Gerona, the four cultivars

that have been most widely planted are P. 'Mainou,' 'Bordils,'

'Poncella,' and 'Polla Carolina.' The last was practically eliminated by disease some 35 years ago. P. alba occurs naturally and

as occasional trees in plantations of other cultivars; it is not

commonly planted. Two cultivars, 'Blanquillo' and 'Nigrito,' are

recognized in the region of Granada. In the Province of Logroño

the most commonly grown poplars are the locally called canadensis

and 'Itálica.' In Aragon, cultivars of the European black poplar

are grown as pollards from which 10 to 15 stems are cut at

10- to 12-year intervals.

471632°—59-
14 HANDBOOK 15 0, XJ. S. DEPARTMENT OF AGRICULTURE

Significance for the United States

One of the objectives of this study was an investigation of the

silvicultural characteristics (such as site requirements, growth

rate, and disease and insect resistance) of the poplars that are

grown commercially or are being tested in Europe. Only empirical

and inconclusive answers are available on these questions, primarily because of the confusion in naming hybrid poplar clones

and the impossibility of exactly identifying taxonomically similar

clones. We should avoid similar confusion in the expansion of

poplar culture in the United States. (See appendix p. 112, Identification and Naming of Poplars.)

The search for, selection, and testing of superior individual

trees of native poplars, and of natural and artificial hybrids, has

greatly increased in recent years in various parts of the United

States. Such selections should always be maintained as single

clones, never as mixtures of ramets from trees that appear to be

similar. Clones should be carried under number till they have

been adequately tested and are ready for commercial distribution.

With a continuous and increasing number of new clones, national

registration will eventually become necessary for the protection

of the grower.

SELECTION AND BREEDING

Clonal Selection

Selection of superior individual trees is a major activity of

poplar specialists and growers in almost every European country.

They generally recognize that there is an essential and continuing

job to obtain better clones for future use and to replace those

that may fall prey to unusual environmental conditions or to new

diseases and insects. Selection offers excellent possibilities in

many sections of Europe because of the many hybrid clones in

the plantations and the ease and frequency of natural hybridization.

In the Rhine Valley near Bonn there is an excellent example of

the possibilities for frequent and abundant natural hybridization

and backcrossing. A female Populus *Regenerata' and a male

'Serotina,' estimated to be at least 90 years old, are growing a

short distance apart on an old estate. These two clones, and

possibly others including early local hybrids, have been planted

extensively in mixed or adjacent plantations. In an open area

near such a 50-year-old stand there were, in 1952, literally thousands of seedlings from the 1949 seed crop. Dr. Müller, director

of the Research Institute of the German Poplar Society, had

selected a large number of the best of these natural hybrids for

clonal tests.

Except for the Swedish tests, very few clonal tests have been

designed for precise statistical analysis. Many new selections are

being multiplied for immediate use on the basis of phenotypic excellence of the ortet and the nursery performance of the cuttings.
PRODUCTION OF POPLAR TIMBER IN EUROPE 15

Where new selections have been tested in plantations, the field layout was seldom designed to permit statistical evaluation of results. One or more factors, such as type of planting stock, time of

planting, planting methods, culture, site différences, or soil heterogeneity within the plantation, are almost always completely confounded. Although such clonal selection will bring some elite

clones into immediate use, these empirical testing methods will

not provide the basic comparisons that are required for scientifically sound clonal evaluation.

The Netherlands.—The clones selected by the late Prof. Houtzagers during the 1930's as the best of those grown in The Netherlands have been under test on 3 different sites for more than 20

years by the Netherlands Land Reclamation Society (fig. 6).

These are among the clones now certified by the State for commercial planting. New clones must be tested for 5 years before

they can be included in the list of poplars that can be certified for

sale.

Photo by G. HOUTZAGEES

FIGURE 6.—A spring photograph in the clonal test plantation of the Netherlands Land Reclamation Society at Eist, The Netherlands. At left is

Populus 'Heidemij,' at right 'Serotina Erecta.' There is considerable

difference in time of leafing.
16 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

Additional certified clones are needed and desired in The Netherlands, but conditions do not favor the production and survival

of natural hybrids in sufficiently large numbers for profitable selection. A relatively few local selections are being made, but the

emphasis is quite correctly put on controlled breeding and on

testing the best clones from other countries. ,

The most promising clones from many countries are bemg

propagated in the central poplar nursery formerly located at Keppel but now at Wageningen. A full-time pathologist is assigned to

investigate disease resistance in these clones, particularly resistance to artificial inoculation with bacterial canker.

Clones that had demonstrated satisfactory growth and reasonable disease resistance in the nursery were outplanted in 1950 in

an extensive and well-designed clonal test on the new Northeast

Polder. It was planned to inoculate trees at regular intervals

throughout the test plantations with bacterial canker and other

native diseases to permit evaluation of clonal resistance to natural infection—the procedure used for more than 25 years m poplar research in the northeastern United States.

Italy.—Clonal selection and testing has been extensively practiced in Italy along two different lines: (1) propagation and testing of seedlings from open-pollinated seed of selected mother

trees; and (2) selection and direct propagation (by cuttings) of

oiit^tandinoi* trees

The Institute for Poplar Research at Cásale Monferrato has

carried on large-scale selection through naturally pollinated seed

from selected mother trees. In past years the Institute has grown

more than 20,000 seedlings per year (in some years 100,000) for

nursery selection. The most vigorous seedUngs that are resistant

to spring dieback and leaf rust and have good stem and crown

form have been tested for rooting ability. After the second year

in the nursery the very best have been planted in a selection arboretum and then multiplied for 15- to 20-year clonal plantation

tests. By 1948 more than 1,800 clones derived from open-pollinated seed of selected mother trees had been established m test

plantations by cooperators in various localities in northern Italy.

Several ramets of each clone were planted in each test plantation.

Selections of the best clones in the oldest of these plots are already

being propagated for commercial planting. ^ ^ c.a. u.-

Prof. Aldo Pavari, director of the Forest Experiment Station

in Florence, established a test plantation of 24 clones at Monsummano in 1941, 1942, and 1943. Ten of the best early selections oí

the Institute at Cásale Monferrato (including '1-214') and 14

named clones of Itahan origin were included in this test. The clone

Populus l-Caroliniano Bianco de Cercenasco' was considered the

best in 1952. ^ ^. .,

Germany.—Since the war there has been tremendous activity

in the selection of excellent local trees in all the poplar-growmg

regions of Germany. Local selections are being propagated by

farmers, nurserymen, estate foresters, State foresters, and research stations. Propagation is usually from single ortets and

these can be registered in several of the States as single clones.
PRODUCTION OF POPLAR TIMBER IN EUROPE 17

Cuttings from several taxonomically similar ortets may also be

propagated under a single name or number, but these must be registered as a clonal mixture—*'Clone Gemisch/' In most cases the

phenotypic excellence of the selected ortets, supported only by

vigorous nursery growth, is accepted as sufficient evidence to warrant multiplication for immediate use in practice.

The Forschungsstelle für Flurholzanbau der Lignikultur in

Reinbek near Hamburg, as the name implies, is sponsoring the

production of wood outside the forest; i.e., windbreak, hedgerow,

and roadside planting of poplars and willows. Although this research station is testing poplar selections from all countries, it is

particularly stressing local selections from northwestern Germany. If these local selections appear satisfactory after several

years in the test nursery they are recommended for use in the locality where the ortet was selected. By this procedure the station

expects to find the best clones for specific localities.

The Federal Institut für Forstgenetik und Forstpflanzenzüchtung, in Schmalenbeck near Hamburg, has a collection of more

than 300 selected clones from Europe and America. The best

clones, as determined by nursery performance, will be tested in

plantations.

In the Niedersachsischen Pappelmuttergarten in Harsefeld, the

State Forest Service has a collection of more than 200 clones, derived from selected local ortets, seedlings from controlled crosses,

and some exotic clones. Small numbers of trees of each of these

clones are tested in Lower Saxony for comparison with the best

registered types. New clones that are as good as or better than

the best clones in local use are then propagated in larger numbers.

This is a continuing program to obtain better clones and to replace

those that may fall prey to unusual environmental conditions, diseases, or insects.

Between 1947 and 1952 the Research Institute of the German

Poplar Society sponsored trial plantings of 18 poplar cultivars in

approximately 150 cooperative plantations scattered throughout

western Germany. During 1951 and 1952 the South German

Branch of the Society established another 150 cooperative test

plantings in Bavaria. The total number of trees in each planting

varied from 100 to 200 and included 10 or 20 trees of each poplar

cultivar. Planting stock (1- or 2-year-old trees) was purchased

from certified commercial nurseries.

Because these were primarily demonstration plantings, their

experimental design will provide only empirical clonal comparisons. The published objectives are (1) to demonstrate the economic possibilities and techniques of poplar culture; (2) to demonstrate choice of correct sites and soils in river bottoms, uplands,

forests, and fields; and (3) to compare the most promising poplar

cultivars under different environmental conditions.

In Baden, Dr. Friedrich Bauer (formerly Oberlandforstmeister,

now at the Forestry Institute of the University of Freiburg) correlates his selections with the site and soil on which the elite ortets (fig. 7) are growing. He has made selections of excellent

phenotypes from good poplar sites, dry sites, extremely heavy
18 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

soils, wet peaty soils, and climatically unfavorable upland sites.

These selections are propagated in 10 small state nurseries in

Baden, each selection being propagated on sites and soils as similar as possible to the site on which the ortet was found (fig. 8).

Soil specifications include depth of water table, fertility, pH

near the surface, and difference in pH between surface and lower

soil level. Baden state foresters who order cuttings are required

to provide standardized information, by numbers and symbols according to a guide sheet, on their proposed poplar planting sites.

The guide sheet covers geographic location, elevation—for climatic information, site information on soil-water relations, soil

structure, fertility, depth, and pH. On the basis of this information, clones from ortets selected from similar sites and soils are

provided for planting. Statistically controlled clonal tests had not

been established to compare the selected clones on similar sites or

on a series of different sites and soils.

Photo by F. BAUER

FIGURE 7.—One of Dr. Bauer's selected ortets in the Rhine bottom lands of

Baden, Germany. The branches have been removed to produce sucker

growth suitable for cuttings.
PRODUCTION OF POPLAR TIMBER IN EUROPE 19

In the Fiillbroch State Forest, Dr. Bauer had an interesting

15-year-old test of clones derived from crooked parent trees. The

ramets had been carefully trimmed and pruned every year to produce straight stems, but the trees were still crooked.

Several specimens of burly native P. nigra were observed in the

Rhine bottom-land forests of Baden. These are called "Maser

Pappel" in Germany and the wood is referred to as "Mappa Holz"

by local furniture manufacturers in Baden. The wood brings a

high price for face veneer, the price depending on the degree and

regular distribution of the burl. The average burly log was said

to bring about $700 per thousand board-feet. There apparently

has been no selection of the best burly trees for clonal propagation.

Photo by F. BAUER

FIGURE 8.—One-year-old rooted cuttings of selected clones in one of the

state poplar nurseries in Baden, Germany.

Many private growers and nurseries in Germany—too many

for individual mention—are selecting and propagating elite trees.

The Harff poplars (fig. 9)—selected and propagated under the

supervision of Dr. Bruno Schmitz-Lenders, who has worked with
20 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

poplars for many years—the Ruskamp poplars, and the Moos

poplars are well known in western Germany.

Sweden.—The search for and selection of natural triploids and

"plus" trees of the native aspen was an important part of the

^::¿ii^j^:

loto by SCHMITZ-LENDERS

FIGURE 9.—Fifty-year-old Harff poplar on excellent bottom land along the

river Erft, District of Cologne, Germany. Its d. b. h. is 45% inches,

total height 140 feet, total volume without bark (to 2.75-inch top) 494

cubic feet, including approximately 400 cubic feet of veneer and saw logs.
PRODUCTION OF POPLAR TIMBER IN EUROPE 21

early work of the Swedish Tree Breeding Institute. The growth

rate of these natural triploids, one clone of which was observed

at the forest estate Satra, is higher than that of the diploids. The

small stand at Satra, about 60 years old, is considered to be of

clonal origin (through root suckers) from a single ortet. Clonal

propagation of triploids and of diploid ''plus'' trees has not been

started on a commercial scale. The selected trees have been used

for intraspecific and interspecific breeding.

It is too early to compare the yield of Aigeiros hybrids with

that of the aspen hybrids, but observations on roadside trees

growing on upland sites in Sweden and Norway warrant the prediction that on good sites Aigeiros hybrids and hybrids between

Aigeiros and Tacamahaca poplars will grow faster than the aspen

hybrids. Such roadside trees have already been subjected to some

degree of selection for adaptability to specific environment. Selection and breeding with these types has lagged in Sweden and Norway because sites suitable for their culture are limited and because the match industry in these countries prefers aspen.

Great Britain,—A collection of poplars for clonal tests was begun by the Forestry Commission in the middle 1930's under the

direction of W. H. Guillebaud, but the war interrupted this work

while most of the collection was still in the nursery. Poplars have

received increasing attention since the end of the war. Largescale field trials of the most promising clones in the collection of

the Forestry Commission Research Branch, which were started

in 1948 by T. R. Peace, are being expanded in various parts of

England, Scotland, and Wales. Some of these are British selections, but the majority are the best clones from other countries of

Europe and from America. All clones are subjected to artificial

inoculation tests for resistance to bacterial canker.

Belgium.—The Dongelberg experimental plot established by

Professor Poskin is apparently the oldest clonal test in this country. This test planting was established with commercial stocks

used in Belgium. On the basis of sex and time of leafing. Professor

Poskm believes there are at least eight clones of P. 'Robusta' in

this plantation. There are also several clones of 'Regenerata,'

most of which, but not all, are susceptible to a disease that closely

resembles, and may be, bacterial canker.

The best clones from all countries are being tested at the Institut de Populiculture of the Union Allumettière in Grammont.

The most promising clones in nursery trials are tested in plantations. This work is too recent for any conclusive results.

In 1952 the Direction Generale de Eaux et Forêts established

an arboretum at Egenhoven-Bos, near Louvain, with five ramets

each of clones that appear promising for commercial use in Belgium.

France.—Between 1949 and 1955 the Direction Generale de

Eaux et Forêts established 15 poplar "forest arboretums"

throughout France. These plantings total 156 acres and contain

11,200 trees. The trees are under regular observation and measurement.
22 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

Previous to 1952 the Department had also established poplar

nurseries at Blois and in southern France to provide certified

cutting stock. It is not clear whether all the stocks in these nurseries represent pure clones derived from cuttings from single trees

or whether they were derived from cuttings taken from several

ortets that have the same taxonomic characteristics. In a letter

dated June 1, 1956, Dr. J. Pourtet stated that a new poplar nursery, representing pure clones, will replace these earlier nurseries.

The National Forest Research Station at Nancy is making selections of native P. trémula in which they recognize two races, a

mountain race and a plains race. The races are reported to differ

in the ratio of leaf length to width, in the lenticels, and m the

character of the bark. Selections of superior straight-boled trees

are being made in both races. .

Switzerland,—Clonsil selection and testing was started m Switzerland by the Federal Forest Experiment Station, located m

Zurich. In 1950, Dr. Fritz Fischer established a poplar arboretum

at Giritz near Koblenz with Swiss selections and some of the best

clones from other countries. Field tests have also been started or

are planned in other localities such as Lac de Neuchatel, Noville,

and in the vicinity of BelHnzona.

In 1954 the Forest School in Zurich took over the work with

Aigeiros poplars and with hybrids within this section for bottomland planting. The Federal Forest Experiment Station is now

handling the selection and testing of Tacamahaca poplars and

their hybrids, including intersection hybrids, for planting on upSpain,—Selection of elite ortets for local use has been a common

practice of the growers in the poplar regions of Spain. This is

being continued by the government foresters responsible for poplar reforestation and by the Forest Research Institute. Favorable

local environments for natural hybridization and survival of seedlings have provided some excellent phenotypes for selection. There

is also good P, alba in southeastern Spain. An excellent specimen

of this species was observed in a 25- to 30-year-old plantation of

Lombardy-type poplars near Hostalrich. This tree, the only white

poplar in the plantation, was 22 inches in d. b. h. and 100 feet m

total height, and it had almost 60 feet of clear trunk.

In addition to selections for vigor, disease resistance, and tree

form, selections are now being directed toward good phenotypes

that are growing on dry sites. The Spanish foresters are also interested in P. euphratica Olivier, which is reputed to grow on

alkaline soils. Unfortunately this poplar does not root from cutSince 1950 many of the best clones from Germany, France, The

Netherlands, Italy, Morocco, Syria, and the United States, and

clones derived from selected Spanish ortets, have been propagated

at the Institute for Forest Research in Madrid. Selections include

burly poplars for furniture face veneer. After preliminary observation and selection in the 20-acre experimental nursery, the

most promising clones are tested in plantations throughout the

country.
PRODUCTION OF POPLAR TIMBER IN EUROPE 23

Controlled Breeding

Denmark.—Poplar breeding in Denmark has been done almost

exclusively with the aspens. As a result of Dr. C. Syrach Larsen's

demonstration of hybrid vigor in crosses between P. trémula and

P. tremuloides, the experimental poplar nursery of the Danish

Match Company at Hellestrup has been converted to a breeding

station for the commercial production of Fi hybrid aspens. An estimated 150,000 two-year hybrid aspen seedlings were being produced here in 1952. This was insufficient to meet the increasing

demand.

Mass pollination is carried out in two greenhouses with lighting facilities for additional artificial illumination to hasten blooming and fruiting. One house is heated for early work in February,

the other is unheated for work later in the season. These breeding

houses have small pollination chambers arranged along one side

for special small-scale pollination work, but the open benches are

used for the mass pollinations.

Approximately one-fourth of the annual seed production has

been obtained from early pollinations on some 400 grafted plants

(in pots) of 15 selected female clones of P. trémula. Three-fourths

of the hybrid seed production has been on large flowering

branches 3 to 41/2 feet long, cut from native P. trémula trees growing in various parts of Denmark. The flowering branches are

brought into bloom in pails of water on the greenhouse bench and

are pollinated by hand dusting. In the past most of the P. tremuloides pollen has been obtained from Canada.

The fruiting catkins are collected before they are fully open

and are placed in glass-covered boxes in which the catkins are

spread out one layer deep on screens until they open. The seed is

cleaned in a small, screened, rotating drum with a fan to blow the

seeds through the wire screen. The cotton stays inside the drum.

The Hellestrup Nursery maintains a record of the parentage of

each seed lot and a record of where the hybrid seedlings are outplanted. The growth and development of the individual progenies

will be checked to select the best parents for future hybridization.

A large number of P. tremuloides clones and seedlings of selected trees from eastern and western Canada, collected by Dr.

Larsen, have been planted in the arboretum at Hellestrup. These

will be used as parent stocks as soon as they come into bloom.

Sweden.—A^^en breeding has been one of the projects of the

Swedish Tree Breeding Institute in Ekebo and the branch stations

m Brunsberg and Sundmo since its founding in 1936. The experimental breeding has included intraspecific crossing of **plus'' trees

of P. trémula, polyploidy breeding, and species hybridization. The

Institute had also mass-produced trémula X tremuloides hybrids

for commercial sale when this did not interfere with research. In

1951 they produced and sold about 20,000 one-year hybrid seedlings.

The progenies derived from crosses between "plus" trees of P.

trémula have not been as rapid growing as the aspen hybrids. For

this reason production of commercial planting stock from mass-
24 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

produced seed of *'plus'' trees of P. trémula has been overshadowed by the demand for Fi hybrids of trémula X tremuloides.

There has been little demand for the mass production of triploid

P. trémula (obtained by crossing diploid X tetraploid) apparently

because of the variability of the seed progenies. The seedlings

range in vigor from those that equal the hybrid aspens in growth

rate to very poorly growing, almost dwarf individuals that aré

graded as nursery discards. Triploid species hybrids, between diploid P. tremuloides and tetraploid P. trémula, are particularly

promising. Selected fast-growing and disease-resistant triploid

clones will undoubtedly find a ready market when commercial

vegetative propagation is started.

The Swedish Match Company maintains Mykinge Försöksgard,

an experimental area near Jönköping, in which some of the Ekebo

aspen progenies are being field tested. A planting of P. trémula X

tremuloides hybrids made in 1943 with 1-year seedlings on an area

logged in 1941 clearly demonstrates their early vigor. Although

this site was covered by a dense stand of root suckers when the

hybrids were planted, the native aspens were completely suppressed by 1950. In the 1947 plantation of 1-year-old seedlings of

tremuloides X trémula, trémula X tremuloides, and crosses between selected diploid trémula parents, the most uniformly vigorous progenies were the tremuloides X trémula hybrids ; the direction of the cross seems to make little difference in vigor.

It is too early to predict the adaptability and disease resistance

of these new hybrids. Test plantings to date indicate that their

early growth rate is extremely promising. Frostcrack and sunscald observed on some of the hybrids indicate the need for continued testing for climatic hardiness. At present the chief risk

with these hybrids appears to be disease, which will be discussed

under that heading.

In 1950 the Match Company built greenhouse and nursery facilities at Mykinge, similar to those in Denmark, for the commercial

production of aspen hybrids; the initial production goal was

200,000 hybrid seedlings per year. The hybridization work is

started in February in greenhouses equipped to provide additional

light to hasten the development of flowers and seed. Thp seed

ripens and is planted approximately 3 weeks after pollination.

The Netherlands—Vo^lSiV breeding is a major project of the

Forest Research Institute in Wageningen, under the direction of

Dr. H. van Vloten. The breeding techniques used at the Institute

stem from, and are basically similar to, the methods used for experimental poplar breeding in Denmark and Sweden.

Male branches, in water, are placed in small glass-partitioned

chambers in a sun porch of the laboratory building and the pollen

is permitted to shed. Female branches are bottle grafted on 1-

year-old rooted cuttings in pots. During the growing season these

pots are plunged outdoors, but the plants are lifted periodically

to prevent the roots from growing out into the soil. Kept rootbound, the plants—aspens particularly—will bloom for several

successive years. On Leuce poplars, pollinations are also made on

flowering branches that are kept in water until the seed matures.
PRODUCTION OF POPLAR TIMBER IN EUROPE 25

Before and after pollination the female grafts and flowering

branches are given artificial illumination between 5:30 p.m. and

7:30 a.m. to hasten flowering and seed maturation (fig. 10).

BOSBOUWPROEFSTATION T. N. O. photo

FIGURE 10.—Control-pollinated bottle grafts maturing seed on greenhouse

bench at the Forest Research Station, Wageningen, The Netherlands.

These potted plants are given additional artificial illumination during the

night to hasten seed maturation.

Special pollination chambers, constructed of glass and aluminum and slightly larger than a telephone booth, are arranged

along the sides of the greenhouse (fig. 11). They are heated by

steampipes along the wall and are connected at the top to an exhaust ventilating system. The temperature of the chambers is

maintained at 20° to 22° C. If the temperature rises too high, the

ventilator is turned on and cool greenhouse air is drawn into the

bottom of the chambers.

Several female parents are brought into a chamber just before

the first flowers are receptive and are pollinated each day as the

successive flowers in the maturing catkins become receptive. Pollen from one tree only, or from a single clone, is used in each

chamber. When the pollinations are completed, the female catkins

are washed with a fine water spray and the potted plants are returned to the greenhouse bench. Each pollination chamber is thoroughly washed down before another set of pollinations is started.

The control-pollinated seeds are planted individually on thin

blocks of pressed peat that are thoroughly soaked in water before

use. After planting, the blocks are kept in a glass-covered greenhouse bench. The germinated seedlings are lifted from the peat

blocks after about 7 days and are transplanted to seed pans or

flats that contain a thin surface layer of heat-sterilized soil. This

method, seen only in The Netherlands, is said to eliminate loss
26 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

BOSBOUWPROEFSTATION T. N. O. photo

FIGURE 11.—Pollination chambers in the greenhouse of the Forest Research

Station, Wageningen, The Netherlands.

from damping-off. The seedlings are given artificial illumination

between 5:30 p.m. and 7:30 a.m. until they are potted and placed

in coldframes.

Many progenies from crosses in the Leuce and Aigeiros poplars

were growing in the nursery in 1952. The poplar breeders in The

Netherlands, as in all other countries, were faced with the problem

of finding land for plantation tests of the large numbers of seedlings that are relatively easy to obtain from controlled breeding.

Great Britain.—The earliest controlled breeding work with poplars was done by Augustine Henry in 1912 and 1914. His bestknown poplar hybrids are the clonal mixture that he named P.

Xdenerosa, Fi hybrids between angulata X trichocarpa. There

has been no recent controlled breeding with poplars in England.

Germany.—Poplar breeding on a comprehensive scale was

started by Dr. W. Wettstein in 1930. Although this project was a

casualty of World War II, a plantation of Wettstein's hybrids—

now more than 17 years old—near Frankfurt includes some excellent individuals that have been selected for clonal testing.

The most extensive controlled-breeding program in West Germany is centered at the Federal Forest Genetics Research Station

in Schmalenbeck near Hamburg. Although emphasis has been on

the breeding of the Leuce poplars, work has also been started on

Aigeiros poplars. Controlled breeding with the Leuce poplars includes breeding with triploid aspen, intraspecific crosses between

selected phenotypes, interspecific hybridization, and since 1954 a

breeding project with triploid P. Xcanescens. A major concern is

to decide how large a progeny should be outplanted for future selection and to find sufficient suitable land for plantation tests.
PRODUCTION OF POPLAR TIMBER IN EUROPE 27

Limited controlled breeding has been done by other German

workers. At Harsefeld, in Lower Saxony, Forstmeister H. Lücke

has made some crosses and has been attempting to produce polyploids by colchicine treatment. Prof. Rohmeder, in Munich, has

progeny from a cross between P. 'Marilandica' (female) and Itálica' (male).

Austria.—Since the end of World War II Dr. W. Wettstein, at

the Federal Forest Research Station in Hadersdorf (Vienna),

has been breeding Leuce and Aigeiros poplars. Breeding and selection for photoperiodic response is considered particularly important because early spring budding and delayed fall dormancy are

the most serious hazards at the higher elevations in Austria. Controlled breeding is done on flowering branches brought into the

greenhouse and pollinated during January and February—the

method devised by Wettstein in his early work in Germany. Approximately 10,000 seedlings were produced in 1956, including

crosses with pollen from Spain and Korea.

Belgium,—The Poplar Experiment Station of the Belgian Match

Company, established and specially designed for poplar breeding,

has excellent laboratory and greenhouse facilities, including pollination chambers, and a good nursery site. The breeding techniques

used by Dr. C. Muhle Larsen, Director of the Station, are essentially the same as those described for The Netherlands. The breeding work is still in its early stages. The major objectives are intraspecific and interspecific breeding to obtain improved clones for

commercial use, and basic genetic information on various taxonomic and physiological characteristics.

France,—In the poplar nursery of the Match Company in Saintine, Mr. Chardenon is growing 27 seedlots of American poplar

species for future breeding work. They have also been experimenting on the grafting of flowering scions of Aigeiros hybrids

from old trees on rootstocks in small pots. These have been treated

m the same manner as aspen grafts in Denmark, Sweden, and The

Netherlands. The oldest grafts were 3 years old in 1952 and all

had failed to bloom after the first year. They have also grafted

scions of black poplar hybrids on willow as a possible method for

getting partially incompatible grafts that might bloom at an early

age.

Intraspecific and interspecific breeding with aspens and white

poplars was started in 1954 by the National Forest Research Station at Nancy. The aspen hybridizations include crosses with pollen of P. tremuloides and P. grandidentata from Canada.

Italy,—A limited amount of controlled breeding is done every

year at the Institute for Poplar Research at Cásale Monferrato

with Italian pollen and pollen from other countries. Tetraploid

pollen has also been used. Exceptional conditions that favor natural hybridization throughout the Po Valley have made available

seed and seedlings in almost unlimited quantity from parents that

have been subjected to severe natural selection resulting from the

prevalence of epidemic diseases. Consequently, only about 2 percent of the clones under test by the Institute have been obtained

irom controlled pollinations. Most of the clones have been derived
28 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

from naturally pollinated seed collected from superior phenotypes.

Several seedlings, from seeds received from our lower Mississippi

region, are considered to be very promising. In 5 years, one of

these (clone 72/51) had grown to a diameter of 13 inches at 3.3

feet above the ground.

Spain.—Controlled breeding with Aigeiros hybrids XP* mgra,

trémula X (^Iba, trémula X 'Bolleana,' trémula X nigra, and alha

X nigra has been started since 1953 at the Forest Research Institute in Madrid. Breeding is directed toward rapid growth, improved wood quality for veneer, lumber and pulp, and disease resistance. All progenies derived from controlled breeding are tested

in the nursery in Madrid and the most promising are then used in

field tests.

Significance for the United States

The possibilities for selection of natural hybrids are limited in

the United States because exotic species or hybrids have seldom

been used in commercial plantations. Exotic species have been

planted in botanical gardens, arboretums, and—to a very limited

extent-—as shade trees. Although the older European hybrids have

been extensively planted as street and shade trees, male clones

were used almost exclusively because the female trees produce objectionably large amounts of cottony seed.

Natural hybridization has undoubtedly occurred where such

European hybrids have been planted within the pollination range

of our native poplars. But the germination and survival of such

hybrid seed and seedlings would depend on a combination of favorable site conditions that are seldom present in residential areas

where poplars are planted as shade trees.

Our native species, P. deltoides, P. trichocarpa, and P. tremuloides, three of the best timber poplars in the world, merit intensive racial studies and clonal selection and testing. Select clones

can be used for planting until tested hybrids are available, but

their principal value will be as parents for controlled breeding and

hybridization. . .i xi ^

The best of the European and Asiatic poplar species, those that

have proved to be resistant to European diseases and insects,

should be introduced and tested in different regions of the United

States for possible use in practice and future breeding. They

should be imported by means of seed, not cuttings; it is impractical, if not impossible, to sample adequately the inherent variation

within an exotic species by the importation of cuttings.^ If possible, seed should be obtained from 5 to 10 trees selected at random in each of a sufficient number of localities to saniple the entire range of a species. This would provide information on possible racial variation as well as on individual variation.

Clonal tests must be set up in comparable, statistically sound,

field designs under a wide variety of environmental conditions be-

' Poplar cuttings can be imported only through the Division of Plant Introduction and must be grown in quarantine at the Plant Introduction Garden

for 1 or 2 years before they are released.
PRODUCTION OF POPLAR TIMBER IN EUROPE 29

fore individual clones of native species or new hybrids can be recommended for commercial planting. The excellent growth and

form of the selected ortet, substantiated only by nursery performance of the ramets, is not an adequate clonal test.

