Twenty-Five Years of 

Managing Vegetation in 

Conifer Plantations in 

Northern and Central 

California: Results, 

Application, Principles, 

and Challenges

Philip M. McDonald and Gary O. Fiddler

United States 

Department of 

Agriculture

Forest Service

Pacific Southwest 

Research Station

General Technical 

Report

PSW-GTR-231

October 2010
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Pesticide Precautionary Statement

This publication reports research involving pesticides. It does not contain recommendations for their use, nor does it imply that the uses discussed here have been 

registered. All uses of pesticides must be registered by appropriate state or federal 

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CAUTION: Pesticides can be injurious to humans, domestic animals, desirable 

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Use all pesticides selectively and carefully. Follow recommended practices for 

the disposal of surplus pesticides and pesticide containers.

Authors

Philip M. McDonald is a research forester (emeritus), Pacific Southwest 

Research Station, Western Forest Management Unit, 3644 Avtech Parkway, 

Redding, CA 96002; e-mail: pmcdonald@fs.fed.us; and Gary O. Fiddler is 

a silviculturist, Ecosystem Conservation Staff; Pacific Southwest Region, 

Vallejo, and stationed at the above address in Redding, CA; e-mail: 

gfiddler@fs.fed.us.

Cover photographs are by the authors.
Abstract

McDonald, Philip M.; Fiddler, Gary O. 2010. Twenty-five years of managing 

vegetation in conifer plantations in northern and central California: results, 

application, principles, and challenges. Gen. Tech. Rep. PSW-GTR-231. Albany, 

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station. 87 p. 

In the late 1970s, the outlook for conifer seedlings in new plantations in the Western 

United States was dismal—too many were dying or growing below the potential 

of the site. This situation was untenable, and a large study aimed at increasing the 

survival and growth of planted conifer seedlings was implemented. This was the 

National Administrative Study on Vegetation Management of which the California 

portion is reported here. This “study” was really a program on plantation release 

with 32 individual studies that resulted in more than 60 publications over a 25-year 

timeframe. The authors emphasized plant community development and biological 

influences to help explain why some direct release methods were effective and 

some were not. Survival and growth of several species of conifer seedlings were 

correlated to the density, foliar cover, and height of various combinations of over 

235 species of hardwoods, shrubs, forbs, and graminoids after application of five 

principal release techniques (herbicides, manual release, mulches, grazing, and 

mechanical [large machines]). Herbicide use was the most effective treatment 

followed by manual release and mulches. Domestic grazing, as currently practiced, 

was not effective, and release with large machines was worthwhile only if followed 

by an herbicide. Genetic enhancement of conifer seedlings showed promise as a 

vegetation management tool at first, but lost its efficacy later. Indirect vegetation 

management by using shade and organic material to reduce the growth and density 

of competing vegetation has potential, but needs more study. Nineteen principles 

and 10 conclusions resulted from this research program, but more work in the form 

of 11 challenges is recommended. 

Keywords: Competing vegetation, conifer seedling growth, northern California, 

plant community dynamics, vegetation management.
Summary

Inadequate reforestation and poor conifer seedling survival and growth led to a 

national research and application program on vegetation management in young 

conifer plantations. Findings from the California part of this program, and more 

specifically on plantation release, are presented here. Results are from 32 study sites 

throughout northern and central California and reported in more than 60 publications over a 25-year timeframe. The study sites represent a wide variety of site 

qualities, soils, slopes, aspects, and vegetation types. Most of the planted conifer 

seedlings were ponderosa pine (Pinus ponderosa Dougl. ex Laws. var. ponderosa) 

and Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco var. menziesii).

We wanted to bring a more holistic perspective to vegetation management, and 

to concentrate on plant community development, biological influences, and operational techniques. The goal was to describe what plant species are present in young 

conifer plantations, how they and the communities they represent develop, and how 

they can be manipulated to achieve vegetation management goals. Consequently, 

in addition to vegetation management, we present information on botany, ecology, 

wildlife, domestic grazing, and economics (costs).