A central clonal nursery, a "clone bank," is needed to supply the

demand for certified cuttings for nationwide tests. Only clones

that have been adequately tested and found superior in at least

the locality of their origin—and only the very best foreign clones,

including those with figured wood—should be accepted and grown

in such a central clonal nursery.

We need many demonstration poplar plantations, similar to

those established by the German Poplar Society, to bring the poplars best adapted to a region or locality to the attention of farmers and landowners.

Controlled breeding and hybridization with our best native

species should be started or expanded in the South and West because we do not yet have the possibilities for extensive natural

hybridization. Large progenies of known parentage can be obtained easily through controlled breeding. Such progenies should

be outplanted for selection of the best ortets for clonal tests after

10 to 15 years.

Aspen hybridization deserves special attention where the native

aspen is at present commercially important or where it is better

adapted to reforestation than other native species. If aspen hybrids prove to be as promising in this country as in Europe, it is

quite certain that their mass production as Fi hybrids would also

be economically feasible in the United States.

NURSERY PRACTICE

European poplar nurseries are invariably on good agricultural

soils, and most nurserymen practice a rotation that allows an interval of 2 to 5 years between successive poplar plantings. Organic

manuring, direct or by cover crops, and the use of mineral fertilizers where necessary, is standard practice. Irrigation is considered essential in many nurseries. Spraying for insects and nursery

diseases is practiced in all countries. Crooked trees are often

staked because only straight stems can be certified for sale where

certification is in effect. Where leaders are lost or damaged, laterals may be tied to vertical splints to obtain straight stems.

Pruning of nursery stock is not uniform in any country. The

most common practice is to prune to a single whip by removing all

side branches just before or immediately after the trees are dug

for sale.

A traditional and common practice in some parts of Germany

and France is to remove all side branches except three or four of

the strongest laterals of the current year's growth. These lateral

branches are cut back to short stubs 2 to 8 inches long (called

Zuchtruten in Germany), which may be removed in the second

year after planting. This heavy pruning is advocated to hold back

top growth during the first year in order not to overtax the temporarily reduced root system. Some of the most progressive Ger471632°—59 3
30 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

man nurserymen prune trees for roadside planting as above, but

prune trees for forest planting to only one-third or one-half their

height.

In The Netherlands, 1-, 2-, and 3-year stock is generally pruned

progressively during the last year in the nursery to avoid retarding grov^th and to obtain rapid callusing. The trees are pruned to

one-fourth their height in early spring, to one-third in June, and

to about one-half in the fall before they are dug for sale. Some

nurseries in France, including the Cacor State Poplar Nursery,

are pruning their trees progressively during the grov^ing season

to approximately one-half or two-thirds of the stem height. Pruning stumped trees to one-half their height is recommended and

practiced in Italy and in England.

Aigeiros Poplars

The black poplars, cottonv^oods, and their hybrids are commercially propagated by cuttings and less commonly by sets. Cuttings are usually 10- to 15-inch lengths of dormant 1-year-old

stems. Sets are dormant whips, 4 to 10 feet or more in length—

most frequently 6 to 8 feet, taken from nursery stools, pollarded

trees, or the top branches of felled trees. They are usually 1 or 2

years old, but may be older where they are taken from the upper

branches of mature trees. Although sets are most often used directly for outplanting, a few nurseries produce 1-year-rooted stock

from 1-year-old sets that are 3 to 6 feet long and are planted 10

to 18 inches deep.

Throughout most of the poplar regions of Europe, cuttings are

obtained from stools, spaced 3.3 by 3.3 feet to 4.5 by 6.5 feet apart,

that are usually cut back to 2 to 12 inches above the ground. Some

growers prefer higher stools (up to 3 or 4 feet) for easier cultivation and handling. The current growth, cut back to the stool

each winter, is used for cutting stock.

Stooled plants, with their many stubs, tend to accumulate such

facultative parasites as Valsa and Dothichiza, and also boring

insects. The buildup of diseases and insects in stools varies from

nursery to nursery, depending on the cultivar, climate, site, and

nursery practice. This has led to wide differences of opinion about

the effective life of a stool planting; estimates range from 4 to

15 years or more. Loss of vigor of the stools is generally attributed

to the annual harvesting of the shoots rather than to facultative

parasites. In all cases that came to my attention, the early decline

in vigor was most probably due to the inroads of Valsa, Dothichiza, wood and bark borers, and other diseases or insects.

In Baden, Germany, Dr. Bauer is testing the possibility of

maintaining healthy stools by very careful trimming of pollards

3 feet high to promote rapid callusing of smooth, rounded heads

without projecting stubs (fig. 12). In some of the Baden poplar

nurseries the weaker shoots are removed from the stools in early

summer to force the stronger shoots into better height and

diameter growth.
PRODUCTION OF POPLAR TIMBER IN EUROPE 31

Photo by F. BAUER

FIGURE 12.—Production of cuttings on 3-foot-high pollards in one of the

state poplar nurseries, Baden, Germany. The current growth is removed

annually for cuttings or sets. The tops of the stools are very carefully

trimmed to promote rapid callusing of smooth, rounded heads vi^ithout

projecting stubs.

There has been much emphasis in Europe on how cuttings

should be made, stored, and planted. Many nurserymen and poplar

experts claim that a good cutting must be cut at an angle, not

transversely; that the upper cut must be immediately above a

bud; and that the cuttings must be made with a sharp knife

because pruning shears make inferior cuttings.

Cuttings (figs. 13,14) are generally made during the winter, tied

in small bundles, and stored in moist sand in a storage cellar,

heeled-in in light soil, or buried in pits until planting time. Some

growers insist that cuttings should be stored vertically with the

butt ends up. There is difference of opinion as to whether cuttings

should be callused before planting.

Length of cuttings varies from 9 to 15% inches, with 9% to

11% inches the most common length where cuttings are planted

flush with, or very slightly below, the surface of the soil. As

soon as there is sufficient growth, all but one shoot is removed and

the soil is hilled-up slightly over the top of the cuttings. This

deep planting, presumably to decrease the possibility of disease

infection (a doubtful control measure) and to favor growth of

the top bud, is widespread but not universal.

Many nurseries in France plant the longer cuttings, 11% to

14% inches long, with two or more buds above the ground. The

Raverdeau Nursery and the match company nursery at Saintine

have been planting 13%-inch cuttings with 3 or even 4 buds above
32 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

4 5 6

I I I I I I I IINCHES

GREAT BRITAIN FORESTRY COMMISSION photo

FIGURE 13.—Type of cuttings recommended by the Forestry Commission of

Great Britain.

Photo by FERNANDO JAIME FANLO

FIGURE 14.—Poplars at the end of their second season's growth from cuttings, in the state poplar nursery at Castelnov on the Martin River,

Province of Teruel, Spain. Average height at the end of the first year

was 9 feet.
PRODUCTION OF POPLAR TIMBER IN EUROPE 33

ground. In June the cuttings are pruned to one good shoot, usually

one from a lower bud. On cultivars like Populus *Regenerata/

which have a tendency to spreading growth, the young shoot is

tied to the top of the cutting to obtain straight-stemmed trees.

Nursery spacing of cuttings varies greatly. Where the rooted

cuttings are hfted and replanted at the end of 1 year, close spacing

is the rule. In Italy they are planted 3 to 4 inches apart in rows

spaced 32 to 40 inches. In northern Europe, cuttings may be

spaced as close as 6 to 8 inches in rows 191/4 inches apart. In

The Netherlands the 1-year rooted cuttings are then lined out

3.3 by 3.3 feet and are grown for 1 or 2 years at this spacing.

Spacings of 15i/^ by 311/^ to 40 inches are used in northwestern

Germany for the production of 2-year stock without transplanting. In France the spacing for cuttings that will be dug as 2-yearold trees is usually l^Yz to 19i/^ inches by 3.3 feet.

Disease conditions have been so serious in Italy that stool

plantings are now seldom used for the production of cuttings. The

recommendation of the Poplar Research Institute at Cásale Monferrato that stumped trees be used for outplanting has been widely

accepted. The stumped-tree system eliminates the hazard of infected stock from diseased nursery stools and saves the nursery

space and labor required to maintain stool plantings.

Stumped plants are produced by cutting back 1-year-old trees

to a short stump with two or three buds, close-pruning the roots,

and replanting the stumps in nursery rows. The stumped plants

(fig. 15) are planted level with the ground and are spaced 16

to 24 inches in rows 5i/4 to 7 feet apart. When the stumps have

sprouted and the new shoots are 10 to 12 inches high, they are

pruned to one shoot and are hilled to cover the stump.

Cuttings are made from the stems of the 1-year-old trees from

which the stumped roots are obtained. The cuttings are planted

with their tops at ground level, and after growth has started the

soil is slightly hilled around the shoots to cover the top of the

cutting. Cuttings are spaced 3 to 4 inches in rows 32 to 40 inches

apart; or to conserve space, they may be planted in single rows

between the rows of stumps.

At the end of the year, the 1-year rooted cuttings are dug, cut

back, and replanted as stumped trees and cuttings. The rooted

trees are dug and sold as 2-f 1 stock (2-year root, 1-year top)

if they have grown to sufficient size; otherwise they are left

another year to produce 3+2 stock up to 24 feet in height. The

demand is heavy for large trees because their use usually brings

the grower a better financial return.

Some Italian planters who raise their own nursery stock on

extremely fertile soils use the largest of their 1-year-old trees

(8 to 10 feet tall) for outplanting and get satisfactory results.

Large 1-year trees are also sold by commercial nurseries where

there is a heavy demand for planting stock, but this practice is

not recommended by the Poplar Institute.
34 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

GREAT BRITAIN FORESTRY COMMISSION photo

FIGURE 15.—Stumped 3+2 (3-year root, 2-year top) ready for outplanting.

Kennington Forestry Commission Nursery, England.

Leuce Poplars

Populus alba, the European white poplar, is usually propagated

from cuttings or sets in the same manner as the Aigeiros poplars.

The gray poplar P. Xcanescens represents a confusing mixture

of natural hybrids between P. alba and P. trémula, and undoubtedly includes many generations of crosses between hybrids and

backcrosses to both parents. The rooting ability of these hybrids

varies, but few clones root well enough for commercial propaga-
PRODUCTION OF POPLAR TIMBER IN EUROPE 35

tion from cuttings. Where there is demand for the gray poplars

they have usually been propagated from root suckers, root cuttings, or by layering rooted stems in a shallow trench with the

tip exposed. By this method most of the buds develop shoots that

root around the bud and can then be cut into separate plants

at the end of the growing season.

W. Gröhn, a nurseryman in Halstenbek, Germany, stated that

he had good success in rooting gray poplars by grafting short

willow cuttings to the base of the poplar cutting before planting.

Both the willow cutting and the poplar cutting produce roots.

When the trees are dug, the willow cutting is simply broken off,

leaving a well-rooted poplar.

P. trémula, the European aspen, is usually grown from seed.

In the very few places where aspens are propagated vegetatively,

the commercial nurserymen use root suckers, root cuttings (fig.

16), or layers, as for the gray poplars.

Photo by W. WETTSTEIN

FIGURE 16.—European aspen propagated from root cuttings. Short pieces

of root are planted vertically or horizontally. Wettstein, in Austria,

reports up to 60 percent rooting, with great variation between individual

trees.

Aspens and gray poplars can also be successfully rooted by

the "RS-cutting" method developed by C. Muhle Larsen. In late

February pieces of root 2 to 4 inches long are placed in moist
36 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

peat moss, lightly covered, in a greenhouse or hotbed. By the

end of March, the root pieces have produced softwood shoots.

Aspen is more prolific than gray poplar ; one 4-inch length of

aspen root produced 38 shoots. The 1- to 2-inch shoots are removed during April and May and are rooted in clean gravel m

the greenhouse, hotbed, or coldframe.

The P. trémula X tremuloides hybrids are grov^n from massproduced hybrid seed. At the Hellestrup nursery in Denmark,

the hybrid seed is sown in outdoor beds on 2 to 3 inches of

thoroughly composted soil that has been steam sterilized in the

beds at 100° C. for 1/2 hour with portable equipment. DampingOÍF has not been serious. Sowing is at approximately 1 gram of

seed per 4 square meters (44 square feet) of bed. At this density

of sowing they can expect approximately 1,000 one-year-old seedlings.^ The seed is covered lightly with composted soil and the

beds are covered with glass for about 1 month. Past practice

was to shade the beds during summer, but in 1952 the nursery

experimented without shades to obtain stouter seedlings.

The 1-year seedlings are transplanted during the fall and early

spring to nursery rows spaced 12 inches apart with 24 inches

between every third and fourth row. This spacing is adapted to

the tillers used for cultivation. The nursery is on a soil that has

high pH and is underlain with chalk.

At Mykinge Försöksgard, Sweden, mass production of hybrid

aspen begins in early February. The seed is sown on sphagnum

moss litter in greenhouse seedbeds about 3 weeks after pollination By early June the hybrid seedlings, 2 to 4 inches high, are

ready for transplanting to outdoor beds. All seedhngs are hfted

in the fall and stored indoors in sand during the winter. The

largest 1-year seedlings are sold in the spring and the smaller

trees are planted in transplant rows to be sold as 1-1 stock.

Significance for the United States

In general, the excellent nursery practice used for the production of poplar planting stock in Europe is applicable to poplar

culture in the United States. ^ . , ^ 4. i -4.

Care and precision in making cuttings (angle oí cut, knite

vs. shears) and in storage of cuttings are not necessary. Between

1928 and 1932, in our work with poplar in the Northeast, we found

that we could obtain practically 100-percent survival (without increasing the hazard of disease infection) by cutting bundles oí

whips into cuttings on a fine-tooth circular saw. We have stored

cuttings successfully in all positions—horizontally, vertically, with

tops up and tops down ; in moist sand ; in peat moss ; and m sawdust at temperatures from 20° to 34° F., sufficiently low to prevent

the production of roots or bud bursting before planting. We have

' It has been estimated that 540 grams of hybrid seed will Produce approximately 400,000 one-year seedlings and 200,000 salable 1-1 seedhngs.

The smallest seedlings are discarded.
PRODUCTION OF POPLAR TIMBER IN EUROPE 37

also found that callusing does not increase the rooting or growth

of clones selected for good rooting ability.

Cuttings 10 to 12 inches long are the most practical for nursery

planting in the United States.

Nursery spacing of 12 to 18- inches by 2 to 3 feet is generally

sufficient for the production of 1-year-old rooted cuttings, 18

to 24 inches by 3 feet for stumped 2+1 stock, and 2 by 6 feet for

stumped 3-f2 stock.

Planting cuttings just deep enough to leave two buds exposed

is preferable to planting level with the ground because this permits easier machine cultivation with a minimum of injury to the

cuttings early in the season. There is no sound evidence that deep

planting will prevent disease or produce straighter trees. Treatment of cuttings with an effective fungicide will give much better

control of disease than deep planting, and straightness of stem

depends primarily on the inherent nature of the clone. Some

clones must be pruned early to a single stem; others produce a

straight stem without pruning.

The Italian practice of using stumped trees for outplanting and

of producing cuttings from the 1-year-old trees deserves strong

recommendation.

The careful hand labor in making cuttings and the cost of

splinting and staking in the nursery to produce straight trees

cannot be justified in this country. Extremely crooked trees

should be discarded ; slightly crooked trees of inherently straightstemmed clones will straighten up after outplanting.

Vegetative propagation of white poplars and aspens by present

European methods, which are barely feasible economically even

in Europe, cannot be recommended for commercial use in this

country. It should be possible to produce P. Xcanescens hybrids

that will root from cuttings under nursery conditions by backcrosses to P, alba parents that have good rooting ability.

Hybrid aspen seedlings could be produced commercially in the

United States by the method used in Denmark, but minor modifications would be needed to permit use of our standard nursery

equipment.

SITE AND SOIL REQUIREMENTS

Aigeiros Poplars

These poplars are generally considered bottom-land species,

and relatively few foresters or growers in the past have dared to

recommend or try commercial culture on upland sites. But in

all European countries, and in America, Aigeiros hybrids have

been freely planted on upland sites as shade or roadside trees.

In Spain particularly, European black poplars of both pyramidal

and spreading types have been grown for centuries along roadsides seemingly too dry for their survival. Growth is very slow

but the trees survive for many years under extremely adverse

conditions.
38 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

Where soil depth, fertility, pH, and moisture conditions are

adequate, the growth of Aigeiros poplars on such upland sites

is as good as on bottom lands. And conversely, on bottom-land

soils of low fertility, low pH, or poor aeration, the poplars grow

as poorly as on infertile or dry upland soils. European poplar

plantations clearly demonstrate that the limiting soil factors for

profitable poplar culture are depth, fertility, pH, moisture, and

aeration.

Soil depth

The Aigeiros poplars require soils 2 to 3 feet deep or more

for their maximum growth. In general, the minimum depth on

which reasonably satisfactory growth was observed in European

plantations was approximately 18 inches. There were occasional

exceptions, along rivers where the water was reported to be rich

in nutrients, where growth was very good on such relatively shallow soils. The soil depth available for the growth of poplars may

be limited by bedrock, hardpan, heavy clay, or stagnant ground

water. Moving, well-aerated ground water in river bottoms or on

slopes does not limit soil depth for the growth of poplars.

Soil fertility

Research on the fertility requirements of poplars has been

started in many places, but the work is too recent to provide an

accurate measure of the fertility level required for profitable

poplar culture. Observations on many European plantations indicate that the most profitable growth rate requires a level of

soil fertility equal to that needed to grow a profitable corn crop

in the United States. The excellent growth of poplars on river

sandbars does not contradict their need for fertile soils. Such

trees are utilizing the fertility of the river water.

The effect that soil fertility has on growth rate of poplars was

strikingly apparent in a 7-year-old plantation of clone Populiis

'1-214' in the Po Valley in Italy. This planting was established

on a river bottom site with a soil gradient, along the rows, from

fertile sandy silt to coarse gravel. Here the same clone varied

from 15 inches d. b. h. on the fertile silt to 6 or 7 inches d. b. h.

on the less fertile gravel. There apparently was not sufficient

variation in pH, permeability, and depth of the water table on this

relatively flat site to account for these growth differences.

Soil pH

Liming experiments on a well-drained upland site at the

Forschungsstelle für Flurholzanbau in Reinbek, Germany, have

demonstrated that on their soils the critical pH for poplar is

4.5 to 5.0. These experiments were based on the growth of nursery

stock on land cleared of a beech forest where the pH was originally

3.5 to 3.8. Below pH 4.5, growth was extremely poor; at pH 5.0

growth was fair: but for maximum growth the site required

liming to approximately pH 6.0.
PRODUCTION OF POPLAR TIMBER IN EUROPE 39

Low pH is seldom a limiting factor in the commercial poplar

regions of Europe because most of these river-bottom soils are

above pH 5.5. The threshold of pH 4.5 to 5.0 is strongly indicated but not confirmed on the relatively few sites with low pH

where poplar has been planted in Europe. Unfortunately, heavy

soils and poor drainage complicate the interpretation of poor

growth on practically all such acid sites.

For example, the clonal test plot of the Netherlands Land Reclamation Society near Zwolle is on polder land where the depth

of the water table is kept constant at approximately 20 inches.

The soil is a highly organic sandy clay (30 to 50 percent clay)

on peat. The soil is 20 inches deep at one side of the test plot

and grades to a depth of 12 inches at the other side. The pH

varies from 4.2 to 5.2. Although the soil is considered moderately

heavy, there appears to be little water movement and inadequate

aeration. Growth of the certified Netherlands clones is extremely

poor and all clones are infected by bacterial canker. On this site

it was not possible to separate the effects of low pH and poor

aeration.

Soil moisture and aeration

The Aigeiros poplars require a moist soil during the entire

growing season, or a water table within reach of the roots. There

are examples of good plantations on sites with light, relatively

dry soils where the summer water table is reported to be 12 to

15 feet deep. On upland sites, the best growth was observed more

often on medium heavy soils, apparently because their waterholding capacity is better than that of light-textured soils.

Soil aeration during the active growing season is essential.

The Aigeiros poplars will grow well on sites that have high

ground water if the topography and a light soil permit free

water movement to provide aeration. Heavy impervious soils on

wet sites, and light permeable soils with a stagnant water table

that does not drop below 18 inches during the growing season, are

unsuitable.

The Chautagne plantations near Aix-les-Bains, France, present

an excellent example of the poor growth of poplar on wet sites

where there is insufficient ground-water movement to provide

adequate aeration. Approximately 1,700 acres of marshy land

below the outlet of the lake were purchased by the government

in the late 1930's, drained, and planted with poplar. The soils,

pH 7.0 to 8.0, vary from deep silty clay alluvium to stratified

layers of peat alternating with thin layers of clay and silt. The

drainage ditches originally installed were too shallow and too

widely spaced. Much of the area carried a heavy growth of reed

(Phragmites communis) in 1952.

Three-year-old rooted trees were originally planted at 13- by

16.5-feet spacing without site preparation and without cultivation after planting. The stock was mostly P. 'Robusta' with a

limited mixture of other cultivars of the 'Virginiana,' 'Regenerata,'

and 'Serotina' types. The growth of all cultivars on the poorly
40 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

drained silty clay soils has been unsatisfactory, and on the peat

soils without site preparation most of the poplars have failed to

survive. 'Robusta' has shown itself the most poorly adapted to

these wet sites. Small '^bleeding'' spots on the stems, resembling

trunk scab, were abundant in these plantations.

In 1950, three experimental plots were laid out in the 1939

plantation on deep, moderately heavy, silty clay, and were given

the following treatments : (1) control, (2) calcium nitrate fertilizer appHed annually, and (3) thinning to half the stand by cutting out every other tree. Annual increase in circumference 2

years after treatment indicated no significant benefit from fertilizer or thinning. This tends to confirm the conclusion that the

principal deleterious factor in these plantations was insufficient

soil aeration due to poor drainage.

It was planned to improve the drainage on these sites by increasing the number and depth of the drainage ditches. To make

the peat soils (peat with alternating thin layers of silt and clay)

more permeable and so to provide better drainage, these areas

were to be plowed 2 to 3 feet deep and permitted to lie fallow for

2 years before planting.

There are examples in France, Germany, and the Low Countries

of attempts to establish poplars on poorly drained sites by planting the trees on mounds from 6 to 10 feet in diameter and from

2 to 3 feet in height. The author did not see any plantings of this

kind where the poplars continued to grow satisfactorily for more

than 6 to 8 years. Mounding the soil in a continuous bank along

the drainage ditches (Rabattenpflanzung) was observed to be

effective on heavy soils where the drainage ditches were not more

than 20 to 25 feet apart and the mounds were IVè to 2 feet high

and 4 to 6 feet wide.

Poplars grow poorly on the banks around the rice fields in

northern Italy, which are flooded from April to September. Apparently these banks are not wide enough to provide sufficient

well-aerated growing space for the poplar roots.

Site-indicator plants

The study of ground-vegetation types as indicators of site quality for the Aigeiros poplars has received considerable attention

in Germany.^ Many of the species recognized in Germany as indicators of good, medium, poor, or nonpoplar sites are native to the

United States, have become naturalized or occur locally as adventive escapes that are not sufficiently widespread to be considered

naturalized. With one exception {Impatiens), all species in the

following lists are found in the United States.

Indicators of the best poplar sites,—The most productive bottomland sites are highly fertile, well drained, and well aerated, and

^Hesmer, H. (editor). Das pappelbuch. Deut. Pappelverein. 304 pp., illus.

Bonn. 1951.

Rüskamp, Gustav. Lohnender pappelanbau. 88 pp., illus. Dülmen (Westfalen). 1952.
PRODUCTION OF POPLAR TIMBER IN EUROPE 41

the ground water is within easy reach of the poplar roots. They

are usually characterized by the predominance and lush growth of

several of the following species :

Circaea alpina L. enchanters-nightshade Indigenous

C canadensis Hill :; do. Do.

Deschampsia caespitosa (L.) Beauv. tufted hairgrass Do.

Galium aparine L. cleavers, goosegrass Do.

Geum rivale L. water or purple avens Do.

G. urbanum L. large-leaf avens Adventive

Glechoma hederacea L. gill-over-the-ground Naturalized

Impatiens noli-tangere L. touch-me-not, jewelweed (®)

Stachys silvática L. hedge-nettle Adventive

Urtica dioicia L. stinging nettle Naturalized

U. urens L. dog nettle Do.

^ Listed because our native species are usually indicators of good poplar

sites.

Indicators of good poplar sites.—Lowland meadows with a high

water table are usually good poplar sites if some of the following

species are abundant. These plants indicate good aeration and a

summer water table at least 18 inches deep :

Ajuga reptans L. creeping bugleweed Naturalized

Athyrium filix-femina (L.) Roth lady-fern Indigenous

Circaea alpina L. enchanters-nightshade Do.

C. canadensis Hill do. Do.

Filipéndula ulmaria (L.) Maxim. queen-of-the-meadow Adventive

Humulus lupulus L. common hop Indigenous

Lysimachia vulgaris L. ^JL. garden loosestrife Naturalized

Myosotis scorpoides L. forget-me-not _. Do.

Ribes nigrum L. European black currant Adventive

Scutellaria galericulata L. marsh scullcap Indigenous

Solanum dulcamaria L. bitter nightshade Naturalized

On upland sites where the water table is below the effective

reach of tree roots the following species are indicators of sufficient

soil moisture and fertility for good poplar growth :

Circaea alpina L. enchanters-nightshade Indigenous

C. canadensis Hill do. Do.

Geum rivale IL, water or purple avens Indigenous

G. urbariMm L. large-leaf avens Adventive

Impatiens noli-tangere L. touch-me-not, jewelweed (®)

Lamiufn m^aculatum L. spotted dead-nettle Adventive

Stachys sylvatica L. hedge-nettle Do.

Stellaria holostea L. greater stitchwort Do.

^ Listed because our native species are usually indicators of good poplar

sites.

Loam to loamy-sand soils with deep ground water can be good

poplar sites. Some loess-loam soils in northwestern Germany

have produced exceptionally high yields of poplar. The following

species indicate safe and productive sites :

Asperula odorata L. . sweet woodruff Adventive

Galium sylvaticum L. baby-breath, Scotch-mist Do.

Stellaria holostea L. greater stitchwort Do.

Indicators of poor poplar sites,—Where loam to loamy-sands

with deep ground water are poorer in structure and deficient in

bases they are less suitable for poplar. Sites that will support only
42 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

moderate to poor growth of poplar are characterized by a decrease

in the above plants and increasing predominance of the following

species :

Anthoxanthum odoratum L. sweet vernalgrass Naturalized

Holcus lanatus L. velvetgrass Do.

H. mollis L. German velvetgrass Adventive

Lonicera periclymenum L. woodbine Do.

Rubus ideaus L. raspberry Naturalized

Sites not suited for poplar.—Common occurrence of the following plants indicates sites that should not be planted to poplar.

These plants indicate that the soil is too wet for adequate aeration

because of a high stagnant water table, is too heavy and poorly

drained, or is deficient in bases :

Carex acutiformis Ehrh. sedge Naturalized

Corynephorus canescens (L.) Beauv. _ clubawngrass Do.

Deschampsia flexuosa (L.) Trin. wavy hairgrass Indigenous

Eriophorum angustifolium Honckeny _ narrow-leaf cottongrass Do.

Festuca ovina L. sheep fescue Naturalized

Iris pseudacorus L. yellow iris Adventive

Milium effusum L. millet-grass Indigenous

Molinia caerulea Moench moorgrass Adventive

Phalaris arundinacea L. reed canary-grass Indigenous

Phragmites communis Trin. common reed Do.

Pteridium aquilinum (L.) Kuhn bracken Do.

Sphagnum acutifolium Ehrh. sphagnum moss Do.

S. cuspidatum Ehrh. ^ do. Do.

Vaccinium myrtillus L. whortleberry Do.

V, vitis-idaea L. mountain-cranberry Do.

Leuce Poplars

The natural range of the European aspen (P. trémula) extends

from Italy to northern Sweden, and there is evidence for the existence of races that may differ in their site requirements. The variation between northern and southern races in their response to

differences in day length is particularly evident.

Aspen has long been an important component of the native

forests for the Scandinavian match industry; yet it is only in

recent years that aspen has begun to be planted in these northern

countries. Plantation culture of aspen in Europe is still in the

experimental stage. Since they can be grown commercially on sites

that are too dry or too infertile for profitable culture of Aigeiros

poplars, the aspens and their hybrids (including P, X canescens

types) can be used to extend the range of profitable poplar culture.

It must be noted, however, that plantations of the so-called

*Tolish'' aspens in Holland and of the hybrid aspens (trémula X

tremuloides) in Denmark indicate that even aspens will make their

maximum growth on fertile, moist, but well-drained soils.

The P. X canescens or gray poplars (the alba X trémula complex) have been planted throughout Europe, usually as shade trees

and windbreaks but seldom for timber production. In northwestern Germany they are preferred for planting near the coast

because they can withstand a coastal climate with its wind and salt

spray better than other poplars.
PRODUCTION OF POPLAR TIMBER IN EUROPE 43

The natural range of P. alba has not been clearly defined. In

localities in southern Europe where the white poplar is abundant

and reproduces naturally, it reaches its best development on welldrained fertile bottom lands. Its optimum site requirements are

apparently similar to those described for the Aigeiros poplars.

Although the white poplar has been widely planted in Europe as a

shade and ornamental tree on upland sites, there is no conclusive

evidence that for profitable culture it is better adapted to drier or

poorer soils than the European black poplar (P. nigra).

Significance for the United States

The natural distribution of the European and American

Aigeiros poplars along river courses may have been responsible

for the traditional notion that these poplars are exclusively

adapted to bottom-land sites. This is true for natural regeneration

by seed. Bare soil, free from sod and weed growth, and continuous

moisture at the soil surface for several weeks at the time of seed

germination, are essential for germination and seedling survival.

Such conditions are most frequent on river bottom lands that are

subject to annual or periodic overflow. They are too infrequent on

upland sites to bring the poplars out of the river valleys by natural

seeding. But poplar culture is not based on the uncertainty of

seed reproduction in such situations; plantations are established

with rooted trees or cuttings.

The Aigeiros poplars can be grown commercially on upland soils,

but research is needed to provide practical criteria for evaluating

sites suitable for their profitable growth in the United States. Such

research should include a study of the reliabiKty of common native

plants as indicators of site quality.

The guidelines outlined in this section, based on European

experience and research, can be safely applied to poplar planting

until more specific information is available in the United States.

It^ is safe to assume that most of the indicator plants associated

with good and poor poplar sites in Europe would be indicative of

approximately the same biological conditions, favorable or unfavorable to poplar, when grown in this country. It is possible,

however, that some of these plants may not require the same depth

of soil necessary for the best growth of hybrid poplar. Soil depth

should be checked on all sites considered for poplar culture.

The European aspen has much the same site requirements as our

American aspens. The American species can be used to extend

profitable poplar culture in the United States to sites that are

not suited for commercial plantations of Aigeiros poplars.

We have only preliminary evidence of racial variation in American poplar species. The evidence of considerable racial differences

in the European species indicates the immediate need for intensive racial studies of our important native poplars.

The gray poplars {alba X trémula) might be used to excellent

advantage m breeding for rapid-growing hybrids adaptable to

drier upland sites. Crosses between the European alba and our

American aspens also offer possibihties for obtaining types with
44 HANDBOOK 150, U. S. DEPARTMENT OF AGRICULTURE

hybrid vigor that would grow on drier and less fertile sites than

Aigeiros hybrids require. ^ .. With the exception of batture lands of our largest rivers, fertile,

well-drained soil for profitable poplar culture is seldom available

in large contiguous areas. But on almost every farm there is some

acreage that meets the requirements for excellent poplar growth

and that is not being used for agricultural purposes. For example,

poplars can be grown around barns and farm buildings ; they can

serve as shade trees and can eventually be cut for timber. With a

succession of younger trees to replace those that are harvested,

such plantings would have a continuous aesthetic and monetary

value. Farm roads and fence rows offer areas for the growing of

poplar. Fertile farmland that is too wet in the spring for cultivation, but is sufficiently well drained during the growing season, is

normally used for hay or grazing. Planted to widely spaced poplars, such land could produce both timber and grass.

OUTPLANTING

Site Preparation

Intensive soil preparation is commonly practiced only where the

land is cropped between poplar rotations or intercropped during

the early years after the poplars are planted. Under such conditions the land is plowed, harrowed, and fertilized for whatever

crops that are to be grown.

Alder- or brush-covered sites are always cleared before planting,

either completely or in wide strips ; but there is usually no additional soil preparation. A typical example was observed in France

where a match company had cleared an area carrying a dense

stand of alder in strips 10 to 13 feet wide on 26.5 feet centers^ and

had planted the poplars 26.5 feet apart through the centers of

these strips. The strips will be weeded for several years by cutting

the new alder sprouts. The alder between the strips will be cut for

fuel after 5 years, and again after 15 years.

Planting Stock

Unrooted cuttings and sets

The use of unrooted cuttings for outplanting was not observed

in any of the poplar regions in Europe. Cuttings are the cheapest

type of planting stock, but they require site preparation equivalent

to plowing and harrowing and they must be cultivated for at least

1 year. They could be used successfully where the poplars are

established on land on which cultivated crops are interplanted for

several years. No convincing reason was advanced anywhere in

Europe for the exclusive use of sets or rooted trees under such

conditions. . , ^ -r^ i ^

Sets are used to only a limited extent m most of Europe, but

there are a few localities where they are planted almost exclusively. Sets are always pruned clean of even the smallest side branches.
PRODUCTION OF POPLAR TIMBER IN EUROPE 45

They are outplanted almost exclusively on sites that have a high

w^ater table because it is generally considered essential that the

butts be in the ground water. Nevertheless, the author saw a few

examples of excellent results with 1- and even 2-year-old sets

planted above the water table in soil that was rather wet until

early summer.

The oldest clonal tests of the Netherlands Land Reclamation

Society were established with one row of rooted trees and one row

of sets for each clone. After 20 years there was practically no

difference in form or growth rate between the trees derived from

these different planting stocks.

In Germany, in Westphalia and the Lower Rhine, 3- and 4-yearold sets, iO to 15 feet long from pollarded trees, have been used

with good results. In the Rhine bottom lands of Baden, sets are

being used with excellent results on favorable sites. Here 1- or

2-year-old sets 6 to 8 feet long are used ; they are peeled a few

inches at the base, and are planted deep enough so that the peeled

butt is in the ground water. On the steep face of a strip-mine

bank in Horrem, Dr. Müller planted sets to a depth of about 3 feet

in the winter of 1950-51. For this sandy fill, survival at the end of

the first year was excellent and the growth was fair (fig. 17).

The Forest Inspector for the region of Venice, Italy, told the

author that in the spring of 1952 he planted branches (sets) from

large trees to a depth of more than 12 feet to reach ground water

on a dune site where rooted trees had failed repeatedly. He said

that in September of the same year survival of these long sets was

better than 90 percent.

In the region of Hostalrich, Spain, the use of sets (plantones) is

traditional and apparently the general practice. They are always

planted with the butts in the ground water, and survival and

growth is excellent. Populus alba, occasionally planted in this

region, is also grown from sets.

There are some excellent 30- to 35-year-old poplars with breasthigh diameters up to 23 inches on the Pratt Estate in England that

were said to have been planted as sets in openings in a bottom-land

hardwood stand.

Rooted planting stock

Rooted trees are most generally used for outplanting in all

European countries, but there is considerable diversity in the age

and size of planting stock even in the same locality. Trees less

than 5 feet high are seldom used : the demand is predominantly for

larger sizes. Rooted stock is of two types: rooted cuttings and

stumped trees. Both are graded and priced either by height or by

circumference. The grades commonly used in Italy are 8 to 11 cm.

(3 to 4.2 inches), 11 to 15 cm. (4.2 to 5.8 inches), and more than

15 cm. in circumference at 1 meter (3.3 feet) above the ground.

Rooted cuttings.—Rooted cuttings are nursery-grown trees that

have not been cut back to the ground, so root and top are therefore

the same age. They are outplanted as 1-, 2-, and 3- and occasionally

4-year-old trees. In northern and middle Europe 2- and 3-year-old

471632°—59 4
46 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

stock up to 12 feet high is most frequently used, although large

1-year-old rooted cuttings are planted with very good results on

favorable sites.

For roadside and vi^indbreak plantings the older 3-year and even

4-year trees have a better chance for survival because of their

larger size and sturdier stems. In southern Europe, where poplars

are grown on nursery soils of high fertility, 1-year-old rooted

trees, 8 to 12 feet in height are used to some extent for outplanting; but, in Italy particularly, most planters prefer the larger,

heavier-rooted, and stronger-stemmed 2- and 3-year stock.

^

Photo by R. MüLLER

FIGURE 17.—One season's growth of unrooted sets on the steep face of a

strip-mine bank in the lignite district west of Cologne, Germany.

Stumped trees.—Stumped trees are rooted cuttings that have

been cut back to two or three buds at the end of the first year, rootpruned, replanted and grown 1 or 2 years longer in the nursery.

The root is therefore 1 or 2 years older than the top. (See Nursery

Practice, p. 33.) Stumped trees have been most extensively used in
PRODUCTION OF POPLAR TIMBER IN EUROPE 47

Italy, and their use is recommended in England and is increasing

in other countries.

In Baden, Germany, the State Forest Service has used stools

from which cuttings are normally taken as stumped trees to fill

failures in plantations. Roots 2 to 5 years old with 1-year shoots

are used, depending on the age of the plantation where fill-ins are

required.

Pruning of planting stock

Planting stock is generally top- and root-pruned at the nursery.

Depending on the size of the tree, roots are ordinarily cut back

to 6- to 12-inch lengths. The degree of top-pruning (described

under Nursery Practice) varies from none to complete removal

of all side branches and sometimes even to cutting back of the

leader.

Observations in many young plantations indicate that severe

pruning may retard early growth. The first- and second-year

growth of some 20 clones that were planted without pruning in

the poplar arboretum near Koblenz was remarkably better than

that observed on any heavily pruned stock established by comparable methods on similar sites in central and northern Europe.

Spacing

The spacing of poplars in rows and particularly in plantations

is one of the most controversial aspects of European poplar culture. The traditional conviction is that the cultivated poplars

are biologically unlike forest trees in that they cannot be grown

in forest stands. But from a biologist's standpoint it is poplar

culture that differs basically from general forest practice because poplar plantations are usually monoclonal stands composed

of genetically identical ramets. There is thus a complete lack of

the genotypic variation that exists in a natural stand or in a

plantation established with seedlings.

Row spacing

Poplars, except the aspens, are extensively planted in single

or double rows along public roads, farm roads, field boundaries,

and fence rows. Double-row planting, two relatively close-spaced

rows with the trees staggered in the adjacent rows, is less common

than single rows.

Many experts and growers recommend that poplars planted

in single or double rows should be spaced widely enough apart

to avoid contact between mature crowns, or should be thinned

as soon as the crowns touch. This criterion permits very close

planting of narrow-crowned trees such as the Lombardy poplar,

and necessitates relatively wide spacing of broad-crowned types.

This practice tends to ignore completely the probability of deleterious root interactions between adajacent trees. There is evidence that the growth of poplar may be more seriously influenced

by the soil volume available for free development of the individual

root system than by top competition for light.
48 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

Row plantings can be found in Europe at almost every conceivable spacing from practically open-grown trees to spacings

as close as 3 feet. There is little evidence of seriously retarded

growth vigor where closely spaced row plantings are on deep,

fertile soil and the tree roots are not limited in their lateral

extension by deep ditches or other obstructions. Retarded growth

was observed in closely spaced row plantings on shallow soils—

where depth was limited by the water table or infertile or impervious subsoil—and where the sidewise extension of roots was

curtailed by water-filled ditches.

Close spacing of trees in rows, or double rows, does have a

serious effect on log quality for lumber and veneer. Insufficient

room for normal concentric crown development results in leaning trees that produce crooked logs with eccentric boles. Such

trees may also have a large amount of tension wood, which further

degrades their value for high-quality lumber and veneer.

Plantation spacing

There is relatively little argument about the spacing of aspens.

They are traditionally considered to be forest trees than can be

grown at relatively close spacing. The biological factors that

affect closely spaced monoclonal plantations are not operative in

aspen plantings because seedling stock is generally used for outplanting. In the Scandinavian countries the aspen hybrids are

usually spaced approximately 10 by 10 feet with alder or other

filler trees. The somewhat higher cost of the hybrid planting

stock apparently has a bearing on plantation spacing.

Throughout Europe, plantations of Aigeiros poplars and their

hybrids are established at almost every conceivable spacing, including square spacings of 5.0, 6.6, 9.8, 13.3, 16.4, 19.7, 23 and

26.3 feet, and rectangular spacings of 13.3 by 32.8 feet, 23 by

32.8 feet and 26.3 by 29.5 feet. There is wide divergence of

opinion on plantation spacing, at the local as well as at the international level. Unfortunately the arguments for close or wide

spacing are often seriously confused by failure to distinguish between the biological and economic aspects of the problem. Present

planting distances are largely the product of tradition and of

observations on the growth of poplar cultivars that are assumed

to be genetically identical and are growing on sites that are

assumed to be similar.

There are two rather adamant schools of thought on planting

distance in Germany. The advocates of wide spacing (20 by 20

feet to 26 by 26 feet) argue that poplars should be planted at

their final (crop-tree) spacing to produce high-quality logs for

veneer and lumber, and pulpwood from the tops and branches.

Such wide planting eliminates the hazard of stand degradation

because of delayed thinning due to negligence, labor shortage, or

lack of good markets for pulpwood. Spacing as close as 6.5 by

6.5 feet, with early and regular thinning, is urged by those interested in pulpwood production ; but the relatively high establishment costs and the low value of the first thinnings for pulpwood

tend to discourage such practice. Studies in The Netherlands
PRODUCTION OF POPLAR TIMBER IN EUROPE 49

indicate that the pulpwood value of a 6- to 8-year-old tree is

less than the per-tree establishment costs.

The most commonly recommended spacings in Europe are from

20 by 20 feet to 26.5 by 26.5 feet. In Italy, there is increasing

use of the 25.5-feet triangular spacing (trees staggered in adjacent rows) recommended by Prof. Piccarolo, and this spacing

is gaining in favor in some other countries. Although the demand

for pulpwood was the principal stimulus for the early interest

in poplar culture in Italy, the expansion of markets for other uses

has affected the trend in spacing. In 10 to 14 years, with wide

spacing, the Italian grower can produce trees with 70 to 75

percent of their merchantable volume salable for lumber and

veneer, and 25 to 30 percent marketable as pulpwood.

In Austria, Dr. Wettstein recommends spacing the poplars 13

by 20 feet where they are planted in mixture with other lightdemanding species. For pure plantings of poplars where there

is a pulpwood market, he is considering 6.6 by 6.6 feet, with early

thinnings to 13 by 13 feet and a final spacing of 26 by 26 feet.

Thinnings in 6-year-old experimental plantations established at

the 6.6- by 6.6-feet spacing have yielded 1,120 cubic feet per

acre of pulpwood, approximately 12 cords.

In widely separated parts of Europe, spacing 16.4- by 16.4-feet

has given excellent yields without thinning. In Baden, Dr. Bauer

recommends this spacing with filler trees at the same interval.

The recommendation is justified by past experience; this planting

distance with fillers has been widely practiced in the bottom lands

of Baden for many years. The 16.4- by 16.4-feet spacing has also

been in general use in southeastern Spain, where the plantations

are intercropped during the first 4 or 5 years. The recommended

15- by 15-feet planting distance in England is not based on firm

local experience, but it falls within this middle spacing range.

Planting

There is more agreement on planting methods than on almost

any other aspect of European poplar culture. Correct and careful planting is strongly emphasized everywhere. Depending on

the size of the stock, planting holes are made 16 inches to 3 feet

in diameter or 16 to 24 inches square, and 16 to 24 inches in depth.

The use of tractor-mounted soil augers to dig planting holes 12

to 24 inches in diameter is being tested in several countries. They

are highly efficient where the soil is not too rocky.

Use of organic or mineral fertilizers, mixed with the soil in

the planting hole, is strongly recommended but is not yet common

practice. It depends on the fertility of the soil and the affluence

of the landowner. Fertilizer is usually applied as a "small" or

"large'' handful. On land that is intercropped for the first few

years, the fertilizer is applied as a part of the crop culture. Manure

or mineral fertilizers are seldom used where poplar plantations

are on meadowland that is cut for hay or used for grazing. Such

lands are most often river bottoms subject to annual overflow,

where fertilization is seldom necessary.
50 HANDBOOK 15 0, V, S. DEPARTMENT OF AGRICULTURE

Some poplar foresters recommend annual or periodic fertilization on sandy soils of low fertility. Research on this problem

has been started in Germany, but there is at present very little

information to guide the grower as to the kind and quantity of

fertilizer required.

Liming is considered essential for soils with a low pH and is

widely practiced in some regions. The lime is broadcast, applied

around the tree, or mixed with the planting soil.

In many places where large planting stock is used, and particularly where newly planted trees are subject to strong winds, soil

is heaped in a conical mound around the tree. The height and

diameter of such mounds varies from place to place. Conical

mounds 3 to 4 feet in diameter and 1 foot high at the tree are

recommended in England. Poplars planted on sites exposed to

high winds are mounded somewhat higher.

First-Year Plantation Care

The intensity of protection and culture during the first year

after planting varies from none to seemingly unnecessary extremes. On windy sites poplars are sometimes individually staked.

Where leaders are broken, a lateral may be tied upright to a

splint to avoid crook.

Where necessary, the poplars are protected against rabbits

by guards made of roofing paper, asphalt paper, or hardware

cloth. Occasionally barbed wire on 3 posts, or even rail fences

5 to 6 feet square, are built around each tree for protection against

deer and cattle. The costs are said to be justified by the income

derived from hunting leases or grazing values.

Cultivation

All poplar experts now agree that grass and weeds inhibit the

growth of newly planted poplars. The most intensive cultivation

is applied to plantations on land that is used for agricultural

crops during the first 3 to 5 years after planting. This is the

most common practice in Italy and Spain (fig. 18) and is practiced to some extent in France—particularly in the south, in the

lower Rhine Valley in Germany, and in Belgium and The Netherlands. The stumps are removed after a poplar rotation and the

land may be replanted immediately to poplar or it may be used

several years, sometimes as much as 5 to 10 years, for agricultural

crops before poplars are planted again.

On some estates in the Po Valley, agricultural crops are often

interplanted primarily to cover the establishment, fertiUzer, and

early cultural costs of the plantation. After 3 to 5 years, when the

yield of crops under a poplar plantation is reduced approximately

50 percent, the land is put into grass for hay and pasture. Such

clean culture and fertilization results in extremely vigorous young

plantations and produces high yields on 12- to 20-year rotations

in southern Europe.
•s ,,

PRODUCTION DP POPLAR TIMBER IN EUROPE 51

>i "v ..- ^ . '} "it . ' -•.*, '••. • r.

FIGURE 18.—Newly planted poplars in the Genii River Valley near Granada,

Spain. Agricultural crops will be grown for several years under the

poplars.

On noncultivated land, or where the land is also used for hay

and grazing, only the most progressive growers hand-cultivate

an area 3 to 6 feet in diameter around each tree during the first

year or two after planting. Examination of the stumps of many

felled poplars in the Parisian Basin, where they are generally

grown on grasslands, showed 3 to 5 extremely narrow rings immediately after planting.

The most important single factor responsible for the retarded

growth of the Parisian Basin poplars is undoubtedly the deleterious eifect of sod on newly planted trees. In view of the value of

poplar timber and the susceptibility of poorly growing poplars

to disease, the saving in labor costs and the small amount of grass

that would be lost by cultivation or mulching around each tree

is not adequate compensation for such drastic curtailment of early

growth.

Mulching

A mulch heavy enough to eliminate weeds and grass is fully

as effective as cultivation. At Mykinge, Sweden, a 10-year-old

experimental plantation of very poorly growing Aigeiros hybrids

on a brushy and soddy site was heavily mulched with straw in

1950. The effect of the late mulch was already apparent in 1952.

The growth rate had increased and the foliage color was much

improved.

It is not necessary to mulch the entire plantation area. In

Great Britain, Peace and Jobling have found a marked early

growth response, on almost every kind of site, to mulching an

area 3 to 4 feet in diameter around each tree. Mulching individual

trees to maintain growth after planting is an old practice but it
52 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

has never become general even in restricted localities. It is now

recommended in Great Britain and is beginning to be more widely

recommended and used on the Continent.

Significance for the United States

Fitted land, plowed and harrowed as for agricultural crops, is

necessary for plantation establishment with unrooted cuttings,

and it will also provide maximum growth conditions where sets

or rooted trees are used. Normally such site preparation is

possible only on cleared land.

On cut-over forest land or on land too brushy for plo\ying, minimum site preparation will require cutting the brush at least 4

to 6 feet around each planted poplar. Strips are most desirable,

and where these are made by machine they should be at least 10

feet wide. If possible, the planting strips should be rototilled or

treated with other machinery, such as a weed-hog or a heavy disk,

to break up the root mat.

There is no sound justification in the European plantings, except those on the most windy sites, for severe pruning to whips—

with or without short lateral stubs. The stubs actually provide

entry for such diseases as Valsa and Dothichiza. Well-rooted

unpruned trees have survived excellently and have made very

good growth with careful planting on good sites.

One-year-old rooted cuttings should be planted in holes 12 to

18 inches in diameter, depending on the size of the roots, and 15

to 18 inches in depth. For 2+1 stumped trees the holes should

not be less than 18 inches in diameter so that the roots need not

be cut back too severely.

The application of approximately i/^ to 1 pound of commercial

fertilizer per tree, mixed with the soil at the time of planting,

will greatly stimulate early growth. How much benefit may be

derived from liming a short distance around each tree on soils

with a low pH is still questionable. Where the soil pH is between

5.0 and 6.0, broadcast liming to bring the pH up to at least 6.0—

and preferably to 6.5—will have a more lasting effect.

Mounding is beneficial where the newly planted trees are subjected to strong winds, but its value under less exposed conditions

needs further investigation. Roots usually form in these mounds

and if the mounds are eventually washed away these roots become

exposed and may be subject to injury and infection by fungi.

Dormant unrooted cuttings have been used successfully to establish plantations of hybrid poplars by the U. S. Forest Service's

Northeastern Forest Experiment Station. But it cannot be too

strongly emphasized that they will be successful only on tilled

land, and that they must be kept free of grass and weeds at least

during the first year after planting.

The use of unrooted sets for outplanting on sites that have a

high water table or on soils that remain moist all summer deserves

wider testing. For best results, the sets should be vigorous shoots

not less than 6 feet long, and they should be planted at least 24

inches deep, preferably with the butts in the ground water. One-
PRODUCTION OF POPLAR TIMBER IN EUROPE 53

year-old whips are preferred but 2-year-old stems are being used

successfully in Europe.

Rooted trees provide the greatest assurance of successful establishment where the land cannot be plowed and harrowed or

given adequate soil preparation. One-year-old rooted cuttings

are suitable for outplanting if they are 5 feet or more in height.

Two + one stumped trees that have been dug and root-pruned at

the end of their first growing season in the nursery will give better

results on difficult sites than 1-year-old rooted cuttings.

Pruning to half the height of the tree has proved best for

successful establishment and vigorous early growth. Progressive

pruning during the last year in the nursery has the important

advantage of smaller branch wounds and complete healing before

the trees are outplanted. This can be recommended as the best

practice on the basis of European experience.

In row plantings, where no thinnings are contemplated, hybrid

poplars should be spaced 15 to 25 feet apart. Where the tree roots

can spread laterally, the spacing depends mainly on the crown

width of the clone a'nd the size to which the trees are to be grown.

Twenty-five feet is a safe distance if these factors are not known

at the time of planting.

The traditional European notion that poplars cannot be grown

in closely spaced forest stands is based on experience with singleclone plantations. Monoclonal plantings at close spacing deteriorate rapidly if they are not thinned before the growth rate is

seriously retarded by the density of the stand. Single-clone plantations can be successfully matured with an original spacing of

4 by 4 feet, provided they are thinned early and frequently. Such

close spacing is desirable to provide early ground cover where

plantations are established with cuttings. Close spacing is also

useful on low sites that are subject to overflow and where it is

advisable to speed up the deposition of silt.

Plantation spacing of 15 by 15 feet, or thinning to this final

spacing, can be recommended as biologically sound for the culture

of Aigeiros hybrids on good soils. Mixtures of hybrids can be

established at closer spacing without undue risk of stand retrogression due to delayed thinning. There is no evidence in Europe

about the growth and development of random mixtures of many

clones in closely spaced plantations. The possibilities of clonal

mixtures are discussed under ^'Management and Silviculture."

If dormant cuttings are used for the establishment of a plantation, the plantation must be kept free of weeds during the first

year by machine or hand cultivation. For satisfactory early growth

it is also necessary to keep a weed-free area, 4 to 6 feet in diameter, around rooted trees or sets during at least 1 and preferably 2 years. The deleterious effect of grass and weeds on newly

planted poplars may continue for 4 to 5 years depending on the

fertility of the site and the depth of the soil. The grass and weeds

may be eliminated by cultivation or by applying a heavy mulch,

such as hay, straw, sawdust, or wood chips. Mulched trees should

be fertilized.
54 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

Site preparation on cut-over forest and brush land will be

possible at reasonable cost with machinery (brush cutters, rototillers) developed in recent years. Where such machinery is not

available or is too expensive, site preparation can be reduced to

cutting the brush for a distance of 6 feet around each widely

spaced planting hole.

Cuttings provide the cheapest type of planting stock but require the most intensive site preparation and cultivation after

planting. They can be cultivated with minimum expense if they

are check-planted to permit machine cultivation in two directions.

The use of cuttings can be recommended for the farmer or landowner who has machinery available for fitting the land and

cultivating the plantation during the first year.

Sets are cheaper than rooted trees but their use is limited to

sites that have abundant moisture during the entire growing

season. The cheapest rooted stock will be 1-year-old cuttings.

The cost of 2+1 or even 3+1 stumped trees should not be excessive in nurseries that are equipped with tree diggers designed

to handle large nursery trees.

The most profitable plantation spacing will depend on many

factors; and among these the markets available to the grower

must be considered. At the present time thinnings are a lowvalue product salable primarily to the cellulose industry, and

therefore close planting usually will be advisable only where a

market for such wood is available.

Close spacing will add relatively little to the cost of establishing plantations with cuttings; but the cost, or the time required,

for thinning young plantations must be considered. A spacing

of 8 by 8 feet should eliminate the need for thinning before a

plantation of mixed clones reaches merchantable pulpwood size.

Since the production of high-quality logs for veneer or lumber

requires proper pruning even in forest stands, plantations for

such products can be established at a final crop-tree spacing

of 15 by 15 feet. Where the land is to be used for poplars and

grass, the distance should be increased to 25 by 25 feet.

The cost of planting rooted trees must be considered in deciding

the plantation spacing distances. If a soil auger mounted on a

tractor is available for digging the holes, the cost of planting

100 to 200 trees per acre would not be excessive. The cost of

V2 to 1 pound of fertilizer per tree, and the cost of liming, is a

reasonable establishment cost for hybrid poplar.

MANAGEMENT AND SILVICULTURE

Leuce Poplars

Management

The European aspen is normally associated with other broadleaf and coniferous species; but, like the American aspen, it

also grows in pure stands after catastrophe, particularly fire. No

instances of aspen management were observed in western continental Europe. In the Scandinavian countries aspen is generally
PRODUCTION OF POPLAR TIMBER IN EUROPE 55

managed extensively, seldom intensively, in mixture v^ith spruce,

birch, or both. Investigations and publications on the silviculture

and management of native aspen stands have been sponsored by

the match industry in Norway and Sv^eden, and interest in more

intensive management has increased greatly in recent years. Aspen

is considered a liability by some foresters managing Scotch pine

in Scandinavia because it is the alternate host of Melampsora

pinitorqua. This rust disease can cause high mortality in Scotch

pine seedlings and serious crooks in the leaders of older trees.

In spruce-aspen stands in Sweden and Norway the aspen is

maintained in the stand during the later years of the longer

conifer rotation as scattered codominants or intermediates, or

through a succession of root suckers that replace trees lost through

suppression. Any merchantable aspens in the spruce-aspen stand

are harvested with the conifers. In the second rotation, the aspen

becomes the dominant species by natural regeneration mostly

from root sprouts. When there is advanced spruce reproduction

or seed trees in these stands, relatively little sivicultural work

is required to develop a fully stocked spruce understory for the

spruce-aspen third rotation.

Managed stands of European white poplar were not observed

or mentioned in western Europe. This poplar forms such dense

natural stands by prolific root suckering in the river valleys of

southern France that it is difficult to estimate the extent of regeneration by seed. In one such stand the largest trees were

12 to 15 years old, 10 to 13 inches in d. b. h., and 60 to 70 feet

in total height. Stem form resulting from early natural pruning

was excellent.

Thinning

The natural aspen stands in Scandinavia, both pure and in

mixture, are maintained at a relatively high density by light

thinnings started soon after the aspen has reached merchantable

size. Although there is little uniformity in the length of the

cutting (thinning) cycle, the following tabulation,^ based on relatively even-aged, naturally regenerated stands in Norway and

Sweden, indicates the general effect of intermediate cuttings on

the density of the aspen :

stands

Age (years): {number)

20 1

30-39 - 5

40-49 6

51-56 3

Trees per acre

Aspen

{number)

680

Spruce, birch,

or both

{number)

93

Total

{number)

773

333 134 467

230 142 372

161 301 462

' Data derived from the following reports :

Anonymous. Sätra bruk. Särtryck ur programmet for Svenska Skogsvardsföreningens 26:te exkursion till Skaraborgs Ian 9-11 juni 1938. 16 pp., illus.

Stockholm. 1938.

Barth, Agnar. Aspen, Dens kultur og behandling for kvalitetsproduksjon.

Fra ingeniar F. H. Fr0lichs fond for aspeskogbrukets fremme Nr. 1. 87

pp., illus. Oslo. 1942.
56 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

The high average of ''other'' trees in the three 51- to 56-year-old

stands represents higher original stocking. The 20-year-old stand

was a mixture of aspen and birch. Mixed stands of aspen-spruce

or aspen-spruce-birch normally carry a higher proportion of the

associated species.

Aigeiros Poplars

Management in forest stands

It is the traditional concept in Europe that Aigeiros poplars

cannot be managed in typical forest stands. There is abundant

evidence that single-clone plantations of the commercially planted

poplars cannot be matured profitably without careful and regular

thinning if planted closer than 16 by 16 feet on the best sites,

or closer than 26 by 26 feet on average sites.

There is no evidence in Europe on management of a random

mixture of many poplar clones of different parentages in closely

spaced (forest) plantations. But there are many examples of

successful management of poplars in bottom-land hardwood

forests.

In Schleswig-Holstein, Germany, hybrid poplars were planted

into openings in a young beech-oak stand on an excellent welldrained alluvial soil. The poplars at 56 years of age averaged

20 to 21 inches in d. b. h. (maximum 30 inches) and 100 to 110

feet in total height. The peeled volume of the largest trees was

estimated to exceed 200 cubic feet. The trees were clean boled for

40 to 60 feet—evidence of an early and continuously dense stand.

On the Rhine bottom lands in Baden the highest quality and most

profitable poplar stumpage is grown in mixed forest stands with

native hardwoods.

The poplars near Yvonaud, Switzerland, are in mixed stands

with alder and other native bottom-land species. At 50 to 60 years

the average height of the poplars is more than 100 feet with a

reported volume (poplar) of more than 9,000 cubic feet per acre.

A poplar-birch mixture approximately 50 years old in the same

locality showed similarly excellent growth of the poplars. The

local foresters were not certain about the history of the birch,

which was younger than the poplar. It may have been interplanted

later, or it may be sprout regeneration from birch planted with

the poplar. In the community forest of Novillo, 2-year-old rooted

poplars were successfully established in openings left by removal

of mature oaks.

Poplars have also been used to convert coppice to high forest.

In France for example, Populus 'Robusta' and 'Serotina' had been

planted in strips cut through an old coppice stand of oak, beech,

hornbeam, and other species on a silty sand in which the water

table was reported to vary in depth from 2 to 3 feet. After 25

years the original coppice species had disappeared almost completely under the poplar canopy.

Near Emendingen, Germany, hybrid poplars were planted into

small openings in a bottom-land stand containing ash up to 43

years old, oak up to 90 years old, and described as coppice with
PRODUCTION OF POPLAR TIMBER IN EUROPE 57

standards. The poplars, at 32 years, had grown up through the

ash and oak and stood 15 feet or more above the forest canopy.