Knowing which are the most competitive species in the plant community 

mandates knowledge on what species are present in young conifer plantations— 

a subject heretofore not well known. In one study on a good site, a total of 71 

plant species were found during the 5-year study, although no more than 62 were 

present during a given year. Every plant species was measured for density, foliar 

cover, and average dominant height every year. After 5 years, density averaged 

112,408 plants per acre, foliar cover 17,433 ft 2

 per acre, and height from 0.8 ft for 

perennials to 2.4 ft for graminoids. In another more extensive study with 21 study 

areas (plantations) each with 10 years of data, the average number of plant species 

in each plantation was 23 at the beginning and 28 at the end. For all the plantations 

combined, more than 235 species in six categories (conifers, hardwoods, shrubs, 

forbs, graminoids, and ferns) were found at the beginning and 213 at the end of the 

study. Core species and genera (defined as being present in at least 50 percent of 

plantations) numbered 3 and 10, respectively.

The morphological and physiological adaptations of some species enable them 

to capture scarce resources such as soil moisture, nutrients, and growing space 

almost from the beginning of their existence. They also can tolerate extreme stress. 

For example, 5-year-old deerbrush (Ceanothus integerrimus Hook. & Arn.) seedlings can withstand an internal moisture stress of 42.7 atmospheres and greenleaf 

manzanita seedlings (Arctostaphylos patula E. Greene) one of 23.8 atmospheres 
without apparent damage. In a 4-year side-by-side study of these two species, 

deerbrush developed a clump of stems the first growing season and greenleaf 

manzanita the second. By the end of the second season, deerbrush had produced 

statistically more stems per plant, was taller, and had wider crowns than manzanita. 

Deerbrush also began to produce flowers and seed during the third growing season, 

but manzanita did not do so during the study. These data suggest that deerbrush is 

the more competitive species, at least for the first 4 years. In another study, biomass 

accumulation of 1-year-old greenleaf manzanita plants on a good site was almost 60 

times that of ponderosa pine seedlings. 

We have found that plants in the early seral community of young conifer 

plantations in California follow most, if not all, of the five classical regeneration 

strategies: windblown seeds, persistent seed banks, persistent seedling banks, rapid 

expansion aboveground, and vegetative elongation belowground. They also embrace 

one or more of the three traditional successional pathways of facilitation, tolerance, 

and inhibition. Together the regeneration strategies and successional pathways mean 

that a large variety of well-adapted plant species, each with unique ability, will be 

present in young conifer plantations.

This paper details results from nearly all the direct vegetation management 

release techniques used in the Western United States. These include herbicides, 

manual release, mulches, grazing (browsing) animals, and mechanical (large 

machines). Also included are genetic enhancement of young ponderosa pine seedlings and a new concept termed “indirect treatments.” We evaluated 21 variations 

of the direct treatments and 96 trials in this part of our research program. 

We found that herbicides are effective for release on a wide range of plant 

communities at a reasonable cost. Manual release generally is effective if the community is mostly forbs and graminoids, or shrubs if very young. The cost of manual 

release is higher than for herbicides because a second treatment is often necessary. 

Mulches are effective if they are large and durable, but are severely limited by cost. 

As presently practiced, the use of domestic grazing animals for plantation release 

showed no significant gain in conifer seedling growth. Grazing is too little, too 

late, and not enough competing vegetation is utilized. Mechanical release of older 

seedlings with large machines is not effective without a followup application of 

herbicides. 

Genetic enhancement to boost the growth of conifer seedlings over that of 

competing species began with promise, but deteriorated to the point that it showed 

little efficacy as a vegetation management tool. Indirect vegetation management 

using shade and organic material to reduce the growth and density of competing 

vegetation has promise, but needs more study. It has the advantage of being low in 
cost and the disadvantage of taking more time to achieve results. No side-by-side 

studies comparing direct and indirect treatments are known. 