Growth of the poplars had been fairly vigorous : the best trees

had reached 24 inches in d. b. h.

Filler trees

Filler trees are interplanted to fill the gaps between widely

spaced poplars on land that is not utilized for intercropping, hay,

or pasture. They are occasionally used in row plantings. The

use of filler trees is most common in the Rhine Valley of Germany,

less common in France and the Low Countries where poplar

plantations are frequently used for hay and forage and more

occasionally for crops.

Where there is a market for fuel or specialty uses such as

turnings, the filler trees often provide an early monetary return

from the plantation, and in some localities they are planted primarily for this reason. But filler trees perform a far more important biological function : they provide the early ground cover

needed to eliminate grass and weeds and to protect and improve

the forest floor. They also function as trainers for the widely

spaced poplars. Good poplar silviculture requires early and regular pruning ; and shading of the pruned poplar stems by the interplanted species prevents or at least greatly reduces epicormic

branching, which is often prolific on trunks exposed to full

sunlight.

Black alder (Alnus glutinosa (L.) Gaertn.) and white or

speckled alder (A. incana) are most frequently used for interplanting (fig. 19). The black alder is usually preferred; but in

plantations on strip-mine banks at Hermülheim, Germany, the

white alder, interplanted at 5 by 5 feet, has been found superior

to the black. Black alder is reported to grow more slowly on these

mine banks and fails to provide a sufficiently early ground cover

at this spacing. Alder, because of its nitrogen-fixing nodules, is

generally considered beneficial to the growth of poplar ; however,

there is still some doubt on this point.

The Research Institute of the Germany Poplar Society, in

Briihl, was investigating the possibilities of a number of species for

interplanting with poplar. They planned to interplant a 3-year-old

poplar plantation with blocks of red oak, beech, black alder, black

locust, hornbeam (Carpinus L.), and linden at a 5 by 5 feet spacing in the spring of 1953.

At Albalate, Spain, a poplar plantation at 13- by 13-feet spacing was interplanted with Finns canariensis (fig. 20). After 5

years the poplars averaged between 45 and 50 feet in height.

They will be removed at commercial maturity, probably at the

end of 12 years, to leave a coniferous stand.

Where there is a market for thinnings, filler trees (particularly

alder) are generally planted at close spacings of 3.3 by 3.3 feet

to 5 by 5 feet because the necessary early and repeated thinnings

are profitable. Present practice on state lands in Baden is to

plant alder, ash, and willow at the same spacing as the poplars.
58 HANDBOOK 150, V. S. DEPARTMENT OF AGRICULTURE

16.5 by 16.5 feet, to eliminate the need for early thinnings. If

these filler trees grow into the crowns of the poplars, they are

headed back at heights of 20 to 35 feet.

Plantation cultivation, fertilization, and irrigation

Cultivation of the entire plantation area during the first 3 to

5 years after planting was observed only where the land is also

used for agricultural crops. Hand cultivation for a distance of

3 to 5 feet around the young trees has been practiced by a few

growers in all regions.

Established plantations are seldom fertilized except where crop

fertilizers are applied annually during the early years of intercropping. Experiments on annual and periodic application of

fertilizers and lime have been started on the strip-mine banks in

northwestern Germany.

In Italy, southern France, and Spain, poplar plantations are

regularly irrigated on sites where irrigation is required.

Photo by G. HOUTZAGERS

FIGURE 19.—Twenty-year-old plantation of Populus 'Marilandica,' at St.

Oedenrode, The Netherlands, with coppice understory of alder.
PRODUCTION OF POPLAR TIMBER IN EUROPE 59

Photo by FERNANDO JAIME FANLO

FIGURE 20.—A poplar plantation underplanted with Pinus canariensis, on

river-bottom land at Albalate del Arzobispo (Teruel), Spain. The plantation was established in 1946; the photo was taken in October 1954.

Thinning

Single-clone plantations can seldom be retrieved successfully

where vigor has been seriously reduced because of too close spacing; the occasional exceptions are on particularly excellent sites

Mvith deep soils. Degenerate plantations resulting from failure

to thin at the proper time can be seen in every poplar region of

Europe. It is primarily for this reason that poplar foresters

recommend planting distances wide enough to eliminate the need

for thinning.

Initial spacing and earliness and frequency of thinning depend on local land use and local market conditions. Close spacings,

when the plantation is thinned frequently and at the proper time,

provide a higher total yield per acre than wide spacings because

they produce additional wood in small sizes from the thinnings

without reducing the growth rate of crop trees. But close spacing is practical only where thinnings can produce a profit, and

this is impossible in many parts of Europe. For example, in The

Netherlands the estimated establishment cost per tree has been
60 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

more than the pulpwood value of an 8-year-old tree, the maximum

age at which a first thinning must normally be made in a 6.5- by

6.5-foot planting.

European recommendations for thinning poplar stands are

derived from general observations, growth studies, and thinning

experiments in plantations at various spacings. There is considerable difference of opinion because the comparisons have been

based on commercial plantings established under different conditions. Differences due to clones carried under a single cultivar

name, site and site preparation, condition of planting stock, year

of planting, planting methods, culture, and other biological variables are hopelessly confounded in such comparisons. The most

common basis for thinning recommendations is to thin without

making overly large holes in the canopy as soon as the crowns

touch.

The most generally accepted final spacing for mature stands is

20 by 20 feet to 26.4 by 26.4 feet. In the Po Valley, triangular

spacing of 24.5 feet is said to provide the most profitable yields.

On the deep alluvial soils along the Rhine in Baden, common

practice has demonstrated that a final spacing of 16.5 by 16.5

feet is biologically and financially sound. The plantations spaced

at 5 by 5 feet in southern Spain are usually thinned to half the

number of stems at the middle of the 12- to 14-year rotation

(fig. 21 and fig. 22).

There is also some difference of opinion about whether release

should be uniform around the entire crown of the residual trees

or whether release can be *'sidewise." This is important where

the original planting distance is relatively wide, because in regularly spaced plantations removal of every other tree—a 50-percent thinning—is necessary to free the residual crowns on all

sides. With initial spacings of 10 by 10 feet or wider, a 50-percent

thinning in a stand without filler trees opens the stand to an

undesirable degree. A lighter thinning allows more gradual release but produces irregular openings that tend to produce crooked

trunks and eccentric crowns. Both conditions may affect the wood

quality through the formation of tension wood.

Mixed stands of poplar with filler trees or volunteer species

usually receive silvicultural thinnings to favor the poplars as crop

trees. In Baden the filler trees are often topped at 20 to 30 feet

to eliminate interference with the poplar crowns and still maintain full ground cover and shade on the poplar trunks to prevent

epicormic branching.

Pruning

Proper pruning is essential even in closed stands for production of the highest quality logs, particularly for veneer. Although

there is general agreement on what constitutes good pruning,

this is probably the most frequently mishandled cultural treatment. Local traditions and uses are often responsible for extremely bad pruning practices.

The severest pruning was encountered in the Mediterranean

region of France and Italy, where the native black poplars, and
PRODUCTION OP POPLAR TIMBER IN EUROPE 61

FIGURE 21.-

Photo by FERNANDO JAIME FANLO

-Nine-year-old poplar plantation at Bordils (Gerona), Spain.

particularly the Lombardy types, have been put to multiple uses

for centuries. The poplars are usually pollarded high and pruned

annually, or severely pruned as standards, for fagots ; and during

dry summers vs^hen forage is scarce, the leaves and young shoots

may be stripped for fodder.

Pruning to small feather-duster tops every year or two, vi'ith

abundant production of watersprouts, is common where fuel is

scarce. Examples of 60- to 80-foot poplars stripped of all branches

to the current year's leader are not uncommon. In the region of

Naples, where pollarded poplars used as vine supports (fig. 23)

also provide fuel and lumber, the most fertile soils support a

3- and occasionally a 4-story culture : poplars, grapes, sometimes

dwarf fruit trees, and an annual crop or forage.

Examples of excessive pruning can be found also in other poplar-growing regions of continental Europe. Where there is a

local shortage of fuel, the immediate need for fagots overshadows

any enhanced timber value. Even where fagots are not a primary

471632°—59 5
62 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

..>.»1i**

r-482563

FIGURE 22.—Merchantable thinnings from 5- to 6-year-old plantations in

the river-bottom plantings at Oliete, Spain.

consideration, farmers frequently prune too high if the poplars

shade cultivated fields.

Local labor costs largely determine the frequency and carefulness of pruning on community and larger private ownerships.

On such properties the best pruning practices were observed where

there is a limited market for fagots and labor is cheap in relation

to the value of high-quality poplar logs. Where there is a good

market for fagots, pruning is often too severe.

Where labor costs are relatively high there is also a tendency

toward less gradual pruning to provide some heavier branch wood

for sale and to reduce the labor costs (ñg. 24). Such delayed

pruning leaves large knots that heal poorly; the removal of 6

to 10 whorls may reduce growth ; and on widely spaced trees such

pruning almost invariably induces a heavy crop of watersprouts.

The result is often a decrease rather than an increase in log

quality.

Pruning is done in many ways and with various tools. Climbing the trees and pruning with a hand ax is still a common

practice, but the use of ladders and pole tools is increasing.

Climbing irons are said to be commonly used in southern France—

with detrimental eifects on log quality.

The best pruning was observed in Germany, The Netherlands

(ñg. 25), Belgium, and northern France. The pruning practice

of the Belgian Match Company is typical of good plantation

management. Eight to 10 years after planting, the trees are

pruned to one-third of their height, between 10 and 15 years to

half their height, and after 15 years the pruned stem is increased

to its final length of approximately two-thirds the total height.

The trees are not pruned regularly each year; for example, a

21-year-old plantation near Boussu was pruned for the fifth and
PRODUCTION OF POPLAR TIMBER IN EUROPE 63

the last time at 19 years. The crowns are also thinned to three

or four branches per whorl between the 8th and 15th years to

reduce the number of knots in the crown section and to "give the

trees more air and light." Such crown thinning was not observed

in other parts of Europe.

FIGURE 23.-

Photo by T. R. PEACE

-Poplars used for vine supports with continuous intercropping.

Vicinity of Naples, Italy.
64 HANDBOOK 150, U. S. DEPARTMENT OF AGRICULTURE

The following rules for pruning poplars, taken from Das Pappelbuch (see footnote 5, p. 40), would be accepted by most poplar

foresters throughout Europe as biologically sound ; but many

would question the economic applicability where the work must

be done by hired labor.

• Start pruning in the 5th year after planting.

• From 5 to 10 years prune one whorl of branches annually.

• From 11 to 30 years prune one whorl every 2 years. At 30

years the final crown length should be 60 percent of the tree

height on open-grown trees, 40 to 50 percent on plantation trees.

• Prune live branches from early spring, but not at freezing

temperatures, until mid-August. Remove new watersprouts in

July. Dead branches can be pruned at any time.

• Use chisel tools only for live branches up to 1.2 inches in diameter; for larger branches use pole saws and handsaws.

F-482565

FIGURE 24.—Twenty-year-old Populus 'Robusta' along a farm road. The

trees are spaced 26.5 feet apart in the rows. Here pruning has been delayed too long. The local market for fagots tempts the grower to delay

this work until the branches are large enough to pay the pruning costs.

Holtwick, Germany.
PRODUCTION OF POPLAR TIMBER IN EUROPE 65

NED, HEIDEMIJ. (AKNHEM) photo

FIGURE 25.—A row planting of Populus 'Robusta' along a secondary road in

The Netherlands. These trees have been properly pruned.

Rotation

The rotation for Aigeiros poplars and their hybrids is extremely

variable—from 12 to 60 or 70 years. The shortest rotations are

practiced where wood is scarce and the climate and soils favor

rapid growth. In the Mediterranean region of Spain and in Italy,

plantations are often harvested 12 to 15 years after planting, but

rotations of 18 to 25 years are recommended as most profitable.

Longer rotations—20 to 40 years, depending primarily on the

site and growth vigor of the clone—are the rule in France, Belgium, The Netherlands, and where wood is scarce in Germany.

The Raverdeau Nursery in the Parisian Basin favors a 20-year

rotation for P. 'Robusta,' which is said to grow rapidly in the first

10 to 15 years; and a longer rotation for 'Serotina,' which surpasses 'Robusta' after 20 years.

Where Aigeiros hybrids are grown in mixed forest stands with

native species, particularly in Germany, the rotation may be as

long as 70 years. Such management produces large, clear logs of

highest value for sliced and rotary veneer.

Natural regeneration

The Aigeiros hybrids are always planted. Regeneration by seed

is not practical because many cultivars are male and the seed of

female hybrids would not breed true. But when the tree is cut.
66 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

most if not all of these hybrids produce root suckers wherever

the surface roots are exposed. Although scarification with a drag

or disk harrow before logging would provide exposed roots for

prolific root suckering, regeneration of plantations by this means

has not been given serious consideration in Europe. The most

probable reasons are that a dense stand of root suckers would

make intercropping or grass culture on the land impossible, and

delayed weeding of such a dense single-clone stand could result

in serious deterioration.

Significance for the United States

The management of American aspen can be recommended only

in regions where there is a market for the wood and where climate

and soils are unsuited for the more rapid-growing Aigeiros or

Tacamahaca hybrids, or for more valuable native species. The

possibility of managing aspen in mixture with spruce or other

native species, as the European aspen is managed in Scandinavia,

deserves study on favorable sites in our northern regions.^

There is abundant evidence in Europe that a single clone should

not be planted in closely spaced plantations unless there is positive

assurance that thinnings will be made at the proper time. But

there is no evidence to indicate that random mixtures of clones of

different parentages cannot be grown at close spacing.

Dense natural seedling stands of our native cottonwood and of

the European black poplar, where they still occur, mature into excellent forest stands by natural thinning. There is, however, a

basic biological difference between such a seedling population in

which every tree is a different genotype, and a monoclonal stand

in which all trees are identical genotypes. With very rare exceptions every tree in a seedling stand differs more or less from its

neighbors in some or all of its physiological characteristics. Such

genetic variation is completely lacking in a monoclonal stand.

Forest plantations should be established with mixtures of carefully selected and tested clones of as many parentages as possible.

In such a stand there will be little chance that an individual tree

will be surrounded by ramets of the same clone.

There is one such example in Maine where approximately

13,000 hybrid seedlings, representing 95 parent combinations,

were planted at 6- by 6-foot spacing in 1927 and 1928. The seedlings were grouped by parentage and the plantations were never

thinned. Many parent combinations have been partially or completely eliminated by climate, disease, and insects; yet there are

some parentages in which survival is still better than 70 percent.

Here the hybrid plantation has followed the same developmental

pattern as a seedling stand of a native species. There has been

sufficient genotypic variation between seedlings of even the same

' Study of mixed aspen-conifer stands, chiefly balsam fir, is under way in

the Lake States. Personal communication from Paul O. Rudolf, U. S.

Forest Service, Lake States Forest Expt. Sta. 1956.
PRODUCTION OF POPLAR TIMBER IN EUROPE 67

parentages to produce a stand of dominant, intermediate, and suppressed trees. Close spacing has not resulted in the uniform loss

of vigor in all individuals with resulting decadence of the entire

stand.

The interplanting of filler trees in poplar plantations is biologically advantageous. The best filler species for use in the United

States would depend on local climate and site conditions. Commercially important species should be used where the objective

is conversion to the filler species or the development of a mixed

stand. Volunteer native species should be retained as fillers, particularly on cut-over forest land and brushland. At present the

planting of alder and other commercially inferior species as filler

trees cannot be recommended.

Hybrid poplars can be grown in forest stands with native species. Mixed stands permit wide spacing of the poplars and silyicultural thinnings to favor them as crop trees. Such stands will

require relatively less cultural investment—particularly in pruning, will reduce the risk by the inclusion of species known to be

adapted to the environment, but will tend to increase the length

of the poplar rotation.

Close spacing, with early and frequent thinnings, will produce

higher total yields of cellulose per acre than wide spacing without

thinning. Wide spacing will usually be advisable where there is

no market for thinnings. Row plantings are preferred to plantations with close spacing for the production of cellulose and quality

logs, because they are less susceptible to deterioration from delayed thinning.

Pruning methods recommended in Europe are biologically

sound. Growth rate can be maintained by gradual reduction of

crown length over a period of 10 to 20 years after the trees are

well established—6 to 8 years after planting. The advantages of

pruning while branches are small to get clean pruning cuts and of

pruning early in the growing season to get rapid healing are obvious. The effectiveness of paints to eliminate the risk of infection

of wounds after winter pruning should be investigated.

Annual pruning can be recommended for the farmer or landowner who can do this work in slack seasons. Since winter pruning fits best into the workload of the American farmer, it may be

done during this season in spite of the risk of infection by diseases and wood stains. Pruning for high-quality veneer logs should

be started when the trunk reaches 6 or 7 inches in diameter, the

size of the core to which peeler logs are turned, rather than at a

specific age. On this basis pruning would start at an earlier age on

a good site than on a poor site, and earlier on fast-growing hybrids than on slow-growing types.
68 HANDBOOK 150, U. S. DEPARTMENT OF AGRICULTURE

DISEASES

Trunk, Branch, and Twig Diseases

Bacterial canker

This is the most serious disease of poplars in northwestern

Europe and Great Britain (fig. 26). It was first reported as epidemic in The Netherlands on the local 'Brabantica' poplar, which

came into high favor about 1860 and was grown extensively because of its excellent growth rate, good form, and desirable wood

quality. As early as 1870 this cultivar was observed to be suffering from disease and by 1875 it was reported that very few

healthy specimens were to be found.

GREAT BRITAIN FORESTRY COMMISSION photo

FIGURE 26.—Bacterial canker on Populus 'Eugenei' in a test planting for

disease resistance in England.

Studies of the disease were reported in 1900 in Belgium, and in

1906 it was described as the most common type of canker in northern France. Bacterial canker became so serious in The Netherlands and in the adjacent Low Countries that it threatened the

commercial growing of poplars. This led to the clonal selection
PRODUCTION OF POPLAR TIMBER IN EUROPE 69

work of Dr. Houtzagers during the 1930's and the compulsory

certification in The Netherlands of clones that are resistant to this

disease.

Bacterial canker is characterized by the initial appearance of

small cracks in the bark, which exude a light-colored bacterial

slime during the first year. Callus forms around the infected area

as the disease progresses, and this produces the characteristic

swollen nonhealing cankers. Trunk cankers may occasionally kill

a tree rather quickly by girdling, but usually the numerous cankers that develop on the branches kill the tree gradually by reducing its living crown over a period of years (fig. 27).

The presence of slime is a definite diagnostic character in young

cankers, but in old cankers slime is not always present. For this

reason it is not possible to diagnose all older branch and trunk

cankers with absolute certainty. In France, the author was shown

^Regenerata' poplars 20 to 25 years old, with unusually large cankers on the lower 15 feet of the trunks that were characterized by

vertical slits in the bark. These have been assumed to be bacterial

cankers.

Typical bacterial branch cankers, as diagnosed from the

ground, were present on some of the trees with trunk cankers, on

others the branch cankers were lacking. A cautious investigator

would not classify such cankers as bacterial without cultural evidence; they may have originated from other primary causes. M.

Chardenon, of the French Match Company, suggested that climatic or other environmental conditions during a particular year

or series of years might account for these unusual trunk cankers.

Bacterial canker has been investigated in several countries;

the most intensive and continuous research was started in The

Netherlands about 1936. It is now generally agreed that a bacterium, Pseudomonas syringae (v. Hall) E.F.S. f. sp. populea Sähet (synonym: Ps. rimaefaciens (Sm. et Towns) Koning), is responsible for at least the initiation of the disease. The evidence in

The Netherlands indicates that the most susceptible period is in

the spring, but this may diflier from year to year as to exact dates.

Although individual clones vary in degree of susceptibility in different years, their relative susceptibility as compared to other

clones is usually the same from year to year.

Netherlands pathologists have found that the typical syndrome

of bacterial canker is not induced by inoculation with pure cultures of the bacterium. Such inoculations produce infections and

swellings that heal over in the first or second year. (These healed

infections resemble, superficially, the trunk scab described on p.

75.) It is necessary to inoculate with the raw slime or, as Sabet reported from England, to supplement the inoculation of the pure

bacterial culture with sterilized slime, to obtain a typical canker.

Bacterial canker is so serious in Great Britain that selection

and inoculation tests have been under way for some years. Old

multiclonal plantings where bacterial canker has been present for

many years indicate that under conditions of natural infection

some individuals are resistant. Disease-free ortets have been selected from such plantings for pathological tests.

The role of other bacteria and fungi that are usually associated
70 HANDBOOK 150, U. S. DEPARTMENT OF AGRICULTURE

GREAT BRITAIN FORESTRY COMMISSION photo

FIGURE 27.—Branches and upper trunk of Populus 'Eugenei' heavily infected with bacterial cranker. Near Farnborough, Hants, England.

with this canker is in doubt. It is possible that the bacterial infection alone, without secondary pathogens, might not result in

the serious losses associated with this disease.

It is now known that there are clones that are highly resistant

to artificial inoculation, but it is not yet proved that there are any
PRODUCTION OF POPLAR TIMBER IN EUROPE 71

completely immune clones. There is some indication that the severity of bacterial canker, and possibly even susceptibility, may be

intensified by poor grov^ing conditions. The Zwolle clonal test of

the Netherlands Land Reclamation Society is on a poorly drained

peat soil. The grow^th of all clones is very poor and bacterial canker is much more prevalent and more severe than on tv^o comparable plantings of the same clones on good sites in the central

and southern Netherlands. Even clones that are highly resistant

to artificial inoculation when tested on a good site shov7 cankers

in the Zwolle plantation in some years when climatic conditions

favor the disease. It is reported that this is the only site in The

Netherlands where trunk cankers have developed on P. 'Serotina.'

There is no information available as to the possible existence

of distinct physiological strains of the primary pathogen (differing in pathogenicity) in various parts of Europe. Inoculum from

the same regional source has always been used in the intensive

inoculation studies carried out in The Netherlands on many clones

over a long period of years.

The fact that bacterial canker has not been reported from

southern France or Italy may indicate natural environmental control of the disease organism, the occurrence of a different syndrome, or absence of the carrier or carriers of the disease. It is

improbable that there has been no opportunity for transport of

the bacterium to southern Europe.

The problem of clonal susceptibility to natural infection has

not been studied with sufficient precision to warrant sound conclusions. Such tests are necessary for new selections. A clone that

is superior in all respects except its susceptibility to artificial inoculation might have other inherent attributes that would protect

it from natural infection. For example, it is not known how bacterial canker is transmitted from tree to tree. If it is carried by

insects, then susceptibility to natural infection v^ould be correlated

with the degree of clonal attractiveness to such insects.

Tacamahaca poplars and their hybrids are generally considered

highly susceptible to bacteriaí canker, and for this reason there

is a strong prejudice against this entire group. The prejudice is

based on observations and tests of relatively few clones in comparison to the number of Aigeiros clones involved in the years of

natural and artificial selection for disease resistance. Inoculation

tests in Great Britain have now clearly indicated that there are

clones of P. trichocarpa, P. balsamifera, and their hybrids, which

are highly resistant to bacterial canker.

There are examples of Tacamahaca poplars within the natural

range of this disease that are free of bacterial canker. Such cases

may be purely accidental but they merit further tests. A row of

healthy, 30-year-old 'Berolinensis' poplars on the old castle

grounds in Schleswig, Germany, averaged 90 feet in height and

approximately 26 inches in d. b. h. There was no excessive borer

injury on the branches of these trees and no evidence of any disease. 'Berolinensis' is reported to be healthy in this locality.
72 HANDBOOK 150, XJ. S. DEPARTMENT OF AGRICULTURE

Spring dieback

This disease—called Defogliazione primaverile, literally translated "spring defoliation"—almost eliminated profitable poplar

culture in northern Italy and was a primary incentive for the establishment of the Poplar Institute at Cásale Monferrato. There

is much confusion over the scientific name of the fungus responsible for this disease. In Italy, Pollaccia elegans Serv. (perfect

stage, Venturia popuUna (Vuill.) Fabr.) has been reported to be

the pathogen on Aigeiros poplars, and a second species Pollaccia

radiosa (Lib.) Bald, and Cif. (perfect stage, Venturia tremulae

Adreh.) on P. alha.

In Spain, the perfect stage of the fungus responsible for dieback ("la defoliación de primavera") is reported to be Didymosphaeria populina Unamuno. It is considered to be distinct from

D. populina Vuill., Venturia populina Fabr., and V, tremulae Kleb.

Napicladium tremulae (Frank) Sacc. and Pollaccia radiosa

(Lib.) Bald, and Cif. have been identified as the cause of this disease in northern Europe.

The disease kills the young leaves and new shoots in early

spring, usually before the leaves have reached their full size.

Young twigs girdled by insects may have the same appearance

but are easily distinguished from spring dieback. The dead,

blackened, immature twigs and leaves may remain on the trees as

"flags" into the summer. Repeated defoliation can be serious

enough to cause the death of the tree, but it is not always certain

whether death is due to this fungus alone or to the invasion of

secondary parasites.

During 1952 the author found only one locality in which heavy

mortality might be ascribed to the primary effect of spring dieback. A 12-year-old plantation of P. 'Berolinensis' in Lolland, Denmark, was in critical condition after several years of heavy infection. It was impossible to determine whether the decline and heavy

mortality was due entirely to repeated spring dieback or to a combination of dieback and other pathogens. There was no indication

of typical bacterial canker on these trees, but they did have small

brownish to black necrotic areas on the trunks much like the

trunk scab of 'Robusta' in Germany. Since this plantation was on

rather heavy and poorly drained soil it is possible also that site

conditions may have been responsible in part for its early deterioration.

Clonal variation in susceptibility was apparent in this plantation. Although the stock had been presumed to represent a single

clone, there appeared to be at least two clones. Among the rather

wide-spreading trees that were heavily infected and dying there

were a few more columnar individuals that were very lightly infected and appeared to be in good health.

In Sweden it is reported that in some years spring dieback

drastically reduces the growth of young trees of P. trémula, particularly trees under 5 years of age. The disease is not considered

to be of practical importance after the trees reach a height of

about 20 feet. Severe spring dieback apparently has not been observed on the trémula X tremuloides hybrids.
PRODUCTION OF POPLAR TIMBER IN EUROPE 73

Isolated cases of spring dieback, too light to have any effect on

the growth of poplars, were observed in practically all European

poplar regions. A few trees of 'Regenerata' and 'Robusta' in the

poplar nursery at Schmalenbeck, Germany, had typical symptoms.

Near Bellinzona, Switzerland, in a new planting of white poplars

from Italy, some of the trees were heavily infected. Dieback was

also present in Spain. In England it is thought to be associated

with dieback of aspen.

Spring dieback is an arrant menace to poplar culture that fortunately can be eliminated by the breeding and selection of resistant clones. Selection of resistant hybrids has eliminated the

danger of epidemic losses in Italy, at least until physiological

strains of different pathogenicity are imported or develop in

Italy. Constant vigilance is necessary because it is uncertain

whether different species or races of this pathogen vary in virulence and pathogenicity.

Canker and stem dieback

Several fungi cause stem dieback or cankers, depending on the

vigor of the tree or the part of the tree that is infected. Where

small and weakly growing stems and branches are quickly girdled

and killed, particularly during the dormant season, the syndrome

is usually called bark necrosis, stem or branch dieback, or simply

dieback. On larger branches or trunks where the infection is limited by callus formation around the dead sunken area, it is called

canker. Such cankers usually increase in size by periodic advances

of the pathogen and new callus formation.

Dothichiza populea Sacc. et Briard.—Stem dieback and cankers

due to Dothichiza can be found in all parts of Europe. It has become so generally associated with dieback of poorly growing trees,

particularly newly planted trees, that growers and foresters who

are unfamiliar with the pathology of the poplars usually attribute

all cases of stem necrosis to this fungus. In all countries, some

examples of Dothichiza disease that were called to the attention

of the author proved, on close inspection, to be due to Valsa.

The fungus is characterized by olive-colored tendril-like sporehorns that are exuded from the fruiting bodies through small ruptures in the dead bark during damp weather. It is particularly

virulent on newly planted trees on unfavorable soils, where grass

is inhibiting their growth, or where the trees have not been properly planted.

The European belief that Dothichiza is a facultative parasite,

highly virulent only on young trees that are growing poorly because of other conditions, is supported by general observations

but has not been proved by research. Observations in a few plantations pose the possibility that more virulent strains of this fungus may be present in some localities.

There is also evidence that Dothichiza can be serious on older

established poplars. Many trees in an 18-year-old P, 'Robusta'

plantation in the lower Rhone Valley had elongated and slightly

sunken cankers on the upper trunks. Although both Dothichiza

and Valsa were found in the tops of felled trees, Dothichiza was
74 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

most abundant. This plantation, spaced 13 by 13 feet, was on a

deep silt soil over coarse sand, and in dry summers the water table

could drop to 12 feet. The last 10 years had been relatively dry;

1947 and 1949 were exceptionally dry years, with 70 rainless days

in the summer of 1949. The detrimental eifect on growth vigor of

the prolonged dry weather may have been responsible for the

prevalence of disease.

Bordeaux mixture and other fungicides are used for control of

Dothichiza in the nursery and in newly established plantations.

The most common way to protect new plantations against this

disease is to maintain the trees in strong vigor by careful planting, fertilization, clean culture, and irrigation where necessary.

The Italian method of taking cuttings from 1-year-old stumped

trees is spreading to other countries. This eliminates the danger

of incipient disease in cuttings from old stools that have accumulated such facultative parasites as Dothichiza and Valsa,

Dothiorella spp.—Dothiorella hsis frequently been reported to

produce much the same syndrome on poplars as Dothichiza. Severe damage by Dothiorella populnea Thüm. and D, populina

Karst, was reported in the Provinces of Burgos and Leon, Spain,

in 1942 and 1943. A plantation of fastigiate, columnar, and medium wide-spreading types at Carrion de los Condes, inspected by

the author in 1952, was especially interesting for the variation in

disease and insect resistance.

Although most of the fastigiate types had succumbed to Dothiorella, there were at least three clones in this plantation in 1952.

Some of the remaining fastigiate trees were heavily infested with

both gall aphid and a small leaf spot resembling Septoria, A few

fastigiate individuals, with larger and darker green leaves than

the infested trees, had practically no aphid galls or leaf spots. The

columnar and wider-spreading types in this plantation appeared

to be free of disease and insects and were making excellent

growth.