Nineteen principles, 11 challenges, and 10 conclusions have come from this 

study. Some of the most important principles concern developmental variables for 

conifer seedlings and competing vegetation. In general, seedling height is a poor 

descriptor of growth, stem diameter is best, and foliar cover of shrubs best explains 

the most variation in pine seedling parameters. Important challenges concern the 

need to better predict what competing species will be present in young conifer 

plantations, and their potential density and size. More knowledge on the belowground ecosystem is critical. One of the most important conclusions is that conifer 

plantations in clearcuttings, and herbicides, burning, and grazing are feasible at 

some place, at some time, and for some purpose. They should remain in the vegetation manager’s repertory of available methods and techniques. 
Contents

 1 Introduction

 4 Location, Site Characteristics, and Treatments

13 Plant Community Development

19 Biological Influences

20 Plant Moisture Stress

22 Competition

25 Adaptation, Regeneration Strategies, and Successional Pathways

32 Biological Control and Exclusion

35 Insect-Vegetation-Animal Interactions

36 Past Stand History

37 Operational Techniques

37 Direct Treatments

49 Genetic Enhancement

51 Indirect Treatments

55 Discussion

55 Plant Community Development

57 Biological Influences

59 Operational Techniques

65 Principles

70 Challenges

72 Conclusions

76 Acknowledgments

76 Metric Equivalents

77 Literature Cited

1

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Introduction

In the beginning there was brush, lots of brush, and the plantation manager 

said: “We won’t worry about it for now. If it gets bad, we will treat it later.” 1 

In the end, the plantation was almost all brush (fig. 1) and the manager said: 

“If only I had acted earlier” 

This heartfelt comment portrays the state of vegetation management in California in its early years. Indecision, frustration, and especially lack of knowledge 

were commonplace among forest land managers who were trying to establish young 

conifer plantations that would be fully stocked and growing at the potential of the 

site. Unfortunately, this goal often was not attained and a trend of disturbingly poor 

conifer seedling survival and growth was all too apparent. In many plantations, and 

for several species, conifer seedling survival often was less than 50 percent after 

three growing seasons. This situation was untenable.

The combination of poor seedling growth in plantations and a court ruling 

limiting the use of herbicides on national forest (U.S. Department of Agriculture 

Forest Service) land prompted the Chief and Staff of the Forest Service in 

Washington, DC, to propose a National Administrative Study in summer 1979 

to manipulate competing vegetation in young conifer plantations. The study 

stressed scientific verification of results from many operational vegetation control 

techniques. In 1980, the California part of the national administrative study was 

implemented by the team of researcher Philip McDonald and silviculturist Gary 

Fiddler, working as a joint endeavor by the Pacific Southwest Research Station and 

the Pacific Southwest Region of the Forest Service. Initial funding was provided by 

the National Forest System in Washington, DC, with subsequent financial support 

by the Pacific Southwest Region. Later, most of the funding was provided by the 

Pacific Southwest Research Station. 

The team was given the broadest possible latitude to develop their study 

program in terms of number and kind of treatments, experimental design, data 

manipulation, and presentation of study results. McDonald and Fiddler emphasized 

plantation release and initially concentrated on chemical (herbicides) and manual 

manipulation of unwanted vegetation. In 1982, they broadened their research 

program to include vegetative manipulation by grazing animals and large machines. 

Still later, mulch mats to inhibit vegetative competition, indirect manipulation 

of competing vegetation, and genetic enhancement of ponderosa pine (Pinus 

ponderosa Dougl. ex Laws. var. ponderosa) seedlings in an attempt to outgrow 

1

 Anonymous. 1983. Comment from audience at 5th annual forest vegetation management 

conference. Sacramento, CA: November 2–3, 1983.

The combination of 

poor seedling growth 

in plantations and a 

court ruling limiting 

the use of herbicides 

on national forest land 

prompted the Chief 

and Staff of the Forest 

Service in Washington, 

DC, to propose a 

National Administrative 

Study in summer 

1979 to manipulate 

competing vegetation 

in young conifer 

plantations.
2

GENERAL TECHNICAL REPORT PSW-GTR-231

Figure 1— (A) Choked with brush, seedlings in this ponderosa pine plantation have little chance of 

growing at the potential of the site. (B) Too much competition and lack of resources have doomed this 

young pine seedling. 

A

B
3

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

competing vegetation, were added. Implicit in this program was the desire to study 

“alternatives” for manipulating undesired vegetation in plantations, not only for 

increasing conifer seedling growth but also for understanding the basic ecology 

of competing vegetation, crop/weed competition, animal damage, and “to explore 

likely future issues.” Because cost is a major concern to practitioners of vegetation 

management, data on time to apply the various treatments and concomitant dollar 

values almost always were recorded. This gave the research study program an 

economic component as well. Based on all of these components, it was recognized 

as “The most extensive program in North America to compare all silvicultural 

alternatives for vegetation management” (Campbell 1991). 