Valsa spp.—Valsa species have practically a worldwide distribution and V. sórdida Nitsch. (imperfect stage Cytospora chrysosperma (Pers.) Fries) is present everywhere in Europe. This

fungus causes dieback and cankers that can be distinguished from

Dothichiza in the field only if the fungus has fruited. The sporehorns of the imperfect stage (Cytospora) are bright orange in

color. If the sporehorns are gone, Cytospora can still be recognized

by the somewhat smaller bark ruptures through which the spores

were exuded.

Although it is normally saprophytic on dead branches. Valsa

sórdida can assume a parasitic role on trees that are in poor vigor

from other causes. In the poplar nursery in Saxony a large section

of the stool planting on a poor site was reported killed by this

fungus.

Valsa nivea (imperfect stage, Cytospora nivea Sacc), which is

normally saprophytic on native Populus trémula, was reported in

1955 ^ as a potentially serious parasite on trémula X tremuloides

® Persson, Arne. Kronenmykose der hyhridaspe. Phytopath. Ztschr. 24

(1) : 5572. 1955.
PRODUCTION OF POPLAR TIMBER IN EUROPE 75

hybrids in southwestern Sweden. This fungus has caused repeated

and severe dieback in the crowns of hybrid aspens at Sofiero and

Ekebo (Lat. 56). The dieback was not found in plantations of the

same age and parentages at Mykinge and Brunsberg, 151 and 390

miles north of Ekebo, respectively. Climatic differences are assumed to be responsible for lack of infection in the more northerly

plantations.

The first symptoms became apparent in the Sofiero and Ekebo

plantations on 10- to 14-year-old trees. The disease is reported to

be just as common and severe on vigorously growing trees as on

poorly growing trees, but the progenies of different parentages

vary in their susceptibility to this fungus.

Nectria spp.—There have been reports of severe losses from

Nectria coccínea Fr., sanguinella Wa., and N. galligena Bres.

major Wa. in a few localities. These fungi were reported to be responsible for the widespread death of poplars in the Lower Rhine

region of Germany some 25 years ago. Only isolated cases—usually branch infections—were observed in the course of these

studies, and at present this disease is not considered serious in

Europe.

Hypoxylon crustaceum (Sow.) Nke. and H. atropurpureum Fr.

have been reported on Populus trémula in Norway, but poplar

foresters did not consider these pathogens a serious problem in

aspen management.

Trunk scab

This name is used here as most descriptive of a spot necrosis of

the bark, which occurs in widely separated regions of Europe. The

cause of trunk scab is not known ; but from observations and limited investigations it appears to be of pathogenic origin. Intensive

research will be required to determine whether trunk scab is

caused by a single organism, or whether different pathogens produce similar syndromes in the same or in different localities.

It is possible that trunk scab represents the effect of primary

invaders that, having limited pathogenicity, produce only local

lesions that the tree can overcome in 1 or 3 years. Where such a

primary infection is followed by secondary pathogens the syndrome could be quite different ; for example, cankers typical of the

secondary pathogen could develop.

Small necrotic areas, usually brown or dark colored, appear on

the bark of the trunk and produce an exúdate during the growing

season. These circular to elliptical areas, .1/2 inch to II/2 inches in

diameter, heal over after 1 or 2 years but new spots continue to

appear each year, giving the trunk a scabby appearance. The disease degrades the wood by producing brown-stain spots under the

healed lesions, with or without included dead bark.

The individual defect areas extend only a short distance horizontally and are somewhat longer longitudinally. Scabby logs produce low-grade veneer because of the regularly spaced series of

stained areas or holes where dead bark was enclosed. This syndrome is called "Braunfleckengrind'' in Germany; in France it is

referred to as the "maladie des taches brunes" and also as the
76 HANDBOOK 150, U. S. DEPARTMENT OF AGRICULTURE

"disease of 'Robusta' in Chautagne'*; and in Italy it is called

*'Batteriosi/'

Trunk scab has been increasing in Germany since 1941. It was

first observed on P. 'Robusta/ which appears to be most susceptible; but it also occurs on 'Regenerata,' 'Serotina/ and 'Berolinensis.' It affects trees on some of the best poplar sites. As early

as 1950 the planting of 'Robusta' was prohibited in the state forests of North Baden and the Palatinate.

In France, trunk scab was observed only in the region of Chautagne. The forest pathologist at Nancy has isolated a number of

bacteria and several fungi. Although the evidence points strongly

to bacterial origin, he is of the opinion that Phytophthora should

not be overlooked as a possible primary pathogen.

"Batteriosi" is a serious disease in Italy; it attacks even the

most vigorous trees of highly susceptible clones. Fortunately there

is clonal variation in resistance to this disease and susceptible

clones, such as '1-488,' are being replaced by resistant clones. Dr.

Vivani of the Poplar Research Institute at Cásale Monferrato has

isolated several bacteria.

Trunk scab was also observed in plantations along the River

Toradera near Hostalrich, Spain, and in a young plantation of P,

'Robusta' in the vicinity of Mons, Belgium. The 'Berolinensis' poplars on the Knutenberg Estate in Denmark, which were heavily

infected with twig blight, also showed evidence of this syndrome.

Leaf Diseases

Leaf diseases, with the occasional exception of Melampsora, are

usually of practical importance only in nursery plantings. The control of these diseases by spraying has become routine procedure

in most progressive poplar nurseries in all countries. The spray

schedule in the state poplar nurseries in Baden is typical of good

European nursery practice. Bordeaux mixture is applied at approximately 6-week intervals. The first bordeaux spray is a 1-

percent solution; later sprays are 2-percent.

Melampsora rust

Six species of Melampsora have been recognized in Europe : (1)

M. larici'populina Kleb.; (2) M. allü-populina Kleb.; (3) M,

larici-tremulae Kleb.; (4) M, pinitorqua Rostrup; (5) M. Rostrupii Wagner; and (6) M, magnusiana Wagner. The alternate

hosts of the above species are Larix species (1,3), Allium species

(2), Pinus species (4), Mercurialis perennis (5), Chelidonium

majus and Corydalis species (6).

Melampsora is easily recognized by the bright orange-yellow

spore masses on the underside of the leaves. Rust infection can

completely defoliate young poplars by midsummer; and after a

succession of heavy infestations for several years such trees may

succumb to secondary pathogens.

The disease can be epidemic in older plantations. One such epidemic was observed in early September 1952 near Bellinzona,
PRODUCTION OF POPLAR TIMBER IN EUROPE 77

Switzerland, in a plantation of about 200 trees approximately 12

years old. This was obviously a mixture of at least two clones that

differ in their resistance to Melampsora rust. Six trees in this

stand were practically rust free ; one tree with a broader habit of

growth showed medium rust infection; and all of the remaining

trees were so heavily infected that they were practically defoliated. It is not known whether this plantation had been heavily attacked in previous years, but such epidemic infection in several

successive years would certainly result in severe growth retardation of a monoclonal plantation.

Fortunately Melampsora can be dismissed as a serious threat

to poplar culture because there are many inherently resistant

clones. For every region there are now clones that are highly rust

resistant, but the same clones are not necessarily resistant in other

regions.

Leaf blotch

Waterman and Cash have given this name to a leaf disease

caused by Septotinia popuUperda.^^ The fungus produces small

brown spots on the young leaves; these spots usually increase

rapidly in size, soon become gray at the center, and have an irregular but sharply defined margin. The imperfect stage of this

fungus had been collected in Latvia and described in 1932 under

the name of Septogloeum populiperdum Moesz & Smarods.

Leaf blotch is not considered a serious disease in Europe. Nurserymen, when questioned on its occurrence in the past, remarked

that it was easily controlled by routine spraying with bordeaux.

Localized, light infection was seen in the poplar nursery at

Keppel, The Netherlands, in mid-July. There had been leaf-beetle

feeding earlier in the season and the leaf blotch appeared to be

associated with the beetle injury. Spraying for beetle control had

apparently controlled the spread of leaf blotch. A few infected

leaves were observed in the state poplar nursery at Harsefeld,

Germany, in the poplar arboretum in Giritz, Switzerland, and on

some of the oldest leaves on an occasional tree of clone P. '1-455'

in a nursery at Fronte, Italy. This disease is considered unimportant in Italy because it has appeared only in nurseries and in late

summer. Noticeable infections have not been found in plantations.

Septogloese

A leaf disease called "Septogloese" in Germany was first observed in 1947 and described by Johannes in 1950 ^^ as caused by

the fungus Septogloeum populiperdum sp. n. (This name had already been applied in 1932 to the imperfect stage of the leaf blotch

fungus by Moesz & Smarods.) Septogloese does not appear to be

" Waterman, A. M., and Cash, E. K. Leaf blotch of poplars caused by a

new species of Septotinia, Mycologia 42: 374-384. 1950.

^^ Johannes, Heinrich. Eine pappelsterben hervorgerufen durch den pilz

Septogloeum populiperdum sp. n. Deut. Pflanzenschutzdienst Nachrichtenbl.

2 (5) : 67-69. 1950.

471632°—59 6
78 HANDBOOK 150, U. S. DEPARTMENT OF AGRICULTURE

the same disease as the leaf blotch reported by Waterman and

Cash; the symptoms are different.

On the current year's growth of nursery stools and newly

planted cuttings, leaves infected by the septogloese disease are

reported first to become mottled (etiolated), then translucent, the

green color being maintained only immediately adjacent to the

veins. After 2 to 3 weeks the margins of the leaves begin to curl

strongly upward. The fruiting bodies of the fungus—small white

pustules about 1 sq. mm. in area^—appear on the leaves during wet

weather. Leaf fall is earlier than normal. In the following spring,

the buds, after normal swelling, fail to develop and the stems die

back. Whether the death of the stem is due to the leaf fungus or to

a secondary parasite such as Valsa was still under investigation at

the time of this survey.

In the poplar nursery at Harsefeld, German poplar foresters

did not consider specimens of leaf blotch with symptoms typical

of those described by Waterman and Cash to be the same disease

they recognize as septogloese. They stated that they had observed

this leaf blotch occasionally on nursery stock.

Other leaf diseases

There are other leaf diseases in Europe that up to the present

time have not been of practical importance. Among these are

Taphrina aurea (Pers.) Fr., which causes blisters and some leaf

distortion ; and leaf spots caused by species of Phyllosticta, Marssoniana, and Septoria,

Root Diseases

Root diseases have not been known to cause widespread damage

in the poplar regions of Europe. Localized cases of severe reduction in growth or of high mortality have been reported as due to

infection of the roots by Armillaria mellea (Vahl) Fr., Rossellinia

necatrix (Hart.) Berl., R, amphisphaerioides L., R. quercina

Hart., R. aquila (Fr.) de Not., Pholiota aegerita, Botrytis cinérea

Pers., and other fungi. The crown gall bacterium, Pseudomonas

tumefaciens Sm. et Towns, also occurs on poplar in Europe. These

root diseases are usually thought to become serious only on heavy

or wet soils.

Unidentified Diseases

Small, elongated, and slightly sunken stem cankers of unknown

origin had been observed for several years previous to 1952 in

test plantings of hybrid aspens at Ekebo, Sweden. In a planting of

two rows each of different interspecific and intraspecific crosses,

the trémula X trémula progenies were free of infection, and there

was also a difference in the amount of infection on two adjacent

hybrid progenies of the same female with different male parents.

The variability of infection in the adjacent trees was indicative

of, but not proof of, inherent variation in susceptibility because

in previous years all diseased trees had been removed as soon as

they showed evidence of cankers, thus reducing the probability of
PRODUCTION OF POPLAR TIMBER IN EUROPE 79

heavy and uniform exposure to infection. The same symptoms

were observed on hybrid aspens in the experimental plots of the

Swedish Match Company at Mykinge and of the Tree Breeding

Station at Brunsberg. In these plantations, as at Ekebo, removal

of diseased trees as soon as they were found had failed to eliminate the disease.

On a hybrid aspen at Brunsberg, a series of necrotic areas extended from an unidentified trunk canker at 6 feet above the

ground to the base of the tree. These appeared to be infections

from the original uppermost canker. The forester at Brunsberg

stated that similar cankers are occasionally found on the native

P. trémula.

There were dying Lombardy poplars in the vicinity of Stockholm, Sweden, with symptoms similar to the "vascular wilt'' of

this cultivar in the United States. Along the highway from Madrid to Burgos many columnar and fastigiate black poplars also

exhibit this syndrome. There is evidence of clonal variation among

these Spanish types not only in growth habit, but also in disease

and insect resistance. Near Quintanar de la Mata, heavy infection

of a leaf spot resembling Septoria was found on some trees whereas adjacent trees with somewhat different growth habit were free

of infection.

In the arboretum at Horsholm, Denmark, a specimen of P. 'Bolleana' was in very poor condition ; it had many dead branches and

knobby swellings on the trunk that resembled the crown gall

(Pseudomonas tumefaciens Sm. & Towns) reported by Cook on

this poplar in the United States. The Asiatic Tacamahaca poplars

in the arboretum appeared unhealthy, with many small dead

branches and twigs. These Asiatic species seemed to be suffering

from much the same condition—environmental or disease, or both

—as in the botanical gardens in the northeastern United States.

A yellow mottling of the older leaves, locally called **mosaic,"

was observed in early September on nursery stock of P. 'Angulata'

in northern Italy. It is said to occur only on young nursery trees

and not on all clones, and is thought to be of physiological origin.

It could be a deficiency symptom or a virus disease. ''Mosaic" is

not considered important.

Possible virus infections that result in premature leaf fall have

been reported from Madrid, Spain. The symptoms have been described as similar to the virus diseases reported by Atanasoff on

P. balsamifera in Bulgaria and by Perisic on "Canadian" poplar

in Jugoslavia. Light green areas, which later become yellow and

dry, appear along the veins of the leaves. There has been no experimental work in Spain to determine whether this "mosaic"

condition is actually a virus infection.

A leaf spot that was similar to but not typical of leaf blotch

was found in the state poplar nursery near Zaragoza. This apparently had occurred late in the season and by the end of September had affected relatively few leaves on the individual plants.
80 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

Wood Decay and Stains

Wood decay

Poplars are susceptible to numerous wood-decay fungi. These

seldom attack trees under 40 years of age and rarely become serious on the best sites in trees less than 50 or 60 years old, the maximum rotation recommended in Europe. Since Aigeiros poplars

are usually cut on a 12- to 35-year rotation, wood rots seldom

cause appreciable economic loss.

Heartwood stains

Heartwood stains, in varying shades of dark brown, red, and

orange, are common in all European poplar regions. The stained

wood is sound; there is no decay and no evidence that the stain

leads to eventual decay. It is uncertain whether the staining is

of pathological or physiological origin. A dark-brown stain is the

most frequent cause of log degrade in Great Britain. Some growers believe that it is correlated with variety or poor sites because

it is often present in P, 'Serótina' and 'Robusta' and in trees on

dry sandy soils or wet peaty soils.

Heartwood stain is common in the poplars grown in Italy. The

color varies: in P, alba it is orange; in the Aigeiros hybrids it is

red to dark brown. This problem is under investigation in several

laboratories. At the Poplar Institute they have found that the bacterial count, pH, moisture content, and shrinkage on drying of

the stained heartwood are higher than in normal white wood. No

pathogen has yet been identified as the cause of staining. It commonly follows borer injury and is said to be more common on

trees grown from cuttings than on seedhngs.

In Italy logs used for pulpwood, lumber, or veneer for matches,

core stock, and boxes are seldom downgraded because of stain.

Where poplar is used for both bleached and unbleached groundwood pulp, the bolts are sorted and the unstained bolts are ground

separately for unbleached pulp. Stained heartwood causes some

manufacturing difficulties and waste in veneer use, but the shortage of suitable timber and the prevalence of stain has prevented

industry from insisting on degrade.

For special uses, for face veneer, and for veneer and lumber in

regions of Europe where other suitable species are available there

is a severe degrade for stained heartwood. Industry is more interested than the grower is in research to eliminate this defect.

Significance for the United States

The history of poplar culture in Europe has demonstrated that

disease can threaten the poplar culture of an entire region or even

of an entire country.

It is apparent that climatic control of the pathogens limits their

epidemic spread both in Europe and in the, United States, but

there may be racial or individual variation in the inherent viru-
PRODUCTION OF POPLAR TIMBER IN EUROPE 81

lence or pathogenicity of the disease-producing fungi and bacteria. It is possible also that extensive planting of a single diseaseresistant clone may, within a relatively short time, lead to the

development of new strains of the pathogen to which that clone

is no longer resistant.

Fortunately, because of the extremely variable germ plasm

available in the genus Populus, it should be possible to produce

hybrid clones that will be resistant to introduced diseases or new

strains of native diseases more virulent than those presently in

existence. Bacterial canker in northern Europe and the spring dieback and rust diseases in Italy would have eliminated profitable

poplar culture in those regions if it had not been possible to select

new inherently resistant clones. Continued vigilance, breeding,

and selection will be necessary in order that the appearance of new

diseases, or more virulent strains of older diseases, may be met

with new resistant clones.

Monoclonal plantations are biologically unsound, because every

ramet of a single clone has all the weaknesses as well as the excellence of the original ortet. A new disease or a more virulent

variant of an unimportant native disease could wipe out extensive

areas of a single clone in one epidemic. Random mixtures of 25 or

more clones of different parentage will provide biological assurance against extensive disease losses because in such a stand every

tree will not be equally susceptible to a new disease, or to a more

virulent form of the native pathogen, against which the clones

had not been tested.

Bacterial canker has not yet been reported in the United States.

Spring dieback has long been recognized as common on our

American aspens but it is not considered important on Aigeiros

and Tacamahaca poplars. In 1956, Bruce W. Dance^^ reported

that two different fungi are responsible for this disease on poplars in the United States and Canada. He concluded that the correct name of the fungus commonly found on aspen is most probably Fusicladium tremulae Frank (== F, radiosum (Lib.) Lind),

perfect stage Venturia tremulae Aderhold. He has identified the

fungus that has been reported only occasionally on Tacamahaca

and Aigeiros poplars in the United States and Canada as Pollaccia

elegans Serv. (= Fusicladmm radiosum (Lib.) Lind var. halsamiferae J. J. Davis), perfect stage Didymosphaeria populina

Vuill. (= Venturia populina (Vuill.) Fabricius).

Dothichiza, Valsa, and most European leaf diseases are present

in this country, but it is possible that European forms may represent strains of different virulence than those in the United States.

European experience indicates sufficient inherent variation to warrant the prediction that poplar clones resistant to practically all

of these diseases can be produced by selective breeding.

'^ Dance, Bruce W. A leaf and twig blight of balsam poplar caused by

Didymosphaeria populina Vuill., imperfect stage: Pollaccia elegans Serv.

Canada Dept. Agr. Forest Path. Lab. Interim Rpt. 70 pp., ülus. 1956.

[Processed.]
82 HANDBOOK 150, V. S. DEPARTMENT OF AGRICULTURE

The best American poplar clones should be submitted to European poplar research stations for tests of their susceptibility or

resistance to the most important European diseases. Clones resistant to European diseases should be brought into the United

States and tested in different regions. They will be invaluable for

planting and for breeding new resistant types if European diseases appear in this country.

Because of the serious risk of introducing new diseases or more

virulent strains of native diseases, extreme care is essential in the

importation of cuttings or nursery stock. Our present quarantine

regulations prohibit the importation of cuttings or plants of poplars except through the Federal Plant Quarantine Station. Since

other countries in this hemisphere do not have equally strict quarantine regulations, and because of the extensive introduction of

European cutting stock into South America, we may expect that

most European diseases will eventually reach this side of the Atlantic.

From the biological aspect, heartwood rots may be dismissed as

of little importance; the rotation for poplar will be sufficiently

short to eliminate any loss from wood rots. Heartwood stain, usually associated with so-called wetwood, is common in poplars in

the United States. The dark-colored heartwood of P. balsamifera

has been associated with bacterial infection, but in hybrid poplars

we know little about the cause of stained heartwood.

It is the opinion of the author, based on observations during

the past 30 years, that stains and wetwood are of pathogenic origin, that apparently various organisms are capable of producing

similar symptoms, and that infection is probably through wounds,

including the cut end of the original cutting. There is no conclusive research evidence available in Europe to invalidate this opinion.

It is apparent in all countries of Europe that poplars growing

under adverse environmental conditions are particularly subject

to disease. This is also true in the United States.

Trunk scab is a relatively new disease in Europe. Shade trees

in the United States occasionally have small "bleeding'' spots on

the trunks, but since these poplars are not used for veneer it is

not known whether this syndrome is associated with typical

trunk-scab defects in the wood. It is possible that the American

trunk-scab syndrome may become of considerable importance in

some parts of the United States when hybrid poplars are grown

extensively in commercial plantations.

The blanket indictment of the Tacamahaca poplars as highly

susceptible to bacterial canker assumes a lack of individual variation for which there is insufficient justification. There are far too

few clones of P. trichocarpa, P, balsamifera, and other balsam

poplars in Europe to warrant the conclusion that there are no resistant Tacamahaca clones. The growth vigor of Tacamahaca hybrids, particularly when crossed with the Aigeiros poplars, justifies their continued breeding and selection.

The economic loss occasioned by epidemic diseases that kill trees

or retard their growth is obvious. Heartwood stains and diseases
PRODUCTION OF POPLAR TIMBER IN EUROPE 83

such as trunk scab, which apparently do not interfere with the

growth of the affected trees, are nevertheless of serious economic

importance. Stained heartwood is entirely suitable for bleached

pulp but it cannot be used for the production of unbleached

groundwood pulp. When used for veneer, stained heartwood associated with wetwood results in manufacturing difficulties due

to excessive shrinkage, splitting, and warping. Aside from the

manufacturing difficulties, it is doubtful whether stained heartwood, because of its color, could find a profitable market in the

United States.

INSECTS

Of the many insects that infest poplars in Europe, only those of

major economic importance and the most common species of

minor importance are discussed here.

Boring Insects

Borers, particularly Saperda, are no doubt the most generally

destructive poplar insects in Europe. There is as yet no good evidence for inherent resistance of individual clones to borer injury.

Usually trees in poor vigor are more heavily infested than those

growing vigorously, but occasional observations on excellent sites

indicate that there are exceptions. Some boring insects can kill

the infested trees ; some may be associated with the spread of poplar diseases ; and all cause more or less serious log degrade.

Saperda carcharias (L.), the large poplar longhorn beetle, is

distributed throughout Europe, but heavy infestations are usually

localized. In some localities it is the most serious insect pest of

poplars. Since the full-grown larvae are more than l^ inch in diameter, the larval tunnels in the wood are of large size, and the

deeply boring larvae cause very serious degrade of lumber. The

burrows are often infected with bacteria and fungi, which cause

discoloration and additional product degrade.

There is some evidence that isolated trees and trees in rows are

more heavily attacked than trees in plantations or in mixed

stands. There are reports from Germany that border trees in

plantations with filler species that increased stand density were

more heavily attacked than trees inside the stand. There is evidence in Italy that poplars on dry sites are particularly susceptible to infestation.

For control, the introduction of a suitable fumigant such as

carbon disulphide or carbon tetrachloride into the burrow is most

frequently recommended. In Italy, a large match with fumigant

at one end (called Antitarlo) is widely used for borer control.

When these matches are pushed into the gallery and sealed with

wet earth, the moisture of the tree releases the fumigant. Sanitation, by removing and burning infected wood, is always recommended. Spraying at the time the adult beetles are flying is practiced in a very few nurseries. It is not a practical control in

plantations except under conditions of extremely heavy infestation.
84 HANDBOOK 15 0^ U. S. DEPARTMENT OF AGRICULTURE

Saperda populnea (L.), the small poplar longhorn beetle, infests

the branches of large poplars or the stems of small trees. The injury caused by this beetle is similar to that caused by our native

S, concolor Lee, the poplar gall Saperda. The eggs are laid in the

bark of small branches or in the leader ; and, as the larvae work

between the bark and the wood, the branch develops gall-like

swellings. The larvae eventually burrow into the center of the

stem. This insect is most serious on young, poorly growing trees

where it may infest the small stem or leader and result in eventual

breakage at the weakened point. How much this insect is involved

in spreading various canker diseases is not known. It seems probable, however, that it is an agent in the spread of bacterial canker

and perhaps other fungus cankers.

Cossus cossus (L.), the goat moth, normally attacks other

broadleaf trees but occasionally causes considerable damage to

poplars. It is widely distributed; but, like the insects mentioned

above, it may be serious only in certain localities. The goat moth

seems to prefer isolated or widely spaced trees. Full-grown larvae

may be as much as 31/2 inches long. The larval stage extends over

2 or 3 years, and during this period the larvae continue to make

larger and larger oval-shaped burrows in the wood. For control

of this insect in The Netherlands, the entrance holes are plugged

with cotton soaked in melted paradichlorbenzene.

Zeuzera pyrina (L.), the wood leopard moth, is reported numerous in the south of England; it injures poplars and other

broadleaf species by attacking stems of small diameter. During

the first season the larvae form a circular burrow in the outer sap-

\vood, and during the second year they burrow a 6- to 8-inch vertical tunnel in the inner wood. Small stems often break off at the

point of injury. This insect is not considered serious on the Continent.

Aegeria apiformis (CL), the hornet clearwing moth, is locally

numerous and destructive in many parts of Europe from England

to Italy. The young larvae feed first in the bark and cambium at

the base of the trunk and then burrow into the wood of the stem

and large roots. Trees that have been heavily infested for several

years have the outer layers of wood honeycombed with the larval

burrows and such trees tend to die back from the top or sometimes are killed outright.

Sciapteron tahaniformis Rott., the dusky clearwing moth, has

been responsible for damage in Germany similar to that of the

horned clearwing moth. It is also considered a destructive poplar

pest in Italy.

Phytobia carbonaria (Zett.), the poplar cambium borer, is of

considerable economic importance in Europe. The larvae bore long

irregular burrows in the cambium, which become infected with

bacteria, discolored, and covered by new wood. New tunnels are

formed each year during an infestation, so the infested logs show

concentric rings of small brown spots of varying width that are

referred to as "pith flecks." The frass-filled tunnels cause serious

log degrade because they constitute weak points in the wood and
PRODUCTION OF POPLAR TIMBER m EUROPE 85

make it unsuitable for the manufacture of matches or plywood.

This insect is responsible for considerable economic loss in England.

Sternochetus lapathi (L.), the poplar and willow borer, is found

in all of the poplar regions of Europe. It is usually associated with

alder and willows but also attacks young poplars, and where infestation is heavy it can cause considerable damage. The hibernating insects resume feeding in the spring in the soft tissue of the

inner bark and outer sapwood. Recently planted trees are particularly susceptible and are often killed by girdhng.

Barypeithes pellucidus Boh. and B, araneiformis Schrank,

short-snouted weevils, have been serious pests in nurseries where

bracken compost was used. These insects infest the bark of young

trees just above the ground level.

Cryphalus asperatus (Gyll.) infests the bark in the tops of poplars that are in poor vigor, often on trees attacked by other insects such as borers and defoliators. This insect may cause dieback and prepare the way for diseases in weakened trees.

^ Agrilus sexguttatus Brahm. This borer, related to the bronze

birch borer, feeds in the cambium and weakens or kills the tree

by partial or complete girdling. It has damaged large trees in Germany. The first sign of damage—discoloration of the outer leaves

of the crown—is followed by early leaf fall and death of the tree.

Gypsonoma sp. Serious but very localized damage on young

trees, particularly in the nursery, was observed in several localities. The insects feed in the buds before entering and hollowing

the young shoots. In Germany this injury is laid to G, acerina

Dup. In Italy, it is reported that there is more than one species of

this insect on poplars. Since the insect overwinters in the soil it

can be controlled by rotation of nursery areas with other crops.

Insects on the Leaves

StilpnoUa salicis (L.), the satin moth, is one of the most serious

defoliators of poplars in southern Europe. The larvae first feed on

the surface tissues but later consume all of the leaf except the

midrib. This insect is periodically epidemic in Italy, locally or

widespread. Heavy infestations usually result in complete defoliation. Normally defoliation is followed by the development of new

leaves, in young trees by increased epicormic branching; since the

growth rate is impaired, a succession of epidemic years results in

serious damage.

Chrysomela popuU L., the red poplarleaf beetle, and C. tremulae

F., the aspenleaf beetle, are distributed over all of Europe. C.

populi has caused serious defoliation in many localities from England to Italy. C. tremulae has been reported locally serious in Germany. The larvae skeletonize the leaves by feeding on the surface

layers; and the adult beetles, which emerge during midsummer,

cut irregular holes in the leaves.

Plagiodera versicolora (Laich.), the imported willow-leaf beetle,

causes leaf damage similar to that described for Chrysomela spp.
86 HANDBOOK 150, V. S. DEPARTMENT OF AGRICULTURE

Phratora spp. Three species of blue poplar-leaf beetles, P. vulgatissima (L.), P- vitellinae (L.), and P. laticollis Suffr., have

been reported on poplars in Europe. These are small beetles i/s to

% inch in length, of a blue-black metallic lustre with shades of

bronze to green, particularly on the under side. Both the larvae

and the adult beetles skeletonize the leaves. The adults cause additional damage by feeding on the very young shoots.

Epidemic infestations of leaf beetles in nurseries and recently

established plantings are easily controlled by spraying. Heavy infestation in estabhshed plantations is apparently rare; no instances of serious plantation damage v^ere brought to the author s

attention. . n A

Dicranura vinula (L.), the puss moth, has occasionally caused

considerable defoliation of poplars in Italy and in England. The

poplar kitten, D. hermelina Goeze, is of minor importance.

Cladus viminalis Fall, the poplar sawfly, and Croesus septemtrionalis L., the birch sawfly, occasionally cause local epidemic

defoliation.

Hyphantria cunea (Drury), the fall webworm. At the 1952 International Poplar Commission meetings in Rome it was reported

that the fall webworm, introduced into Yugoslavia from America,

feeds on about 70 species of plants and is a very serious threat

to poplar. It has two and possibly three generations per year m

Yugoslavia. This introduced insect is spreading rapidly m Hungary and adjacent countries.

Leaf miners are distributed throughout all Europe but are generally considered of minor economic importance. There is no information on how damaging heavy infestations by Phytagromyza

populi Kalt, (the poplar leaf miner), Zeugophora scutellans

Suifr., and other leaf miners may be to the growth and general

health of poplars. In mid-August 1952 the author observed heavy

infestations on P. xcanescens in the vicinity of Stockholm,

Sweden, and on native aspen along the highway from Oslo, Norway, to the Swedish border. Infested trees were easily recognized

by the gray appearance of the foliage.

Chalcoides aurata (Marsh.) and C. helxines (L.), flea beetles,

are occasionally numerous enough in nursery plantings to require

control by spraying. .