Throughout, the study approach was one of constant fine-tuning, based on 

countless hours of thought and discussion. We strived to incorporate ideas from 

other natural science disciplines, and from scientists at universities, the Weed 

Science Society, and vegetation management cooperatives. Results from local and 

international vegetation management conferences, as well as from related research 

endeavors such as the Forestry Intensified Research program in southwest Oregon 

(Tappeiner and others 1992) and the Garden of Eden study in northern California 

(Powers and Ferrell 1996) were consistently monitored. We were especially interested in bringing a more holistic perspective to managing vegetation in young 

conifer plantations. Consequently, in addition to vegetation management, we had 

research components in botany, ecology, fuels, wildlife, and domestic grazing in 

our study. We also became increasingly aware, particularly in the later years of the 

study, of the increasing concern about climate change and carbon sequestration. 

Although we could not change the design of the basic program or the experimental design of the individual studies, we did incorporate these concerns into our 

manuscripts. 

The objectives of this paper are to summarize the methodology, results, and 

knowledge gleaned from more than 30 studies on the release of young conifer 

seedlings from undesirable vegetation. Specific objectives were to: 

•	 Present several fundamental processes that influence the development

of vegetation.

•	 Determine	plant community development and species richness in

young conifer plantations.

•	 Evaluate manual, chemical, grazing animal, mulch, mechanical,

and indirect release treatments and genetic enhancement of young 

conifer seedlings.

•	 Understand conifer seedling growth versus weed quantity relationships.
4

GENERAL TECHNICAL REPORT PSW-GTR-231

•	 Arrive at “best” treatments in terms of conifer survival, growth, and cost.

•	 Present some principles of vegetation management derived from	this	

research program and some challenges facing forest vegetation managers 

in the near future. 

Because of the extensiveness of our study, most of the material in this paper 

will be from it, but numerous other references are used to augment a finding, 

make a point, or broaden the discussion. Results from two related longer-term 

studies (28 and 31 years) in particular are used to help explain important biological 

relationships. Consequently, this paper is not a summary of all that has been tried 

by others or is known on manipulating vegetation in young conifer plantations. It 

is our experience.

Although wide in scope, the program had one major and two minor limitations. 

The major limitation was that we concentrated on plantation release and not on site 

preparation. An early, but extensive, survey found that site preparation in the California region was effective and generally well done, but many questions were being 

asked about plantation release. The first of the minor limitations was that the study 

was installed in young conifer plantations 1 to 4 years old, except for the mechanical treatment, which involved large machines (Trac Mac, Hydro-Ax2

) in plantations 

older than 12 years. The second minor limitation was time. The duration of studies 

was 10 years if possible, which was about the time that the conifer seedlings would 

begin to compete with themselves.

A strength and a limitation of the program is its extensive nature. More than 

60 publications have been produced. The limitation is that data from some of these 

publications are not included, nor referenced, in this paper. However, every publication contributed to the results, implications, and discussion presented here. 

Location, Site Characteristics, and Treatments

When this research and application program began, research on the ecology and 

manipulation of many, if not most, of the plant species in young conifer plantations 

was scant and the information that was available was badly fragmented, anecdotal, 

or from small unreplicated studies. The strength of this paper is that it is based on 

plant presence and response data from 32 different study sites located throughout 

northern and central California (fig. 2). Forty-two studies were planned and 

located on the ground, but 10 of them were lost for a variety of reasons, including 

forest fires, chronic deer browsing, and lack of cattle fencing. The 32 studies were 

2

 The use of trade or firm names in the publication is for reader information and does not 

imply endorsement by the U.S. Department of Agriculture of any product or service.