Chaitophorus spp., poplar-leaf aphids. Two species, C. popuh L.

and C. leucomela Koch., are often found in large numbers on young

leaves and shoots. Insects of the second species produce blisters on

the leaves, and in England they are believed to be associated with

infection by the fungus Taphrina aurea. Damage by leaf aphids is

seldom serious. . j? i •-,

Pemphigus spp., gall aphids. There are several species oí aphids

that are responsible for galls on the leaf blades or petioles. The

most common of these belong to the genus Pemphigus. The leaigall aphid (P. affinis Koch.) produces galls on the leaf blades; P.

spirothecae Passerini (the spiral-gall aphid) attacks the leaf petioles; and P. bursarius L. produces galls on the petioles and on the

midrib of the leaf. In Spain, heavily infested columnar P. mgra

types were growing in proximity to wide-spreading P. mgra
PRODUCTION OF POPLAR TIMBER IN EUROPE 87

trees that were practically free of these aphids. This probably indicates clonal differences in susceptibility.

Harmandia loewi (Rubs.), the poplar gall-midge, produces galls

on the leaf petioles. Aspen is reported to be the favorite host.

Bydiscus spp. Two species of these leaf rollers, B, populi (L.)

and B, betulae (L.), have been reported responsible for some injury to poplar in Germany and Italy,

Insects on the Bark

The poplar-bark aphid (Pterochlorus salignus Gmelin.) and the

willow-bark aphid (Melanoxantherium salicis L.) are widely distributed on the bark of poplars. Vigor of the tree is drastically

curtailed by very heavy infestation and it is also possible that

these insects may be a link in the spread of bacterial or fungous

diseases. A woolly aphid (Eriosoma) is sometimes very heavy on

poplars in southern Europe.

Phleomyzus passerinii Sign., the woolly aphid of poplar, is a

serious pest in Italy. Heavy infestations have necessitated felling

thousands of young trees. Dr. Vivani's studies indicate inherent

variation in susceptibility to this insect. Clones P. '1-28' and *I-65'

are highly susceptible. Clones 1-214,' 1-455,' the cultivar 'Carolin,' and P. alba are reported to be ''exempt from infestation."

Cimbex variabilis Klg. The male of this sawfly gnaws the bark

of poplar shoots, forming deep spiral grooves that sometimes completely girdle the young stems. Where the leader is completely

girdled it usually breaks off before the end of the year. Occasional injury by this insect was observed in most of the poplar

regions of Europe.

Significance for the United States

There is observational evidence in Europe of possible clonal

variation in susceptibility to scale insects, leaf-gall aphids, and

leaf beetles. 1^ There is practically no evidence of inherent variation in susceptibility to boring insects.

In sufficiently large numbers, borers that feed in the cambium,

such as Agrilus species, can kill even the largest trees by girdling.

Wood borers that live in the bark in their early stages (Saperda)

seldom kill large trees, but small trees may break off at the point

of injury. These boring insects are particularly damaging because

their large burrows may be so numerous that the timber can be

used only for cellulose.

Leaf-feeding insects seldom cause high mortality even under

epidemic conditions unless such epidemic feeding is continued for

several years. But these insects are important because heavy defoliation seriously retards growth and may weaken the trees to

such a degree that they are highly susceptible to attacks by other

insects and diseases. Leaf aphids seldom cause appreciable damage

"" Research at the Northeastern Forest Experiment Station, U. S. Forest

Service, has demonstrated inherent clonal variation in susceptibility to the

Japanese beetle. (U. S. Dept. Agr. Yearbook 1949: 157.)
88 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

except in nursery plantings where they are easily controlled. Infestations of aphids and scale insects on the bark can be heavy

enough to injure or kill plantation trees.

Several of the European poplar insects have been introduced

into this country. The leopard moth (Zeuzera pyrina) is known to

occur in the Northeastern States from Philadelphia to the northern border of Massachusetts. Although elms and maples are the

favorite host plants in the United States, many other deciduous

trees—including poplars—are attacked.

The horned clearwing moth has also been introduced into this

country. It is widely distributed throughout the northern part of

the United States and has been reported as the cause of severe injury to the large roots of poplars in the vicinity of Revere, Mass.

There is also a native species of this insect that attacks poplars

and willows ; it is similar in appearance to the European species

and is often mistaken for it.

The poplar and willow borer (Sternochetus lapathi) has become

established from Maine west to Ontario and northern Wisconsin

and south to Virginia. In recent years it has also been found m

Washington and Idaho and it is said to be spreading to other

areas. In the United States it attacks willow, alder, poplar, and

birch. It has proved particularly serious to poplar and willow

nursery stock and newly planted trees.

The short-snouted weevil, Barypeithes pellucidus Boh., is recognized as a strawberry pest in the United States.

The satin moth (Stilpnotia salicis) is now widely distributed

in New England and the Maritime Provinces of Canada. It is also

present in the Pacific Northwest and British Columbia west of the

Cascade Range. It has been responsible for serious local defoliation of poplars in New England since it was first discovered m

Massachusetts in 1920. The damage to poplars in New England is

less serious than in southern Europe because in New England

the insect produces only one generation per year. TT -^ ^

Several European leaf beetles have also appeared m the United

States: the European leaf beetles (Chrysomela interrupta F. and

C. tremulae F.) and the imported willow leaf beetle (Plagiodera

versicolora Laich.). The latter is now common in New England

and extends into western New York and south to Virginia. Severe

infestations in the United States, as in Europe, are not common;

but when they do occur they cause considerable damage.

The European poplar sawñy {Trichiocampus viminahs Fall.) is

now distributed from New Jersey through the Northeastern

States into eastern Canada and west into British Columbia. Serious defoliation of shade and ornamental poplars has been renorted.

The' cottonwood leaf-mining beetle, Zeugophora scutellaris

Suffr., has also been introduced into the United States.

In addition to the introduced pests we also have related native

species of several important European poplar insects. The native

poplar borer (Saperda calcarata Say) produces the same damage

as the European longhorn poplar borer. Saperda concolor Lee,

our native poplar-gall saperda, is similar in its attack on poplar
PRODUCTION OF POPLAR TIMBER IN EUROPE 89

to the European Saperda populnea L. We also have native cambium miners in the United States, which cause damage similar to

that of the European poplar cambium borer (Phytobia) on many

species, including poplars. The bronze birch borer, Agrilus anxius

Gory, attacks poplars and aspens in the United States with the

same effect as the European member of this genus.

The giant American sawfly (Cimbex americana Leach) has

been reported as occasionally infesting poplars. Although the

European puss moth has not been reported in this country, we

have native species on poplar that are related to this insect. There

are three native species of gall aphids (Pemphigus) that attack

poplars in the United States. Our native scale insects can cause

serious injury to newly planted trees; in 1950 the author saw a

young plantation in Michigan where the poplars were dying back

from extremely heavy scale infestation.

Control of boring insects whose burrows open through the bark

is possible by the injection of fumigants. Insects that feed under

the bark, without such openings, can be controlled by spraying

with DDT or other suitable insecticides when the insects are moving. Sanitation—removal and burning of infested branches and

trees—is a practical control measure. Heavy infestations of leaf

insects in plantations can be controlled by modern spraying

methods. Control of leaf miners and aphids will seldom be necessary in plantations.

It is apparent in Europe that under conditions of intensive poplar culture both defoliator and boring insects can cause serious

economic loss by degrading the timber, by killing the trees, or by

severely retarding their growth. Control by modern methods, using mist blowers and airplanes for spraying or dusting, should

be economically feasible for poplar plantations. Spraying for the

control of boring insects will require exact knowledge of the life

cycle of the insects because the control must be applied when the

mature insects are moving and laying their eggs.

DAMAGE BY OTHER AGENTS

Mistletoe

Poplars in north-central France are often heavily parasitized

by mistletoe (Viscum album L.). *Regenerata' is particularly susceptible, but other cultivars are also attacked. Mistletoe can be unbelievably heavy on branches, and it also occurs on the upper part

of the trunks. It is said that mistletoe does not appreciably retard

growth, but the trunk wood penetrated by the haustoria cannot be

used for veneer or lumber. This represents a relatively minor loss.

Environmental and Climatic Damage

Environmental conditions that reduce growth vigor are often

the primary cause of decadence. Abnormally dry weather, sites

that are too dry or too wet, soils that are infertile or too acid, and

other environmental conditions such as the inhibiting effect of
90 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

sod, or crowding in plantings of a single clone, can condition the

stand for further injury by insects and diseases. Borers and such

facultative parasites as Valsa and Dothichiza may damage and

kill many trees that have been weakened by adverse environment.

Poplars are susceptible to wind breakage, but on deep soils such

damage is usually limited to the branches—rather than uprooting

of the trees. Although recovery from mechanical injury is rapid

on good sites, the broken branches provide possible entry points

for fungi and bacteria. There is occasional local damage to tops

and branches from the accumulation of snow, ice, or sleet. Local

hailstorms can be extremely damaging. One- and 2-year-old plantations, on the Plaine de FOrbe between Lake Neuchatel and Lake

Geneva, were completely ruined by a hailstorm so severe that large

branches of old trees were stripped of their bark.

Late spring frosts occasionally defohate susceptible clones by

killing the young leaves and shoots. Although there is seldom appreciable retardation of the current year's growth, there is no

information about the extent to which such frost damage may

facilitate invasion by insects, fungi, and bacteria. Frost damage

can be avoided by the selection of frost-hardy clones.

Frost-crack on the lower trunk was observed in several countries. In some localities frost-crack damage is severe on susceptible clones—for example, on Populus 'Robusta' in northwestern

Germany—but it is not generally serious throughout Europe.

Salt-spray injury is common on the Aigeiros poplars where

they are used for windbreaks in maritime areas. Populus Xoanescens is more resistant to such conditions and is used extensively

in northwestern Germany and in Denmark for windbreaks in

coastal areas.

Damage by Animals

Browsing damage by cattle and deer, and girdling by rabbits,

are a menace in many parts of Europe where grazing and hunting

are major sources of income from the land. Under such conditions

each newly planted poplar is protected against cattle and deer by

high woven-wire guards, by barbed wire loosely wrapped around

the stem, or even by single-bar fences supported on 3 or 4 posts

around each tree. Wire mesh or tar-paper guards are placed

around each tree for protection against rabbits. Repellent sprays

are sometimes used, but they are not generally considered reliable.

Significance for the United States

The environmental and climatic factors described as deleterious

to poplars in Europe are equally damaging to poplars in this country. We have long recognized adverse site condition as the most

important limiting factor for profitable poplar culture. For both

biological and economic considerations, poplars should be planted

only where site and climate are favorable for the maintenance of

their normal growth rate.
PRODUCTION OF POPLAR TIMBER IN EUROPE 91

The risk of wind, snow, ice, and hail damage must be considered

in localities where trees in forest stands or in rows are subject to

such injury. The rapid recovery of poplars from top breakage

should be one of the factors included in the calculation of the economic risk.

Frost and frost-crack injury can be avoided by selection of

clones tested for adaptability to the local climate.

On the basis of European experience, P. Xcanescens cultivars

should be selected for maritime exposures.

Poplars will require protection against animals, particularly

deer and rabbits, where the animal population is high. Fencing,

as practiced in Europe, will not be practical for extensive plantings under present conditions. The development of effective repellents offers the best possibilities for economical protection.

GROWTH AND YIELD

Aigeiros Poplars in Plantations

Comparable and precise data on growth and yield of Aigeiros

poplars are rare in Europe. Variations among different clones

designated under the same cultivar name, and inadequate information about planting stock, planting methods, and early care of

plantations permit only empirical growth-and-yield comparisons

among countries, cultivars, sites, spacings, and culture.

Excerpts from published data on yield of poplar in Spain, Italy,

and Germany are presented in table 1. The close spacing and/or

intensive culture on excellent soils are reported to produce average annual increments as high as 490 cubic feet per acre on 11- to

12-year rotations in Spain, and 500 cubic feet per acre on 25-year

rotations in Italy. Average annual increment for 25-year rotations

on site class I in Germany is reported as 392 cubic feet per acre ;

on site class III, as 120 cubic feet per acre.

Productivity of plantations examined by the author

More than 150 plantations were inspected by the author, but

sufficient information for a reasonably accurate estimate of

growth and yield was available for only 64 plantations (table 2).

The ages listed in the table are the years since plantation establishment—1, 2, or 3 years less than the total age of the trees. Descriptions of site conditions and spacing for the individual plantations are given in the appendix, p. 119.

Summary by cultivar and country

The minimum, average, and maximum annual increments for

cultivars and for countries, based on the 64 cases in table 2, are

given in table 3. P. *Marilandica' showed the lowest average annual increment. The fact that the 13 mixed plantings produced

the highest average increment substantiates the biological soundness of clonal mixtures.
92 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

TABLE 1.--Excerpts from published data on yield 'of poplar in Europe 1

Spain Italy

Age

(years)

Granada Logroño 162 trees per acre 101 trees per acre

Trees

per acre

(average)

Total

yield

per acre

Average

annual

increment

per acre

Trees

per acre

(average)

Total

yield

per acre

Average

annual

increment

per acre

Total

yield

per acre

Average

annual

increment

per acre

Total

yield

per acre

Average

annual

increment

per acre

6-

No.

1,240

> 840

• 660

Cu.ft.

2,401

' 3,015

3,658

4,287

4,873

5,416

, 5, 887

6,259

6,516

6,588

6,602

Cu. ft.

400

430

457

476

487

492

491

481

465

439

413

No.

324

Cu.ft.

2,801

Cu.ft.

476

Cu. ft.

1,358

1.686

2,058

2,472

2,929

3,430

3,965

4,494 •

5,009

5,502

5,973

7,674

9,288

Cu.ft.

226

241

257

275

293

312

330

346

358

367

373

384

372

Cu.ft.

1,100

1,436

1,829

2,272

2,772

3,358

4,073

4,687

5,402

6,130

6,873

9,660

12,518

Cu.ft.

183

7 205

g 229

9 324 3,658 406 252

10 277

II 305

12 324

324

5,487

5,259

457

405

339

13 361

14 386

15 409

1 fi 430

20 253 7,802 390 483

25 501

Germany

Age

(years)

Site class I Site class II Site class III

Trees

per acre

(average)

Total

yield

per acre

Average

annual

increment

per acre

Trees

per acre

(average)

Total

yield

per acre

Average

annual

increment

per acre

Trees

per acre

(average)

Total

yield

per acre

Average

annual

increment

per acre

10

No.

324

162

81

40

40

40

40

Cu.ft.

1,600

4,687

7,545

9,803

11.375

15,176

19,334

Cu.ft.

160

312

377

392

379

379

387

No.

324

162

81

40

40

40

40

Cu.ft.

1,029

2,286

3,458

4.544

5,373

7,759

9,746

Cu.ft.

103

152

173

182

179

194

195

No.

648

324

162

81

40

40

40

Cu.ft.

457

1.486

2,515

3,344

3,915

4,844

6,002

Cu.ft.

46

15 99

20 126

25 134

30

40

130

121

50 120

1 Data derived from the following reports:

Anonymous. VIII Sesión de la Comisión Internacional del Chopo en España. Ministerio de Agricultura,

Patrimonio Forestal de Estado. 460 pp., illus. Madrid. 1955.

Piccarolo, Giacomo. Il pioppo. norme pratiche di cultivazione. Ramo editoriale degli agricoltori. 130

pp., illus. Rome. 1952.

Hesmer, H. (editor). Das pappelbuch. Deut. Pâppelverem. 304 pp., illus. Bonn. 1952.

The figures for The Netherlands may be somewhat low because

the stands on which data were obtained in this country were all

P. 'Marilandica/ a relatively slow-growing clone.^^ The high values for Italy and Spain are a reñection of the favorable climate,

long growing season, highly fertile soils, and intensive culture.

'' They are, however, higher than the 6.5 cubic meters "average annual

increase per hectare" (93 cubic feet per acre) reported by G. Houtzagers

in 1937.
PRuu-DCTION Ot FOPLAR TIMBER IN EUROPE 93

TABLE 2.—Productivity of European poplar plantations

(Volume estimates based on peeled wood to 2.75-inch top diameter)

NETHERLANDS

,

Residual stand Thinnings Total

Case No. and cultivar i

Trees

per

acre

Age

Average

d. b. h.

Average

height

Stand

volume

per

acre

Average

annual

increment per

acre

Age

Volume

removed

per

acre

Volume

Average

annual

increment per

acre

1. P. 'Marilandica'.

No.

68

79

55

53

Yrs.

21

32

35

32

Ins.

15.4

16.1

18.7

17.3

Ft.

81

92

95

96

Cu. ft.

2,501

3,558

3,344

2,844

Cu.ft.

119

111

96

89

Yrs. Cu. ft. Cu. ft.

2,501

3,358

3,344

+2, 844

Cu. ft.

119

2. Do 111

96

+89

3. Do

4. Do -

27 0)

BELGIUM

5. 'Regenerata'.

6. Do -

7. 'Robusta'

100

80

12.2

13.4

2,115

2,201

2,415

GERMANY

8. P. Xcanadensis.

9. Do---

10. Unnamed

11. Do -

Do

Do

Do

Do

12.

13.

14.

15.

16. 'Robusta'.

17. Do._.

18.

19.

20.

21.

22.

23.

DoDoDo.

Do.

DoDo-.

24A. Do

24B. Do

25. Do -.

26. 'Regenerata'

27. Do

28. ' Regenerata Harfl'

29. Do

30. Do

31. ' Marilandica'

32. Unnamed

33. Do

34. 'Marilandica'.__.

35. 'Regenerata'

36. P. Xcanadensis..

37. Do

53

115

115

53

260

132

337

146

162

700

214

223

223

135

42

115

45

132

105

134

90

177

67

92

131

204

329

218

260

133

215

152

270

90

101

105

123

381

374

390

114

97

106

166

200

(})

10

13

{')

(0

14

0)-

32

(0

(0

0)

(0

(0

0)

972

272

457

400

(0

3,330

+2, 415

0)

43

257

(9

527

(})

1,715

1,043

2,715

(})

i})

(})

1,143

5,059

9,932

672

1,015

4,201

2,444

5,259

5,016

+3,116

1,729

4,287

7,102

3,658

1,972

900

1,872

+3, 773

+1, 258

2,242

2,529

3,450

10, 260

10, 260

6,040

+5, 931

5,016

7,705

+5, 277

+5,073

+4,344

4, 786

10,003

193

158

+134

90

177

67

92

131

204

329

218

+260

133

286

284

215

152

134

+270

+90

132

105

123

190

187

195

+160

122

135

+88

+89

+106

218

200

471632°—59-
94 HANDBOOK 150, U. S. DEPARTMENT OF AGRICULTURE

TABLE 2.—Productivity of European poplar plantations—Continued

GERMANY—Continued

Residual stand Thinnings Total

CaseNo. andcultivari

Trees

per

acre

Age

Average

d.b.li.

Average

height

Stand

volume

per

acre

Average

annual

increment per

acre

Age

Volume

removed

per

acre

Volume

Average

annual

increment per

acre

38. P. Xcanadensis.--

39. P. Xcanadensis

mixture

40. Do

No.-

32

140

121

101

101

Yrs.

60

17

18

28

38

Ins.

39.4

13.0

11.8

16.5

18.1

Ft.

138

89

85

95

102

Cu.ft.

11,432

4,301

3,044

4,930

6,202

Cu.ft.

191

253

169

176

163

Yrs. Cu.ft. Cu.ft.

11, 432

4,844

+3,044

+4,930

10,203

Cu.ft.

191

(0

(1)

(1)

543

(})

(1)

4,001

285

+169

41. Do +176

42. Do 268

FRANCE

43. P. Virginiana'.-

44. 'Robusta'

45. 'Serotina de

Champagne'---

46. 'Regenerata',

'Robusta'

47. 'Virginiana'

48. Do

49. 'Robusta'

50. Do

51. 'Itálica' mixture52. Do

53. 'Regenerata',

'Robusta*

83

83

87

42

75

81

253

112

14.6

18.5

8.3

11.8

19.3

22.0

14.6

9.0

11.0

13.8

66

98

126

105

70

2,572

4,373

612

1,629

2,815

8,088

3,544

3,044

4,501

4,087

3,144

198

219

100

180

177

169

180

204

2,572

4,373

612

1,629

2,815

8,088

3,544

3,044

4,501

4,087

3,144

SWITZERLAND

64. P. Xcanadensis

mixture -- 55 9,289 169 (0 (1) +9, 289

ITALY

55. P. '1-214'

56. Do

57. Do

58. Do

59. Mixture of clones.

60. '1-214'

61. ' Monilifera' mixture

62. Mixture of clones.

115

115

115

115

130

128

162

115

8.0

6.0

10.0

15.0

16.0

8.0

9.0

17.7

112

65

108

770

486

1,386

4,930

6,859

1,128

3,087

7,174

198

448

429

1,386

4,930

1,128

3,087

7,174

SPAIN

63. Unnamed --.

64. Mixture of clones. 162

8.3

10.0 70

1,872

2,229

374

203

1,872

2,229

1 One or more thinnings were made but data were not available on age of thinning or volume removed.

2 Volume estimates based on 40 trees per acre.

3 These values are for inside trees only; border trees average 14.2 inches in d. b. h. and 79 feet in height.

4 Measurements of the best trees; volume estimates based on 20 trees per acre.

5 Volume estimate based on 40 trees per acre.
PRODUCTION OF POPLAR TIMBER IN EUROPE 95

TABLE 3.—Summary of poplar growth hy cultivar and country

Item Cas5es

Annual increment per acre i

Minimum Average Maximum

Cultivar:

*Marilandica'

No.

6

6

3

15

5

3

5

13

4

11

3

1

37

8

2

Cu.ft.

+89

105

100

69

67

90

187

69

68

+89

44

+134

Cu.ft.

+111

+137

159

+182

+176

175

191

211

+236

+104

154

+162

169

+171

281

288

Cu.ft.

160

193

198

286

374

218

195

448

429

119

219

193

'Regenerata'

'Virginiana'

'Robusta' _ _

Unnamed

P. Xcanadensis

'Regenerata Harff*

'1-214' _

Mixed plantings

Country:

The Netherlands .

France

Belgium-. _

Switzerland

Germany 67

69

203

329

448

374

Italy.- _

Spain ___ _

1 Based on the annual increments listed in table 2.

An owner of three poplar estates of approximately 100 acres

each in the Seine Valley in France has obtained the following

yields :

Average annual Ö •]. Rotation Total yield per increment per

'^"^^' (years) acre (cu.ft.) acre (cu.ft.)

Very best , „ „. 25 6,300 252

Good and deep 25 4,650 186

Good but shallow (average) 30 1,860 62

These figures are not inconsistent with the average for the 11

French plantations listed in table 3; but the 252 cubic feet of annual increment on the very best sites on these estates in the Seine

Valley does exceed the maximum for the 11 plantations listed in

the table. It is not so high as the maximum for Germany.

Comparisons of site, culture, spacing, and age

Site,~The broad site classifications **good" and "poor" are based

on the author's observations, including soil borings where necessary and feasible, and information provided by the local foresters

or owners. Enough information was available for classing 38

cases as good and 7 cases as poor (table 4). The difference in

average productivity was 106 cubic feet per acre per year. Statistically, this was highly significant.

Culture.—To gage the effects of culture, 32 stands that were

given at least 1 year of cultivation or intercropping after planting,

or were established on sites with a forest floor, were compared
96 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

with 24 stands in which the poplars were planted in grass without

cultivation (table 4). The difference in average annual increment,

32 cubic feet per acre, was not statistically significant.

Grass retards the growth of newly planted poplars during the

first years after planting. This inhibiting effect is particularly

serious because the weakly growing trees are highly susceptible

to adverse cHmatic conditions, diseases, and insects. On deep soils

the trees that have escaped serious injury usually recover their

normal growth rate after 3 to 5 years. It is most probable that

these growth-and-yield data are not suflSciently precise for evaluating this early reduction in productivity because they are confounded with other unknown or unmeasurable environmental factors. . ^ ^. ..

Spacing,—For comparing the effect of spacing on productivity,

the data were grouped into three classes based on the average

growing space available per tree: (1) less than 269 square feet

(approximately 16.4 by 16.4 feet) ; (2) 269 to 538 square feet;

and (3) more than 538 square feet (approximately 23.3 by 23.3

feet). Comparisons of the average annual increments of these

spacing classes are listed in table 4.

The difference in average annual increment between spacmgs

of less than 269 square feet and more than 538 square feet is statistically significant. The difference between 538 square feet and

269 to 538 square feet falls just short of statistical significance.

The average annual increment for spacings of more than 23.3

by 23.3 feet is 64 and 54 cubic feet per acre less than with closer

spacings. This loss of more than 25 percent in productivity reflects incomplete use of the total available growing space.

Age.—The age comparisons for Italy and Spain were analyzed

separately (table 4) because of the short rotations in these countries. The difference of 155 cubic feet per acre per year in favor

of the 10- to 19-year-old plantations indicates the soundness of

Italian and Spanish recommendations for rotations of 20 to 25

vears

The data for all cases except Italy and Spain (table 4) show the

average annual increment of three age groups to be as follows :

" Average Cubic feet per

Years: ^ö'^ acre per year

10-19 14.5 173 +

20-29 23.3 167+

29+ 46.2 149+

Although the differences among these age groups are not statistically significant on the basis of these data, these figures do

suggest that the maximum volume productivity in the observed

plantations was reached some time between the 14th and 23d

years. Although there is a further decrease in average annual increment between the 23d and 46th years, this is not large enough

to preclude the profitable production of large, high-quality timber. The high price of large high-quality veneer logs in Europe

will provide a good profit on longer rotations.
PRODUCTION OF POPLAR TIMBER IN EUROPE 97

TABLE 4.—Comparisons to show effect of site, culture, spacing, and age

SITE

Item Cases

Average

annual

increment

per acre

Difference between means; computed

value of t; value of t needed for significance

Good sites -.

No.

38

7

Cu. ft.

205

99

Difference=106 cubic feet/acre

¿ = 2.82 1

t (for P=0.01)=2.69

CULTURE

Forest, or early cultivation or intercropping.

Poplars in grass

Difference=32+ cubic feet/acre

¿=1.27

t (for P=0.05) =2.00

SPACING: GROWING SPACE

269 to 538 square feet 26

18

+201

+191

Difference = 10+ cubic feet/acre

Less tban 269 SQ uare feet - -

¿=0.34

t (for P=0.05)=2.01

269 to 538 square feet _- - 26

13

+201

+137

Difference=64+cubic feet/acre

IVTorp than 538 so uare feet _ _ _

¿ = 1.99

¿ (for P=0.05)=2.02

18

13

+191

+137

Difference=54+ cubic feet/acre

More than 538 square feet

¿=2.25 2

t (for P=0.05)=2.04

AGE

Italy and Spain:

Less than 10 years (average 6.5 years)

10 to 19 years (average 13.8 years)..

6

4

221

376

Difference=155 cubic feet/acre

¿=2.06

t (for P=0.05)= 2.26

All cases except Italy and Spain:

10 to 19 years (average 14.5 years) _- - 21

15

+173

+167

Difference=6+cubic feet/acre

20 to 29 years (average 23 3 years)

¿=0.23

t (for P=0.05)=2.04

10 to 19 years (average 14 5 years) - - 21

20

+173

+149

Difference=24+ cubic feet/acre

More than 29 years (average 46.2 years) _ - -

¿=0.99

¿ (for P=0.05) =2.02

20 to 29 years (average 23 3 years) 15

20

+167

+149

Difference=18+ cubic feet/acre

More than 29 years (average 46.2 years).--

¿=1.01

¿ (for P=0.05)=2.04

1 Significant at 1-percent level.

2 Significant at 5-percent level.
98 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

Aigeiros Poplars in Row Plantings

There are thousands of row planting's in Europe (fig. 28 and

fig. 29). In the poplar regions of France and Italy it was possible

to inspect 10 to 20 such plantings in a day's drive. Unfortunately,

information sufficient for comparative evaluation of their productivity was seldom available. Data on 23 of the more than 300 row

plantings examined by the author are presented in table 5. A description of the plantings is given in the appendix, p. 123.

Estimation of productivity of row plantings on a per-acre basis

is impractical; therefore volume and average annual increment

have been estimated on the basis of 100 feet of row.

TABLE b.—Productivity of European poplar in row plantings

(Volume estimates based on peeled wood to 2.75-inch top diameter)

Case No. and cultivar ing

65. P. 'Heidemij'..-.

66A. 'Serotina'--

66B. Do

66C. Unnamed

67. 'Eobusta' .

68. Do

69. Do

70. ..f Do.i

[P. alba

71. 'Robusta'

72. Do ._-.

73. Do

74. Do

75. 'Serotina'

76. Do

77. Unnamed

78. Do

79. Do

80. Mixture of clones.

81. 'Serotina'

82. 'Robusta'

83. Do

84. 'Regenerata'

85. 'Serotina'2

Ft,

15

7

7

7

16

21

13

33

33

20

26

23

20

20

21

23

33

49.

28

13

25

25

25

M6

4 16

Age

Average tree

Diameter

breast

high

Yrs.

35

15

16

17

13

13

15

18

18

18

18

19

20

20

22

23

38

60

23

19

29

28

28

27

27

Ins.

31.5

11.8

12.0

16.1

12.6

13.4

11.0

18.5

11.4

15.0

17.3

13.1

15.4

15.7

17.7

18.5

26.0

43.2

22.0

17.7

25.2

21.3

23.2

14.6

15.7

Height Volume

Ft.

90

66

69

75

79

66

82

75

77

79

75

75

100

1Ö0

131

82

79

110

90

90

82

82

Cu.ft.

171

19

20

38

21

24

17

52

18

35

44

24

122

426

75

47

127

74

Average

annual

increment

100 feet of row

Trees

Cu.ft.

4.9

1.3

1.3

2.2

1.6

1.8

1.1

2.9

1.0

1.9

2.4

1.3

1.8

1.8

2.1

2.7

3.2

7.1

3.3

2.5

4.4

2.6

3.2

1.3

1.4

Volume

No.

6.7

14.3

14.3

14.3

6.3

4.8

7.7

3.0

3.0

5.0

3.8

4.3

5.0

5.0

4.8

4.3

3.0

2.0

3.6

7.7

4.0

4.0

4.0

6.3

6.3

Cu.ft.

1,146

272

286

543

132

115

131

156

64

175

167

103

180

180

221

271

366

852

270

362

508

296

356

214

239

Annual

increment

Cu. ft.