When this research and 

application program 

began, research 

on the ecology and 

manipulation of many, 

if not most, of the 

plant species in young 

conifer plantations 

was scant and the 

information that was 

available was badly 

fragmented, anecdotal, 

or from small 

unreplicated studies.
5

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

located mostly on land administered by the USDA Forest Service, but also on land 

managed by the USDI Bureau of Land Management, the state of California, Hoopa 

Indian Reservation, and private industry (Sierra Pacific Industries). Twenty-one of 

these studies had at least 10 years of data, and several had 5 years of record. All 

studies had from three to six replications, intensive statistical analysis, and all have 

been published. Map reference number, study names, species of conifer seedling, 

elevation, site quality, treatment, and soils (series and texture) describe each 

study location (table 1). Throughout the paper, the study name is used to reference 

specific areas.

Figure 2—Location of 32 study areas in northern and central California. 
6

GENERAL TECHNICAL REPORT PSW-GTR-231

All but one of the studies were located in conifer plantations, with seedlings 

that were planted by hand as bare root or container-grown stock, as opposed to 

naturally-seeded seedlings. Natural regeneration may occur when seed sources 

are present at the time of disturbance, but it may not provide the desired species 

composition, distribution, or density in a timely manner. Planted seedlings almost 

always accomplish these goals. Ponderosa pine and coast Douglas-fir (Pseudotsuga 

menziesii [Mirb.] Franco var. menziesii) were most commonly planted, with Jeffrey 

pine (Pinus jeffreyi Grev. & Balf.), California white fir (Abies concolor var. lowiana

Table 1—Map (fig. 2), reference number, and information for study areas in northern and central California

Map Site

reference Study area Speciesa Elevation qualityb Treatmentsc Soil series Soil texture

Number Feet

1 Signal DF 1,500 M C, H, M Aiken Loam, clay loam

2 Rocky DF 2,550 M C, H, M Hugo-Clallam Gravelly loam

3 Henry Bell DF 2,400 M C, H, M Holland Shale

4 Hamburg PP 3,100 L C, H, M — Gravelly loam

5 Trapper PP 6,200 M C, M Scheld-Iller Sandy loam

6 Stephens Pass A WF 5,600 M C, M Avis —

7 Stephens Pass B RF 5,600 M C, M Avis —

8 McBride PP 4,750 L C, M Neer Sandy loam

9 ElkSprings PP 4,800 H C, H, M Shasta Loamy sand

10 Harrison Gulch PP 3,800 L C, M, Mu Marpa Gravelly loam

11 Hoopa TO 2,690 H I — Loam

12 Mad River DF 3,250 H C, M Holland Sandy loam

13 Logan PP 5,000 M C, H, M Crazier Loam

14 Latour PP 5,200 M C, H Windy —

15 Deans JP 4,500 H C, M Skalan Sandy loam

16 Lee's Summit PP, DF 5,000 M C, H, M Skalan Sandy loam

17 Sheep II JP 6,600 L C, G, H, M Haypress Sandy loam

18 Third Water PP, J P 5,500 M C, H, Me Gibsonville Sandy loam

19 Mulches DF 3,600 M C, Mu Holland Sandy loam

20 Alumagel DF 3,600 M C, M Holland Sandy loam

21 Votaw PP 4,000 M C, H, Mu Cohasset Loam, clay loam

22 Bear Trap DF 1,400 H C, M, Mu Honeydew Gravelly loam

23 Nooning Creek DF 2,300 H C, M Honeydew Gravelly loam

24 Boggs PP 3,550 M I Aiken Loam, clay loam

25 Snag PP, DF 3,600 H C, H, M Aiken L oam, clay loam

26 Cattle II PP 4,250 H C, G, H Crozier Loam, clay loam

27 Sheep I PP, DF 4,900 M C, G Dubakella —

28 Big Tunnel PP 4,600 M C, H, Me McCarthy Sandy loam

29 Bear Clover PP 4,000 H C, H Cohasset Loam

30 Chaix PP 3,400 M Ge Mariposa-Jocal Loam, clay loam

31 G rapevine PP 3,850 M C, H Josephine —

32 Big Collars PP 5,900 M C, H, Mu — Sandy loam

a

 DF = Douglas-fir, PP = ponderosa pine, WF = California white fir, RF = California red fir, TO = tanoak, JP = Jeffrey pine.

b

 M = medium, L = low, H = high.

c

 C = control, H = herbicide, M = manual, MU = mulch, ME = mechanical, G = grazing, GE = genetics.