32.7

18.1

17.9

31.9

10.2

8.8

8.7

8.7

3.0

9.7

9.3

5.4

9,0

9.0

10.0

11.8

9.6

14.2

11. 7

19.1

17.5

10.6

12.7

^.9

1 Mixed planting of Populus alba and 'Robusta'. The data on P. alba are included for comparison of its

slower growth rate with that of 'Robusta'; they are not included in the computations or comparisons in

the text.

2 4-row planting, rows 20 feet apart.

3 Trees in inside rows.

* Trees in outside rows.

Average^ minimum, and maximum yields

These data should be used only for empirical comparisons. They

are too meager to warrant rigorous statistical analysis.

The average annual increment for all cases is 13.8 cubic feet

per 100-foot row, and the average age is 22.5 years. This is an
PRODUCTION OF POPLAR TIMBER IN EUROPE 99

NED. HEIDEMIJ. (ARNHEM) photo

FIGURE 28.—Row planting of Populus 'Marilandica' in The Netherlands.

The crooked stems are characteristic of this cultivar. Much timber in

The Netherlands is grown along the roads and highways.
100 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

—' '. .^ -« .,:'r '.;.,..ify. *- .. _ _

Photo by G. HOUTZAGERS

FIGURE 29.—Roadside planting of Populus 'Heidemij' on the new Northeast

Polder, The Netherlands, underplanted with alder to be managed as

coppice. The alder had been cut a year before the photograph was taken.

average total yield of 310 cubic feet (approximately 3.4 cords) per

100-foot row in 22.5 years. Minimum production (case 73) is 103

cubic feet (approximately 1.1 cords) per 100-foot row in 19 years.

Maximum productivity (case 65) is 1,146 cubic feet (approximately 13 cords) per 100-foot row in 35 years.

Comparisons of cultivar, age, and spacing

Cultivars.—The average annual increment per 100-foot row for

the cultivars represented in these row plantings is as follows :

Cases Increment

Cultivar: (no.) (cu.ft)

'Robusta' 10 9.8

Mixed clones 1 11. 7

'Regenerata' 1 12. 7

'Serotina' 7 13.4

Unnamed 4 16.9

'Heidemij' 1 32.7
PRODUCTION OF

TABLE 6.—Productivity

POPLAR TIMBER IN EUROPE

of aspen stands on good sites in Sweden

101

Type of stand

33- and 34-year-old stands of aspen

and—

Spruce

Spruce, birch, alder, ash

43- and 44-year-old stands of aspen

and—

Spruce

Spruce, birch, alder, ash

51- and 52-year-old stands of aspen

and—

Spruce

Spruce, birch, alder, ash

Stands

No.

2

2

Average

annual

increment

of aspen

per acre

Cu. ft.

75

56

101

71

Trees per acre i

Aspen

No.

2,660

2,725

1,734

1,600

1,213

1,025

Asso- I Total

ciates

No.

1,582

542

877

220

3,315

1, 870

No.

4,242

3,267

2,611

1.820

Basal area per acre >

Aspen

SQ.ff.

92

76

106

79

4.528 106

2,895 72

Associates

I ft.

31

12

Total

Sq.ft.

123

115

128

90

1 Figures for number of trees and basal

intermediate cuttings. Trees lost through

are the totals for the residual stand plus trees removed in

natural mortality are not included.

Age.—The average productivity per 100-foot row for each of

four age groups is as follows :

Average age (yrs.) :

16.51

(13-19)1

24.71

(22-29)/

36.51

(35-38)J

60

Cases

{no.)

11

Average annual

increment Total volume

(cu. ft.)

13. 4

10. 9

21.2

14. 2

{cu. ft.)

221

269

774

852

Spacing.—The average annual increments per 100-foot row for

three spacing classes are as follows :

Average spacing (ft.) :

121

(7-16)1

23l

(20-28)J

38l

(33-49)J---

(no.)

8

12

3

Increment

{cu.ft.)

17. 5

10.5

10. 8

The d. b. h., height, and volume data for the average tree do not

indicate any trend toward reduction in growth rate due to the

closer spacings in these row plantings. The larger number of trees

per 100-foot row is primarily responsible for the higher average

yield of the 7- to 16-foot spacings. The effect of close spacing on

log quality—crook, lean, eccentric growth—has been discussed in

an earlier section.
102 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

Native Aspen in Scandinavia

The intensity of aspen management varies widely in Sweden

and Norway. The following yields are frequently cited ^^ as overall

averages for aspen at 50 years of age :

Site class I: Total production—2,860 cu. ft./acre (57 cu. ft./acre/year).

Site class II: Total production—2,144 cu. ft./acre (43 cu. ft./acre/year).

The productivity of well-managed stands in southern Scandinavia is much higher than these overall averages. The native aspen stands in Skaraborg shire, between latitudes 58"" and 59° (the

latitude of northern Manitoba), are considered to rank among

the best in Sweden. The information in table 6, based on sample

areas on sites rated as good for aspen, is from a forest estate located between Vener Lake and Vetter Lake in Skaraborg.

The average annual increments, from 56 to 101 cubic feet per

acre (0.62 to 1.1 cords), reflect intensive management. The consistently higher yields of aspen in association with spruce—in

contrast to the lower yields in mixture with spruce, birch, alder,

and ash—is particularly noteworthy. Since the sites are rated as

good, the differences in yields between stands of practically identical ages might be predicated on differences in stand density, species composition, or both.

Differences between stands of similar ages in the number of

aspens per acre are hardly sufficient to account for the higher

yields and larger basal areas in the aspen-spruce association. The

number of trees and the basal areas of the associated species are

consistently lower in the aspen-spruce-birch-alder-ash association.

There is, therefore, little or no justification for attributing the

poorer growth of aspen in association with spruce, birch, alder,

and ash to unfavorable stand density or to the tree size of the associated species. It is possible, if not probable, that species composition—the individual or collective effect of birch, alder, and

ash—is responsible for the lower aspen yields.

Significance for the United States

Aigeiros poplars,—There is no doubt whatever that with intensive management we can obtain, and perhaps even surpass, the

present European yields of Aigeiros poplars under similar climatic and site conditions. This will not necessarily require such

extremely intensive culture as agricultural intercropping. We

have one of the most productive natural poplar species, the American cottonwood, distributed throughout the eastern United States

from Canada to the Gulf of Mexico. And we have under test many

hybrids potentially as productive as any in Europe.

^^ Barth, Agnar. Aspen. Dens kultur og behandling for kvalitetsproduksjpn. Fra ingeni0r F. H. Fr0lichs fond for aspeskogsbrukets fremme Nr. 1.

87 pp., illus. Oslo. 1942.
PRODUCTION OF POPLAR TIMBER IN EUROPE 103

The fast-growing Aigeiros cultivars that are in general use in

Europe today stem from hybrids between the American cottonwood and the European black poplar. The cottonwood (P. deltoides) is the more productive of these two parent species. It produces its highest yields on the batture (overflow) lands of the

lower Mississippi River Valley. In fully stocked natural stands,

annual yields of 4 to 5 cords—approximately 360 to 450 cubic feet

—per acre have been reported ^^ as not uncommon. The average

yield of fully stocked natural stands on these batture lands has

been estimated to be 5,000 cubic feet at 20 years, 250 cubic feet

per acre per year. The following yields per acre have been estimated for fully stocked natural stands on good sites in the Mississippi Delta region :^^

At5 years: 6 cords (approximately 540 cubic feet) (108 cubic feet/acre/

year).

At 10 years; 24 cords (approximately 2,160 cubic feet) (216 cubic feet/

acre/year).

At 15 years: 50 cords (approximately 4,500 cubic feet) (300 cubic feet/

acre/year).

These estimates are for unmanaged stands. Managed stands,

properly and regularly thinned, would exceed these yields. The use

of élite native clones, selected for rapid growth, would further increase the productivity of this species in managed plantations.

And the fact that most European hybrids exhibit hybrid vigor

provides a sound basis for predicting that hybrids, bred and selected for our southern environments, would exceed the productivity of the fastest growing native clones in the optimum range of

this species.

The yield of cottonwood is somewhat lower in the northern part

of its range and on sites that are less fertile and drier than riverbottom lands subject to periodic overflow. Under such environmental conditions the European hybrids may be better than the

natural species. The best European cultivars should be introduced,

through strict plant quarantine, for the reasons mentioned in

earlier sections. They would require at least 10 to 15 years of welldesigned field testing before they could be safely recommended for

commercial planting in this country.

There is no indication that the European cultivars will be more

productive, at least in the northern half of the United States, than

the best of the hybrids now under countrywide test by the Northeastern Forest Experiment Station. These hybrids were originally

selected for survival and growth under New England's climatic

conditions. The growth of the best hybrids in the original test

plantations, estabhshed in 1927 and 1928 in western Maine, justifies the prediction of a minimum total yield of 40 cords (3,600

''Williamson, A. W. Cottonwood in the Mississippi Valley. U. S. Dept.

Agr. Bui. 24, 62 pp., illus. 1913.

^^Bull, Henry, and Muntz, H. H. Planting cottonwood on bottom lands.

Miss. Agr. Expt. Sta. Bui. 391, 18 pp., illus. 1943.
104 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

cubic feet) of pulpwood per acre in 15 years (240 cubic feet per

acre per year) with reasonably good plantation management on

suitable upland soils.

Under intensive management on the best soils the yield could be

much more. Open-grown hybrids on excellent sites in Massachusetts and Pennsylvania have reached 22 inches d. b. h. in 11 years.

In row plantings, spaced 25 feet, the yield of such hybrids in 11

years would be approximately 240 cubic feet (or 21.8 cubic feet

per year) per 100-foot row. This is comparable to the best growth

of Aigeiros cultivars in European row plantings.

The higher average yield of clonal mixtures in European plantations strengthens the case against monoclonal stands.

Aspen.—Within its commercial range, the timber form and productivity of our quaking aspen (P. tremuloides) is fully as good

as that of the European aspen. Studies in the Lake States have

shown that at 50 years on good sites a yield of 47 cords per acre

(84 cubic feet per acre per year) may be expected for unmanaged

stands. The results of thinning experiments indicate that intensive

management could raise the average annual increment on good

sites well above 100 cubic feet per acre in less than a 50-year rotation.

Species hybridization, and possibly also racial hybridization,

can be expected to provide new types of greater productivity than

the native species.

Effect of associated species,—There is practically no experimental evidence on the possible inhibiting effect of associated species

of forest trees on the growth of poplars. All poplar species and

hybrids are classified as very intolerant to shading. Nevertheless,

we have an experimental area on which root sprouts of hybrid

poplars have survived for periods of 4 to 7 years under the complete shade of loblolly pines planted 2 by 4 feet apart. These poplars outgrew and overtopped the loblolly when the pines were between 10 to 18 feet in height. This indicates far greater tolerance

to shading than is generally accorded to poplars; and, most significantly, it indicates absence of any seriously deleterious root interactions between the poplars and the pines. . On the other hand, during the past 30 years the author has observed plantings on equally good sites where the growth of poplars was severely retarded by brush and hardwood sprout growth

that was mowed periodically to prevent overtopping the poplars.

Since the poplars had full sunlight, the growth-retarding effect

was apparently due to inimical root interactions. Experiments to

provide answers to this problem are basic to any future recommendations for the management of Aigeiros species and hybrids,

and of aspen, in mixture with other species.

On suitable soils hybrid poplars can be managed to produce a

greater volume of wood per acre, and larger individual trees, in

a shorter period of time than any other timber tree in the United

States. Large-diameter, knot-free, high-quality veneer logs can be

produced in 15 to 25 years. We have no other timber tree that can

equal this combination of growth and.wood-quahty potential. But

from the economic aspect such production must have commercial

use and reliable markets.
PRODUCTIOlSr OF POPLAR TIMBER IN EUROPE 105

On the basis of present utilization, covered in the next section,

it is safe to predict that in areas where an adequate supply can

be maintained, the demand for poplar logs will keep pace with

greatly expanded production. The problem of marketing small

local production has been discussed in the introduction.

The establishment of hybrid poplar plantations and their management for maximum quantity and quality production will require a somewhat higher investment than plantations of biologically less exacting timber species. But the yields on relatively

short rotations will be sufficiently high to warrant the expectation

of profitable dividends on the added investment.

For industrial and nonfarmer ownerships, poplar production

will necessitate a cash investment that will require the maximum

use of mechanical equipment to reduce to a minimum the use of

hand labor. For the farmer, the investment in poplar production

will usually represent the use of machinery available on the farm,

his own supervision and labor, and the labor of his regular farmhands. This will seldom interfere with agricultural activities because most of the work with poplars can be handled as off-season

jobs.

CHARACTERISTICS OF THE WOOD

The anatomical characteristics of some species of poplar are

sufficiently distinct for reasonably accurate identification, but the

wood of individual Aigeiros hybrids cannot be identified by this

means. Yet there is evidence of differences among the wood of the

various cultivars in their use requirements, the qualities and properties important for their conversion and use for veneer, lumber,

pulp, and particularly for specialty products. These differences

are due both to inherent variations and to the effect of environmental conditions under which the trees were grown.

Trees of the same cultivar are reliably reported to have different wood qualities, depending on the rate of growth, spacing,

and possibly other environmental conditions. Research is under

way in several European laboratories to evaluate the technical

qualities and properties of the cultivars in common use. These

studies are complicated by the fact that the same cultivar name

may be applied to several clones, by the complex nature of some

wood qualities, and by the effect of environmental factors that

cannot be accurately measured and correlated with wood quality.

The following characteristics and qualities apply, in general,

to the wood of the poplars grown in Europe. The wood is light in

weight, light-colored—unless heartwood stain is present, free of

resins, usually straight grained, and normally uniform in texture

and workability because there is little difference between sapwood

and heartwood and between spring and summer wood. In comparison with many other timber species it is a low-strength wood, but

its toughness and strength properties are relatively high in proportion to its light weight.

Poplar holds nails and screws well, and its resistance to splitting by nails makes it a good box wood. It can be glued readily
106 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

except where tension wood (gelatinous fiber) is present. Because

of its white color it can be easily stained, and it takes paint and

varnish well. It is generally odorless, though a Spanish report

states that the wood of Populus alba ^'supplies good building material" but is not "valued for packing because it imparts to the

goods a disagreeable smell.''

Poplar wood is soft (easily dented) ; it does not splinter and it

is easily veneered without previous stearning. It is a poor fuelwood

because of its low density and fast burning. Nevertheless, in localities where fuel is scarce there is an excellent market for

branch wood, prunings, and fagots.

The sapwood is preferred to heartwood for many uses. For this

reason there is often a better market for fast-growing wood than

for slow-growing timber that has a high percentage of heartwood.

There are exceptions: in France the author was informed that

roadside trees make better matchwood, because of their slower

and more uniform growth, than trees grown on alluvial bottom

lands.

Utilization in Europe

The uses of poplar wood in Europe are manifold. Although log

sizes required for different uses vary from country to country,

and even from locality to locality within a country, the following

specifications for minimum diameters—without bark, small end

of the bolt or log—represent a fair average for all of Europe :

2.3 inches: Round poles for construction use.

2.75 inches: Pulpwood. Some mills take peeled wood as small as 1.5 inches.

6.0 inches: Wooden shoes.

6.0 to 10.0 inches : Box and building lumber. Specifications for saw logs are

extremely variable. The smaller sizes are frequently marketable in southern Europe; the minimum diameter in northern Europe is seldom under

8 inches.

7.5 inches: Matchwood.

10.0 inches: Lowest grade veneer, for chip baskets, etc.

15.7 inches: Rotary veneer.

19.0 inches: Highest grade sliced veneer.

Log-length specifications are extremely diverse, depending on

local custom and factory requirements. Bolts of many lengths up

to 8 feet are specified for specialty and veneer use, but many factories buy poplar in tree lengths.

Specialty uses

Poplar wood has many specialty uses because of its white color,

lack of odor and resins, workability, softness, light weight, relatively high strength in proportion to its weight, and resistance to

splintering. It is used for artificial limbs and is extensively used

for wooden shoes. Minor uses include turnery products and kitchenware such as ladles, wooden spoons, and bowls. It is also used

for the production of excelsior.

Large quantities are used for boxes and crates of all kinds and

for slack cooperage. It is particularly useful for containers because of its lightness, the relatively high strength in comparison
PRODUCTION OF POPLAR TIMBER IN EUROPE 107

to its weight, and its nailing quality. In Germany it is preferred

for crating large mirror glass. It is in demand for tubs, pails, and

barrels to be used as food containers because of its lack of odor

and resins. Since the wood does not splinter it has been used as a

safety measure for trim in railway cars. Because of its light

weight and freedom from splintering it is used for wagon construction, for the bottom of truck bodies, and for the manufacture

of various types of handcarts and wheelbarrows.

Poplar is used in all countries for the manufacture of furniture,

particularly for interior members. The Forest Research Institute

in Spain has produced some excellent samples of molded furniture

from poplar veneer.

Because of its softness and even texture, poplar is frequently

used for drawing boards ; and it is preferred for carving and toys.

P, alba is used by at least one manufacturer in France for drafting instruments.

In recent years compressed-board products have been made

from poplar. The compression of the wood raises the strength

quality but also increases the density.

Veneers

Poplar is used in all countries for the production of both rotary

and sliced veneer. It is veneered cold, without steaming or soaking, up to 6 months after cutting—and sometimes longer. Rotary

veneer is used in the manufacture of matches (fig. 30), match

boxes, fabricated plywood doors, plywood sheets for building purposes, cigar boxes, boxes for various packaging purposes, chip

baskets, and woven baskets. It is used in Germany for storagebattery separators because of its freedom from chemical contaminants.

Some manufacturers of veneer baskets and boxes saw out short

planks clear of knots to the width and length of the required veneer pieces. These blocks are then sliced on a guillotine-type machine that can cut through as much as 12 inches of poplar plank.

The baskets or boxes are made immediately, without seasoning the

veneer.

Sliced veneer is used to a considerable extent as a face veneer

for furniture. Burly poplar, called Maser Pappel, commands a

premium price for such use in Germany.

The presence of tension wood in poplar will make it unsuitable

for veneer production. Tension wood causes warping and buckling of veneer; and it produces rough fuzzy surfaces when the

wood is machined. Such surfaces are extremely difficult to work

and are reported to require 60 percent more glue. The causes of

tension wood are not definitely known but it is usually correlated

with lean, eccentric growth, and possibly with the mechanical effect on the stemwood of swaying by wind. Other important defects that make the wood unsuitable for veneer use are insect and

fungous damage, large knots, numerous small knots produced by

epicormic branching, and, occasionally, spiral grain.
108 HANDBOOK 15 0, XJ. S. DEPARTMENT OF AGRICULTURE

F-482669

FIGURE 30.—Poplai- logs for matchwood, Saintines, Prance.

Lumber

The wood of the European black poplar has been used for centuries as round or sawed lumber for interior building construction where high strength is not required and where it is not subject to moisture. The Lombardy is preferred in the Mediterranean

region of France for building construction because the wood is

believed to be stronger than that of other poplars grown in the

region. Round poles are used for rafters, stringers, and frequently

for staging in the construction of buildings.

Poplar lumber must be carefully stacked and seasoned because

it is subject to serious warping and twisting (fig. 31). Many mills

saw the poplar logs into halves or into thick planks for seasoning,

and resaw the seasoned lumber to the finished product. The lumber is stable after seasoning, and there is seldom warping after

manufacture; but seasoned wood is somewhat more difficult to

work than green wood.

Pulp

Poplar is used for both mechanical and chemical pulp, but in

some localities the demand and higher prices for veneer and saw

logs leaves only a small proportion of the supply available for

pulpwood use. Practically all grades of paper, from writing paper

and magazine stock to newsprint and wrappings, are made in

Italy with a furnish containing poplar groundwood or chemical

pulp. A mill in the Po Valley, visited by the author, was making
PRODUCTION or POPLAR TIMBER IN EUROPE 109

F~482670

FIGURE 31.—Members of the International Poplar Commission inspecting

poplar lumber in the yard of a sawmill near Granada, Spain. All the

lumber in this yard is poplar.

newsprint reported to be 50 percent unbleached poplar groundwood, 20 percent spruce groundwood, and 30 percent spruce sulfite; they also made a supercalendered sheet described as 70 percent bleached poplar groundwood and 30 percent spruce sulfite. A

mill in southern Italy was using a furnish of 75 percent poplar

groundwood and 25 percent coniferous chemical pulp.

Utilization in the United States

Our native poplars are utilized for the same purposes as the

European poplars, with relatively few exceptions. Cottonwood

lumber has been used in the upper Mississippi basin since pioneer

days ; and where it reaches its best development it has been an important veneer species for a long time. Records show that the excellent cottonwoods along the Hoosic River in northwestern Massachusetts and southwestern Vermont brought premium prices

100 years ago.

On the other hand, until very recently our native aspen was

generally considered a weed tree, suitable only for limited pulpwood use. During the war years, this situation changed in the

Lake States region. Since 1947 the Lake States Forest Experiment

Station has published 22 reports by 9 cooperating agencies on the

availability and supply, properties and uses, and management of

aspen.

In recent years the use of poplar, with the exception of pulpwood in certain regions, has been increasing as rapidly as the sup471632°—59 8
lio HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

ply and log quality would permit. The most profitable future increase may be expected in the demand for high-quality logs for

the veneer and lumber industries.

Utilization of poplar will surely keep pace with even the most

optimistic predictable increase in the production of high-quality

logs. Steady and reliable log markets will be available only where

there is an assured supply of good poplar timber. Intensive management of native cottonwood and aspen can assure such supplies

in some regions. But intensive culture of thoroughly tested hybrid clones will eventually provide the most profitable, constant,

and uniformly high-quality timber supply.

Although hybrid-poplar culture in the United States is still in

the experimental stage, European utilization provides complete

assurance that properly selected and tested hybrids will meet, and

possibly surpass, the use requirement of our native species.

Pulp and paper

All species of poplar are easily pulped by the standard chemical

or mechanical methods, and they are also suitable for mechanical

defiberizing processes. Poplar is used in high-grade papers (book,

tissue, and specialities), and also for fiberboard, wallboard, and

impregnated building board or felt.

The chemical pulp yield from poplar wood is lower, on a weightper-cord basis, than the yield from the heavier hardwoods. During

the past 25 years, most pulp mills in regions that have abundant

native hardwood species have changed, in part or entirely, to the

utilization of these heavier species. Poplar is still in demand in

regions like the Lake States where there is a high concentration of

paper and pulp industries. Increased utilization of the short-fibered woods for groundwood and chemigroundwood pulp may

bring poplar into increasing competition with the heavier hardwoods.

Veneer

Among veneer woods, cottonwood ranked seventh on a national

basis in 1944. As a core stock for doors, building panels, and furniture, poplar is competing successfully with other native species

such as yellow-poplar (Liriodendron tulipiferw L.). Large volumes of poplar veneer are used for food containers such as cheese

boxes, egg cases, fruit and berry boxes, and soft drink and beer

cases. The use of poplar for veneer is presently limited by the supply of veneer-quality logs.

Boxes and crating

The use of poplar lumber and veneer for shipping containers of

all sorts was greatly expanded during the war, when its adaptability for such utilization was adequately tested. Since the war it

has gained steadily in preference over other box woods for many

uses.
PRODUCTION OF POPLAR TIMBER IN EUROPE HI

Lumber

Poplar lumber is used locally in building construction for rafters, stringers, studding, sheathing, shiplap, flooring, and interior

moldings and trim. Its use as lumber has been limited by spotty

supply of logs of suitable size and quality, poor manufacture and

seasoning by small mills, lack of standard grading rules, and relatively little effort to expand the market.

Furniture and specialty uses

Poplar is used for painted furniture, furniture shelving, backs,

drawer ends and bottoms, and dust separators. Specialty uses in

the United States include excelsior, turnery products, toys and

novelties, and v^agon-box boards.

Farm uses

Home-grown poplar has been used in the construction and repair of dwellings, barns, and other farm buildings. A Massachusetts apple grower obtained several years' supply of excellent apple-box lumber from a few large hybrid poplars originally planted

as shade trees. Poplar wood is too susceptible to decay to be used

for fence posts unless treated with a preservative; pressuretreated aspen posts in test plots were in good condition after more

than 15 years. Creosoted aspen has also given good wear as bridge

planking.
APPENDIX

Identification and Naming of Poplars

The genus Populus is divided into 5 generally accepted sections,

only 3 of which are of importance in European poplar culture.

Aigeiros poplars (Section Aigeiros Duby; spelled also Aegirus)

include the cottonwoods and black poplars. Leuce poplars (Section

Populus; known commonly as Section Leuce Duby ^^) are the white

poplars and aspens. The use of Tacamahaca poplars, also called

balsam poplars (Section Tacamahaca Spach) has been very limited.

Taxonomic studies of the European poplars were neither contemplated nor attempted in connection with these investigations.

The native poplar species of Europe are reasonably well defined,

but it will be very difficult to establish the validity of natural varieties of Populus nigra L. and P. alba L. because of the extensive

natural hybridization and probable introgression within the sections to which these two species belong.

European poplar culture, with the exception of aspen in the

Scandinavian countries, is based on the use of clones. Its economic

importance stems primarily from the possibility of maintaining

hybrid vigor and other superior qualities of a single individual

through vegetative propagation. This exclusive use of clones is

unique in the practice of timber and cellulose production.

Although the term clone is now in general use, it is still necessary to stress its significance as Stout ^^ did in 1929 :

In considering the status of a clone it should be constantly held in mind

that an entire clone, even though it comprises thousands of plants, is

merely one seedling plant that has been multiplied by vegetative propagation. . . . the tendency has been to give clones a specific or a varietal

rank that is not warranted, and to bestow on them scientific names that

ignore the horticultural status of the clone.

In the same paper Stout also proposed that the word ramet (from

the Latin ramus, meaning branch) be used for a member (an individual tree) of the clone. To indicate the original seedling plant

from which the clone is derived he suggested that the word ortet

(from the Latin ortus, meaning origin) may be used. These terms

are used in this report.

In the poplar regions of Europe, particularly in France, Germany, and Italy, literally hundreds of clones have been planted.

Unfortunately the extent and degree of this heterogeneity can

^« Under the International Code of Botanical Nomenclature (Art. 22. 1956),

as amended in 1950, the section of a genus including the type species repeats

the generic name without citation of an author. If Populus alba L. is taken

as the type species of this genus, the section of white poplars and aspens

should be called Populus section Populus. However, the common name Leuce

poplars, long in international usage, can be retained for this section.

"" Stout, A. B. The clone in plant life, N. Y. Bot. Gard. Jour. 30: 25-37.

1929.

112
PRODUCTION OF POPLAR TIMBER IN EUROPE 113

never be accurately determined because closely related clones may

be practically identical in their taxonomic characters. Differences

in physiological characteristics often appear only under special

conditions such as unusual climate, disease epidemics, or insect

infestations. Growth rate of individual trees in the same planting

can seldom be used as a distinguishing physiological characteristic

because of the effect of local soil variability.

The International Poplar Commission apparently recognized

the impossibility of distinguishing many closely related clones

when it originally proposed to name the cultivated poplars with a

binomial followed by a *'forma" name. Later, at its 8th session in

Madrid (1955) the Commission adopted, for naming the cultivated

poplars, the International Code of Nomenclature for Cultivated

Plants.2^ Under this code the "forma'' is replaced by the category

^'cultivar'' (abbreviated cv.) or the restrictive category ''clone''

(abbreviated cl.).

Since a cultivar name may include any number of clones that

are taxonomically similar, the use of cultivar names will not solve

the very practical problem of separating taxonomically similar

poplar clones that are physiologically different. This can be accomplished only by the selection of single trees as ortets for vegetative propagation, and the distribution of all ramets from a single

ortet under strictly controlled clone numbers or names. It is practically impossible to establish the clonal identity of taxonomically

indistinguishable poplars without proof of their origin from the

same ortet.

Thousands of ortets are being selected for clonal tests by European foresters, nurserymen, farmers, and landowners; and in

most cases the selectors maintain the clonal identity of their

ramets. Since the ortets are often plantation trees it is to be expected that within a locality, and even in different countries, genetically identical clones are being propagated and tested under

different clonal designations. Such duplication is unavoidable but

will not lead to serious difficulties in research or practice. Yet too

large a minority of poplar selectors are still propagating mixtures

of ramets from two or more ortets that are taxonomically indistinguishable. These mixtures may be given new names, or they

may be included under an older cultivar name if they fit the general description. In either case this can lead only to further confusion in practice and research.

The International Poplar Commission, in its 1955 session, instructed the Committee on Nomenclature and Registration of Poplar Names to begin work on an International Register of cultivar

names in accordance with the International Code of Nomenclature

for Cultivated Plants. In 1957, at the 9th session in Paris, the

Commission accepted the proposal that the following names, accompanied by the corresponding description card and under the

numbers indicated below, be entered in the special register that

^° Stearn, William T., ed. International code of nomenclature for cultivated

plants. 29 pp. Royal Horticultural Society, London. 1953.
114 HANDBOOK 150, U. S. DEPARTMENT OF AGRICULTURE

will be opened at the Secretariat of the International Poplar Commission :

57/1—Populus Xeuramericana 'serótina d'Exaerde\ Common name: Poplar "Bleu d'Exaerde".

57/2—Populus Xeuramericana 'serótina de Selys'. Common

name: de Selys poplar.

57/3—Populus y^euramericana 'serótina'. Common name:

Swiss poplar.

57/4—Populus yieuTamericana 'serótina de Champagne'.

57/5—Populus X^'^'^(^'¡^ericana 'serótina du Poitou'. Common

name: White Poitou Poplar.

57/6—Populus deltoides 'angulata de Chautagne'.

57/7—Populus deltoides 'carolin'. Common name: Carolin

poplar.

57/S—Populus deltoides 'virginiana de Frignicourt'. Common

name: Virginia poplar.

57/9—Populus Xeuramericana 'robusta'.

57/10—Populus Xeuramericana '1-154'.

57/11—Populus Xeuramericana '1-214'.

57/12—Populus Xeuramericana '1-262'.

57/13—Populus X^uramericana '1-455'.

Selection of poplars in the United States has already resulted

in the description and naming of at least a dozen cultivars, and

this number may be greatly increased within the next few years.

To avoid confusion, such names and descriptions should conform

to the revised International Code of Nomenclature for Cultivated

Plants published in 1958.^1 Because the 1953 Code was prepared

primarily for horticulture, a revision more applicable to cultivated

plants in general was needed. The revision was made by an International Commission composed of horticulturists, agronomists,

and foresters.

Poplar Names Used in This Report

Botanical names

The botanical names of the poplar species used in this report

follow Little 22 for the United States and Rehder ^^ for others.

^ International Commission for the Nomenclature of Cultivated Plants.

International code of nomenclature for cultivated plants, 28 pp., rev. Utrecht.

1958.

^^ Little, Elbert L., Jr. Check list of native and naturalized trees of the

United States (including Alaska), U. S. Dept. Agr., Agr. Handb. 41, 472

pp. 1953.

^ Rehder, Alfred. Manual of cultivated trees and shrubs hardy in North

America, Ed 2. 996 pp. New York. 1954.

Bibliography of cultivated trees and shrubs hardy in the cooler temperate regions of the northern hemisphere, 825 pp. Jamaica Plain, Mass.

1949.
PRODUCTION OF POPLAR TIMBER IN EUROPE 115

American and most European taxonomists accept the validity

of Populus X(^oMadensis Moench (P. deltoides X nigra)^^ European poplar experts have objected to this name as being a source of

confusion and the International Poplar Commission has officially

accepted Populus Xeuramericana (Dode) Guinier,^^ published in

1950,26 as the collective name for hybrids of P. deltoides X nigra.

According to Rehder,^^ Moench's Populus canadensis is apparently the oldest name for the hybrid between P. nigra and P. deltoides, and his description, though meagre, is adequate.

G. Houtzagers,28 commenting on the use of the name Populus

Xcanadensis Moench, was of the opinion that it is taxonomically

and genetically incorrect to name all new forms of P. deltoides

X nigra, or of their hybrids, as varieties of P. x canadensis

Moench, but agreed if *'varietas" were changed to "forma'' the

name P. X<^(^'^(^ensis Moench forma serótina, forma regenerata

etc. would be in accordance with the International Rules of Botanical Nomenclature. Nevertheless, he suggested that the name

P. xcci'^cidensis be rejected because it has been used erroneously

by various authors.

Under the present International Code (Arts. H.l and H.5) a

collective Latin binomial includes the hybrids of all generations,

including backcrosses. It is not genetically incorrect to use such a

collective hybrid name, provided its collective significance is

clearly understood. Although a name may be rejected if it is used

in different senses and so has become a long-persistent source of

error, rejection of the name P. X^ci'^^^densis Moench would require acceptance of the next oldest name, perhaps the name P.

X^erotina Hartig.

The Netherlands taxonomist, B. K. Boom, has presented the

soundest evidence for maintaining the name P. Xoanadensis

Moench.29 In 1956, Boom visited the park and the Natural History

Museum at Kassel to search for some of Moench's original material. He reported that the original trees are no longer in existence

and that Moench's herbarium was destroyed during World War

II. However, he did find an old xylothec with leaves and fruit, and

sufficient documentation to state : 'Therefore I am convinced that

we may safely accept the contents of box 188 of Schildbach's

^ Moench, Konrad. Verzeichnis ausländischer bäume und Sträuche des

lustschlosses weissenstein bei casseL 144 pp. [p. 81.] Frankfort and Leipsig.

1785.

^ Report of the Standing Executive Committee on the application to the

genus Populus of the rules laid down in the International Code of Nomenclature for Cultivated Plants. UN FAO, Internatl. Poplar Comn., 8th Sess.

(Madrid), 13 pp. 1955. (Mimeog.)

''UN FAO Internatl. Poplar Comn. Rpt., 4th Sess., p. 16; Geneva, 1950.

^^ Rehder, Alfred. New species, varieties, and combinations from, the herbarium and the collections of the Arnold Arboretum. Arnold Arboretum Jour.

4 (2) : 111. 1923.

^* Houtzagers, G. Het geslacht Populus in verband met zijn beteekenis voor

de houtteelt. 266 pp. Wageningen, The Netherlands. 1937.

^® Boom, B. K. Populus canadensis Moench versus Populus euramericana

Guinier, Acta Bot. Neerlandica 6: 54-57. 1957.
116 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

xylothec as neotype of P. canadensis Moench. Which clone it presents is not absolutely certain, but the leafshape and the female

character suggests that it may be cv. *regenerata\''

Cultivar names

The poplar names used in this, report conform to the International Code of Nomenclature for Cultivated Plants and accordingly have been placed in single quotation marks. For the sake of

brevity, the cultivar names directly follow the generic name, as

permitted by the code.

Table 7 lists the briefed cultivar names as they appear in this

report, the full name in accordance v^ith the International Code of

Nomenclature for Cultivated Plants, and the names most commonly used in the poplar literature.

TABLE 7.—Summary of poplar cultivar names used in this report

ames used in this report

Full name in accordance with the

International Code of Nomenclature for Cultivated Plants

Names most commonly used

in the literature

Populus 'Allenstein'

P. 'Angulata'

P. 'Angulata de Chautagne'

P.XbejoUnensis Dipp._

P. Xcanaáensis 'Allenstein'

P deltoides 'Angulata'

Allenstein

P.Xeuramericana 'Allenstein*

P. deltoides angulata

P. deltoides 'Angulata de Chautagne'.

P.Xberolinensis Dipp. (=P. laurifolia X nigra 'Itálica').

P.Xcanadensis 'Bietigheim'

P. nigra 'Blanc de Garrone'

P. nigra 'Blanquillo'

P. aZöa'Bolleana' --_ - -__

P. deltoides 'Angulata de Chautagne'

XP. berolinensis

P. 'Bietigheim' Bietigheim

P. 'Blanc de Garonne'

P.Xevramericana 'Bietigheim'

Blanc de Garonne

P. 'Blanquillo'

P. 'BoUeana' _

Blanquillo

Blanco

P. alba bolleana

P 'Bordils' P. nigra 'Bordils' Bordils

P. 'Brabantica' - _ __ P. Xcanadensis 'Brabantica'

P.Xcanadensis Moench (=P. deltoides X nigra)

P.Xcanescens (Ait.) Sm. (=P.

alba X trémula)

P.Xdeltoides 'Carolin'

XP. brabantica

P. Xcanadensis Moench

P Xcanescens (Ait.) Sm

P.Xeuramericana 'brabantica'

XP.canadensis

P. Xeuramericana (Dode) Guinier

XP.canescens

P. 'Carolin' _- -- --_ XP. carolinensis

P 'Drömling' P.Xcanadensis 'Dromling' -

P. deltoides 'carolin'

Dromling

P 'Eckhof - P.Xcanadensis'Echoî' - -- _

P.Xeuramericana 'Dromling'

Eckhof

P'Eugenei' - - P.Xcanadensis'Eugenei'.-- .- -_

P.Xeuramericana 'Eckhof

XP. eugenei

P 'Eugenel Feminine' P.Xcanadensis 'Eugenei Feminine'.

P.Xcanadensis 'Eukalyptus'

P. Xcanadensis 'Flachslanden'

P.Xcanadensis 'Gelrica'

P. Xeuramericana 'eugenei'

X P.eugenei feminine

P 'Eukalyptus'-

P.Xeurameiicana 'eugenei feminine'

Eukalyptus

P. 'Flachslanden'

P.Xeuramericana 'Eukalyptus'

Flachslanden

P 'Gelrica'

P.Xeuramericana 'Flachslanden'

XP. gelrica

P "Xgenerosa Henry P.XGenerosa Henry (=P. trichocarpa X angulata)

P. Xeuramericana 'gelrica'

XP. generosa
PRODUCTION OF POPLAR TIMBER IN EUROPE 117

TABLE 7. Summary of poplar cultivar names used in this report—Contimied

Names used in this report

P. 'Grandis

p 'Heidemij'

p. '1-28' _

p. '1-37 __

p. '1-65'

p. '1-137'

p. '1-154'

p. '1-214'

p. '1-262'

p. '1-455'

p. '1-477'.:

p. '1-488'

P. 'I-Caroliniano Bianco de Cercenasco'

P. 'Isar'

P. 'Itálica'

P. 'Leipsig'

P. 'Lloydii'

P. 'Löns'

P. 'Mainou'

P. 'Manitobensis'.

P. 'Marilandica'__

P. 'Monilifera'

P. 'Neupotz'

P. 'Nigrito'

P. 'Poncella'

P. 'Polla Carolina'.

P. 'Regenerata'

P. 'Regenerata Harff'.

Full name in accordance with the

International Code of Nomenclature for Cultivated Plants

P.Xcanadensis 'Grandis'..

P.Xcanadensis 'Heidemij'.

P.Xcanadensis '1-28'

P. Xcanadensis '1-37'

P.Xcanadensis 'I-65'__

P. Xcanadensis '1-137'

P. Xcanadensis '1-154'

P.Xcanadensis '1-214'

P. Xcanadensis '1-262'

P. Xcanadensis '1-455'

P. Xcanadensis '1-477'

P. Xcanadensis '1-488'

Names most commonly used

in the literature

P. Xcanadensis 'I-Caroliniano bianco de Cercenasco'

P. Xcanadensis 'Isar'

P. nigra 'Itálica'

P. Xcanadensis 'Leipsig'

P. Xcanadensis 'Lloydii'

P. Xcanadensis 'Löns'

P. Xcanadensis 'Mainou'

P. Xcanadensis 'Manitobensis'_

P. Xcanadensis 'Marilandica'..

P. deltoides 'Monilifera'

P. Xcanadensis 'Neupotz'

P. Xcanadensis 'Nigrito'

P. nigra 'Poncella'

P. deltoides 'Polla Carolina'.

P. Xcanadensis 'Regenerata'

P. Xcanadensis'RegeneratSi Harfï'

Grandis

P.Xeuramericana 'Grandis'

P. deltoides missouriensis

P. deltoides 'missouriensis'

P. 'Heidemij'

1-28

P.Xeuramericana '/-28'

1-37

P.Xeuramericana '1-37'

1-65

P.Xeurameiicana '1-65'

1-137

P.Xeuramericana '1-137'

1-154

P.Xeuramericana '1-154'

1-214

P. Xeuramericana '1-214'

1-262

P. Xeuramericana '1-262'

1-455

P. Xeuramericana '1-455'

1-477

P. Xeuramericana '1-477'

1-488

P. Xeuramericana '1-488'

I-Caroliniano bianco de cercenasco

P. Xeuramericana 'I-Caroliniano

bianco de cerenasco'

Isar

P. Xeuramericana 'Isar'

P. nigra itálica

P. nigra 'itálica'

Leipsig

P. Xeuramericana 'Leipsig'

XP. lloydii

P. Xeuramericana 'lloydii'

Löns

P. Xeuramericana 'Löns'

Mainou

P. Xeuramericana 'Mainou'

XP. manitobensis

P. Xeuramericana 'manitobensis'

XP. marilandica

P. Xeuramericana 'marilandica'

P. deltoides monilifera

P. monilifera

Neupotz

P. Xeuramericana 'Neupotz'

Nigrito

P. Xeuramericana 'Nigrito'

Poncella

Polla Carolina

Carolina

XP. regenerala

P. Xeuramericana 'regenerata'

Harfï poplar

P. Xeuramericana 'regenerata

Harfï'
118 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

TABLE 7.—Summary of poplar cultivar names used in this report—Continued

Names used in this report

Full name in accordance with the

International Code of Nomenclature for Cultivated Plants

Names most commonly used

in the literature

P. 'Kegenerata d'Hautervive'.

P. 'Robusta'

P. 'Robusta Bachelieri'--.

P. 'Robusta Vernirubens'.

P. 'Robusta Zeeland'

P. 'Serotina' - ---

P. 'Serotina de Champagne'--

P. 'Serotina d'Exaerde'-.

P. 'Serotina Erecta'

P. 'Serotina Keppelse Groene'.

P. 'Serotina du Poitou'

P. 'Serotina de Selys'.

P. 'Vert de Garonne'.

P. 'Virginiana'

P. Virginiana Carolin'

P. 'Virginiana de Firgnicourt'.

P. Xcanadensis 'Regenerata

d'Hautervive'

P. Xcanadensis 'Robusta'

P. Xcanadensis 'Robusta Bachelieri'

P. Xcanadensis 'Robusta Vernirubens'

P. Xcanadensis 'Robusta Zeeland'.

P. Xcanadensis 'Serótina'-

P. Xcanadensis 'Serotinp de Champagne'

P. Xcanadensis 'Serotina

d'Exaerde'

P. Xcanadensis 'Serotina Erecta'_

P. Xcanadensis 'Serotina Keppelse

Groene'

P. Xcanadensis 'Serotina du

Poitou'

P. Xcanadensis 'Serotina de Selys'.

P. nigra 'Vert de Garonne'.

P. deltoides 'Virginiana'

P. deltoides 'Virginiana Carolin'. _.

P. deltoides 'Virginiana de Frignicourt'

XP. d'Hautervive

P. Xeuramericana 'regenerata

d'Hautervive'

XP. robusta

P. Xeuramericana 'robusta'

XP. Bachelieri

P. Xeuramericana 'Bachelieri'

XP. vernirubens

P. Xeuramericana 'robusta vernirubens'

XP. robusta Zeeland

P. Xeuramericana 'robusta Zeeland'

XP. serótina

P. Xeuramericana 'serótina'

XP. serótina de champagne

P. Xeuramericana 'serótina de

Champagne'

P. Xeuramericana 'Serotina

d'Exaerde'

XP. serótina erecta

P. Xeuramericana 'serótina erecta'

P. Xeuramericana 'serótina Keppelse Groene'

XP. serótina de Poitou

P. Xeuramericana 'serótina du

Poitou'

P. Xeuramericana 'Serotina de

Selys'

Vert de Garonne

P. deltoides virginiana

XP. virginiana

P. deltoides 'Virginiana Carolin'

XP. virginiana Carolin

P. deltoides 'Virginiana de Frignicourt'

Insect Names Used in This Report

American and European entomologists do not agree on the

legitimate scientific names of all insects. The names used in this

report are those officially accepted by the entomologists of the

United States Department of Agriculture who are responsible for

insect identification. The following tabulation lists the insects

mentioned in this report under names that differ from those used

in Europe or in the older American literature.

Names used

in this report

Aegeria apiforonis (CI.)

Chalcoides aurata (Marsh.)

Chalcoides helxines (L.)

Chrysomela populi L.

Chrysomela tremulae F.

Cryphalus asperatus (Gyll.)

Names used in Europe or in

the older American literature

Sesia apiformis Cl.

Crepidodera aurata Marsh.

Crepidodera helxines L.

Melasoma populi L.

Melasoma tremulae F.

Cryphalus binodulus Ratz.
PRODUCTION OF POPLAR TIMBER IN EUROPE 119

Names used

in this report

Dicranura vinula (L.)

Gypsonoma acerina Dup.

Harmandia loewi (Rubs.)

Phratora vulgatissima (L.)

Phratora vitellinae (L.)

Phratora laticollis Suffr.

Phytagromyza populi Kalt.

Phytohia carbonaria (Zett.)

Sciapteron tahanifornáis Rott.

Stemochetus lapathi (L.)

Names used in Europe or in

the older American literature

Cerura vinula L.

Semasia acerina Dup.

Diplosis tremulae Wtg.

Phyllodecta vulgatissima L.

Phyllodecta vitellinae L.

Phyllodecta cavifrons Thomas

Napomyza populi Kalt.

Agromyza carbonaria Zett.

Sesia tabaniforme Rott.

Cryptorhynchus lapathi L.

Description of Poplar Stands Listed in Table 2

Cases i to ^.—Vicinity of St. Oedenrode, The Netherlands. A

rather wet site well drained by many deep ditches. Soil is a heavy

fertile loam ; pH tested 6.5 to 7.0. Ground covered by grass, weeds,

and nettles. Case 2 is on a slightly higher site with somewhat more

slope than case 3. Original spacing: Case 1, 23 by 28 feet; case 2,

23 feet square ; case 3, 26 by SO feet.

Case U,—Near Neunen, The Netherlands. On deep sandy loam

of excellent texture but probably low in fertility; pH 6.0. This

land, originally in oak coppice, was cleared and used several years

for crops before the poplars were planted. It has been in grass for

many years. Thinned at 27 years. Original spacing : 13 by 30 feet.

Case 5,—Vicinity of Mons, Belgium. On deep alluvial silt loam

with a high organic content. Summer water table about 2.5 feet.

Often flooded in winter. Grass and poplar management. Thinned

at 10 years. Original spacing : 15 feet square.

Case 6,—Near Mons, Belgium, on the same excellent silt loam

as case 5. This site has been poorly drained for the last 10 years

because of a change in drainage control. Grass and poplar management. Thinnings at 10, 11, and 17 years. Original spacing: 13

feet square.

Case 7.-—Grammont, Belgium. A bottom-land site subject to only

occasional overflow and well drained by ditches. The soil is a

rather heavy silt loam, with grass and weed cover. At least one

thinning. Original spacing: 11 feet, triangular.

Cases 8 and 9,—Schleswig-Holstein, Germany. Cases 8 and 9

represent two locations in the same stand on an excellent bottomland site. Soil is well drained with a rather high water table. The

poplars were planted in openings in a 20-year-old beech-oak stand.

No information about the original spacing, stand density, or how

the poplars were released. Forest ground cover.

Cases 10 to 15,—Along the river Erft in the vicinity of Grevenbroich, Gustorf, and Frimmersdorf, Germany. On excellent bottom land with considerable local variation in soil. These sandy

loams to loamy sands are generally fertile ^but usually too wet in

the spring for agriculture. Under grass and poplar management.

Original spacing: Case 10, 23 feet square; case 11, 26 feet square;

case 12, 23 by 26 feet; cases 13 and 14, each 16 by 23 feet; case

15, 23 by 26 feet.
120 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

Case 16,—Bottom-land site in the Ems lowlands near Glandorf,

Germany, on deep, moist, organic sand with ground water at about

2 feet. Original spacing: 10 feet, triangular.

Case 17.—Near Warendorf, Germany. On deep humus-poor

sand with ground water at 6.5 to 8.0 feet. Trees fertilized at the

time of planting. Original spacing: 20 feet, triangular.

Cases 18 and 19,—On strongly loamy sand, so-called ''Senkelboden," near Everwinkel, Germany. Ground water at about 2

feet. Case 18 includes inside trees only. Border trees averaged

14.0 inches d. b. h. and 79 feet in height. Original spacing: Case

18, 10 by 13 feet; case 19, 16 feet square.

Case 20,—Vicinity of Dingden, Germany. An excellent site that

was formerly a pond. Light sandy soil, more than 3 feet deep,

drained by numerous deep ditches to a larger ditch with constantly

flowing water. An underplanting of Norway spruce at wide spacing has produced a good profit from Christmas tree sales. Original

spacing: 16 feet square.

Cases 21 and 22,—Lignite district west of Cologne, Germany,

Plantations on strip-mine banks, with filler trees of white alder,

locust, and maple. The trees were fertilized at time of planting.

The fill is a mixture of sand and gravel with some clay particles

but without organic matter. Roots of poplars planted on new

mine banks, loose fill, are reported to penetrate 33 feet deep within

5 years after planting. Original spacing: Case 21, 8 feet square;

case 22, 14 feet square.

Case 23,—Lignite district west of Cologne, Germany, on a stripmine bank; gravelly sand rich in mineral nutrients, with some

clay but no organic material. White alder planted 5 by 5 feet

as filler trees; cut at 10 years. Poplars thinned twice. Original

spacing: 10 feet square.

Cases 2Í and 25,—-Lignite district near Hermulheim and

Frechen, Germany, respectively. Plantations on strip-mine banks ;

loamy sand, somewhat gravelly without organic matter; pH reported to be 7.0 to 8.0. Planted the year the banks were completed.

No fertilizer was used. White alder, spaced 5 by 5 feet as filler.

trees ; cut at 12 years ; one thinning of poplars. Case 24A is on

a low terrace with only 6.5 to 10 feet of fill over the original heavier soils. Case 24B is on a higher terrace that has 16.5 to 23 feet

of fill. Original spacing: Cases 24A and B, and 25, each 10 feet

square.

Case 26,—Vicinity of Wavelinghoven, Germany. On excellent

bottom land originally too wet for agriculture because of flooding

in the spring. Much of it has been drained through river improvement. Grass under the poplars. Original spacing: 26 by

39 feet.

Case 27,—This stand near Grevenbroich, Germany, had been cut

because of heavy shell damage during the war. An excellent

bottom-land site. Original spacing: 16 by 23 feet.

Cases 28 and 29,—Erft River bottom land, Harif, Germany. On

excellent sandy silt loam and loess-loam soils high in lime. Poplars

interplanted among widely spaced bottom-land hardwoods.

Original spacing not known but poplars are said to have **sufi^ered
PRODUCTION OF POPLAR TIMBER IN EUROPE 121

from the pressure of an old stand now cut." Measurements

represent the best trees. Forest ground cover.

Case 30.—On upland site near Harff, Germany. Loess-loam

soil, water table very deep, annual rainfall 21.7 inches. Heavy

losses from rabbits and war damage. Original spacing: 20 by

27 feet.

Case 31,—Bad Rothenfelde, Germany. On deep organic diluvial

sand with weak "bänderton'' at about 3 feet. Ground water at

approximately 2.5 feet. Two thinnings. Original spacing: 16

feet square.

Cases 32 to 35.—Rhine valley near Karlsruhe, Germany. Approximately 15 inches of clay loam varying to sandy and silty

clay over coarse sand and gravel. The moderately heavy to heavy

texture of these soils is not favorable for growth of poplars. The

pH at 15 to 25 inches is reported to be approximately 7.0. Cases

32 and 33 are sample plots in the same stand, originally interplanted with 2 rows of ash and 1 row of alder between the poplar

rows. First thinning was at 41 years, at least one thinning between 41st and 57th year. Case 34 interplanted with ash and

alder. Forest ground cover in all cases. Original spacing : Cases

32 and 33, 20 by 39 feet each; cases 34 and 35, unknown.

Case 36.—Rhine Valley near Rappenwört, Baden, Germany. On

fertile reclaimed bottom-land soil. Mixed hardwood understory;

forest ground cover. Original spacing unknown.

Case 37.—Rhine Valley near Karlsruhe, Germany. On deep,

fertile, silt loam to silty clay loam, subject to overflow; pH 6.0+.

Filler trees of ash and alder. Forest ground cover. Original spacing^: 26 by 49 feet.

Case 38.—Rhine Valley near Neuburgweier, Baden, Germany.

On deep, fertile, well-drained silt loam to silty clay loam; pH

6.5-7.0. Interplanted with Acer pseudoplatanus. Forest ground

cover. Original spacing : 26 by 49 feet.

Cases 39 to U2.—Bottom land at the juncture of the Danube and

Isar Rivers, Bavaria, Germany. Very fertile soils, 6.5 feet or

more of silt loam subject to annual overflow; pH reported 5.5

to 6.0. Early culture of the older stands uncertain. In recent

years the land has been in grass; the younger stands were established in grass. Original spacing: Cases 39-41, each 11 feet

square; case 42, unknown.

Cases U3 and U.—Marne Valley near Perthois, France. On

excellent bottom-land soils subject to frequent overflow Soil is

highly fertile, deep, silty clay loam; pH 6.0+. Said to be one

of the best soils in this area. P. 'Virginiana' (case 42) and 'Robusta' (case 43) on adjacent areas. Grass and poplar culture

Original spacing: each 23 feet square.

Case i5.—On bottom land in the Voire Valley, France. Rather

heavy alluvial soil. Hay and pasture under the poplars. Original

spacing: 23 feet square.

Case J,6.—VoÍTe Valley, France. Alluvial bottom land with

about 12 to 20 inches of gravel underlain by a compact clay subsoil. Grass and poplar culture. Original spacing: 23 feet square.

Case ^/.—Voire Valley, France. Bottom land with about 2
122 HANDBOOK 15 0, V. S. DEPARTMENT OF AGRICULTURE

feet of light loamy sand over gravel on a clay subsoil. Organic

content high in upper 10 to 12 inches of the soil. Site, originally

in oak coppice, was cropped before it was planted to poplars.

Original spacing : 26 by 30 feet.

Case ^5.—Near Wassy (Blaise Valley), France. On a very

deep, alluvial, marly, clay loam subject to overflow. Original

spacing unknown.

Case i9,—Blaise Valley, France. Bottom land with mediumheavy alluvial soil over clay. Grass under the poplars. Original

spacing: 23 feet square.

Case 50.—Rhone Valley near Rochemaure, France. On excellent

deep silt loam over coarse sand on a terrace well above the river

and not subject to overflow; pH above 6.0. Grass ground cover.

It is reported that in dry summers the water table could drop to

13 feet. The last 10 years have been dry ; 1947 and 1949 reported

to have been particularly dry years—70 days of drought in the

summer of 1949. Original spacing : 13 feet square.

Case 51.—Near La Palud, France, in the Rhone Valley. On low

wet land adjacent to the river side of the protection dike. Soil

is a deep, fertile alluvium subject to overflow. Early culture uncertain. Original spacing: 10 feet square.

Case 5^.—Rhone Valley near Bourg-St.-Andeol, France. On

fertile bottom land subject to overflow. Well-drained silt loam;

pH above 6.0. Grass and herbaceous ground cover. Original spacing: 10 by 16 feet. ^ ^ ^^

Case 53.—Garonne Valley near Toulouse, France. On bottomland terrace with light sandy soil, low in fertility, and subject

to overflow only at very high flood stage. Summer water table 3

to 6 feet; in dry years considerably deeper. After supporting a

previous stand of poplar, the land was used for crops for about

10 years before it was replanted to poplar. Present stand intercropped 4 years after planting, followed by hay and later grazing.

Original spacing: 20 feet square. , ^ ., , . rm • Case 5^.—Reclaimed land near Yvonaud, Switzerland. This

land was lake bottom and swamp land until reclaimed through control of the river Aare. A very fertile sandy loam to silt loam with

a high organic content. Early culture not known, apparently mterplanted with other bottom-land species including alder. Forest

ground cover. Original spacing unknown. ^ , \

Cases 55 to 59.—Fo Valley, Cásale Monferrato, Italy. On bottom lands subject to overflow. Early culture with crops 3 or 4

years, followed by grass and weeds. Soils are variable even m

the same plantations. Cases 56 and 57 are in the same plantation rows where there is a soil gradient from poor loamy sand

(case 56) to a fertile silty loam with a good ground cover of

nettles (case 57). Original spacing: Cases 55-58, each 11 by

33 feet; case 59, 13 feet, triangular, in double rows 39 feet apart.

Cases 60 and 61.—Fo Valley near Isolone, Italy. Excellent

fertile alluvial soil subject to overflow. Case 60 is on land previously used for poplar, cleared and cropped for 1 year before replanting to poplar, then intercropped and irrigated during the

first 3 years after the poplars were planted. Case 61 was inter-^
PRODUCTION OF POPLAR TIMBER IN EUROPE 123

cropped for 4 years after planting, then cultivated for an additional 3 years. Original spacing: Case 60, 11 by 30 feet; case 61,

16 feet square.

Case 62.—Po Valley near Piacenza, Italy. On excellent welldrained bottom land subject to overflow. Mixture of about 60

clones selected at Cásale Monferrato from open-pollinated seed

from selected mother trees. Intercropped during the first 3 or 4

years; now in grass and weeds. Original spacing: 11 by 33 feet.

Case 63,—Albalate del Arzobispo, Spain. One-year-old trees

planted on excellent fertile bottom land subject to overflow and

previously used for garden crops. Interplanted with Pinus halepensis and P. canariensis. Cultivated until plantation was closed.

Original spacing: 13 feet square.

Case 6Í.—Toradera River Valley near Hostalrich, Spain. Plantation established with unrooted sets. On well-drained loamy sand

to sandy loam with a low summer water table. Plantation is

still irrigated when necessary with water pumped from the river.

Intercropped in the early years followed by light stand of grass

cut for fodder. Original spacing: 16 feet square.

Description of Row Plantings Listed in Table 5

Case 65.—Vicinity of Sutphen, The Netherlands, on sandy loam

between a paved road and a 3- to 4-foot-deep highway ditch. No

information on water table; probably not lower than 4 to 5 feet

in summer.

Cases 66A-C,—Near Meppen, Germany, on well-drained sandy

soil around the border of a pasture that was fertilized at irregular

intervals. Water table not lower than 3 feet. The poplars were

not fertilized at the time of planting.

Cases 67 to 70,—Nos. 67 and 68 are single rows, 69 and 70 are

double rows, in the vicinity of Warendorf, Germany. They are

all on deep sand deflcient in organic matter. The planting stock,

2-year-old trees, was fertilized with 11 pounds of Thomasmehl

mixed with the soil in each planting hole. Ground water on sites

67 and 68 was at 6.5 to 8 feet, on sites 69 and 70 at 9.5 to 10 feet.

Case 71,—Trees along an old filled road near Everswinkel,

Germany. The soil is a loamy sand with ground water at about

2 feet.

Case 72,—A double row in Holtwick, Germany, on deep sandy

loam of good texture and fertility. Considerably better soil than

case 73.

Case 73,—Holtwick, Germany. Deep loamy sand ; ground water

at 2 to 2.5 feet. Soil rated as "medium sour" ; the poplars have

been limed.

Cases 7Jf to 76,—Vicinity of Glandorf, Germany. On deep moist

sand with considerable organic content and ground water at about

2 feet. Rows 75 and 76 established with unrooted sets. Case 75

is a row along a brook bank and is underplanted with alder.

Case 77,—Frimmersdorf, Germany, on excellent fertile bottom

land subject to overflow. Grass and poplar culture.
124 HANDBOOK 15 0, U. S. DEPARTMENT OF AGRICULTURE

Case 78,—Near Baden-Baden, Germany, on a fertile bottom-land

soil of medium texture.

Case 79,—Vicinity of Karlsruhe, Germany. Fertile overflow

land with 2 to 2.5 feet of loam or sandy loam over coarse sand

and gravel. Excellent poplar site.

Case 80.—Very fertile bottom land subject to frequent overflow

at the juncture of the Danube and Isar Rivers, Bavaria, Germany.

Same site as cases 39 to 42.

Cases 81 and 8^.—Seine Valley, St. Hilaire, France. Fertile

silty clay loam with high organic content; pH 6.0 to 7.0; subject

to occasional overflow. Depth of water table varies from 1 to 2.5

feet, never below 3 feet in dry summers.

Cases 83 and 8U>—Vicinity of Saint-Urbain, France. Row plantings along the Marne canal on excellent bottom-land soil. Grass

under the trees.

Case 85,—Voire Valley, Puellemontier, France. Four rows

along a low embankment across good bottom land. The rows are

20 feet apart.

U. S. GOVERNMENT PRINTING OFFICE : I 9*59