— = No data.
7

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

[Gord.] Lemm.), and California red fir (A. magnifica A. Murr.) present in a few 

studies. Scientific and common names of trees are from Little (1979), and all other 

vegetation follows the nomenclature of Hickman (1993).

Studies were located throughout northern and central California, specifically 

from N 41°52'56", W 123°3'26" to N 36°11'23", W 118°39'56". Study locations 

spanned about 500 mi north to south and about 135 mi east to west. The studies 

were located in the California Coast Mountains (including the King Range), 

Klamath Mountains, southern Cascade Range, Diamond Mountains, and Sierra 

Nevada. Site quality ranged from very high to very low, with most study sites 

having average quality. An average site would support ponderosa pines that were 

about 70 ft tall at 50 years of age (Powers and Oliver 1978). The “lowness” of the 

poorest site was ameliorated somewhat by being on a north slope and receiving an 

average rainfall of 40 in annually. 

Soils were derived from a broad range of parent materials, including volcanic, 

meta-sedimentary, and igneous rocks. Some soils were deep, fertile, and of high 

water holding capacity, whereas others were shallow, rocky, and prone to rapid 

moisture loss. Most were in-between. Textures ranged from loam, clayloam to 

sandy and gravelly. Taxonomically, soils ranged from Ultic Haploxeralfs to Typic 

Dystroxerepts.

The climate of the study areas is mediterranean with long, hot, dry summers 

and cool, moist winters. In most of the plantations, May through October often 

are dry with little or no effective precipitation. Consequently, the vegetation has 

adapted over millennia to both a long period of drought and frequent wildfires. 

Even near the Pacific Ocean, the summers are hot and dry, albeit of shorter duration. Soil moisture is the limiting environmental factor and drought the primary 

cause of mortality for most plants. Daily summer temperatures varied from 60 to 

106 °F. and winter temperatures from -25 to 50 °F. Elevations ranged from 1,400 

to 6,600 ft with most occurring in the 4,000- to 5,000-ft range. Average annual 

precipitation varied from 25 in at a study on the slopes of Mount Shasta to 125 in 

at a study near the Pacific Ocean. Most studies received precipitation as both rain 

and snow. 

Many species of birds and animals inhabited or roamed through the study 

areas, but rarely were they seen. Deer (Odocoileus spp.) and pocket gophers 

(Thomomys spp.) were the most common species, and in a few instances caused 

damage. Indeed, one study had to be abandoned because of chronic deer browsing; in another, damage by pocket gophers was serious. Crouch (1979) stated that 

pocket gophers are “a major cause of reforestation failures in many western conifer 

forests.”
8

GENERAL TECHNICAL REPORT PSW-GTR-231

The first study plantation was measured in fall 1980, and final data were 

collected in the last study in 1998. The studies were located in the general forest 

zone with rocky areas, riparian zones, burned slash piles, and areas near roadsides 

excluded. All areas had received some form of site preparation or had recently 

burned in a wildfire. Almost all hardwood trees, clumps of shrubs, logging slash, 

and unmerchantable conifers were removed during site preparation. Thus, the 

plantations were characterized initially by full sunlight and only minor amounts 

of vegetation aboveground. No study sites were treated with soil-active herbicides 

during site preparation.

As noted earlier, the plantation release treatments included the five classical 

direct methods of (1) herbicides, (2) manual release, (3) mulches, (4) mechanical 

(large machines), and (5) grazing animals, as well as genetic enhancement and two 

indirect techniques (shade and organic material). Each individual study had three 

to six treatments, usually four. Many treatment variations existed, especially in the 

herbicide and manual applications. For example, herbicides could consist of any of 

five different products, and range from application to the whole plot or to either of 

two different radii. Every study had an untreated control, which of course, had no 

plantation release cost, and most studies had a near free-to-grow treatment 3 that 

was the most expensive to install. Other treatments were intermediate and varied 

in biological effectiveness and cost. 

Herbicides—

Commonly applied herbicides were soil-active Velpar, foliage-active Garlon 4, 2,4-D, 

Roundup, and cutting plus cut-surface-active Garlon 3A. One or two trials were with 

Arsenal, Escort, and Pronone (granular Velpar). A licensed applicator applied the 

herbicides, usually in the morning, which most often was the time when specific 

conditions of wind and temperature were met. Spraying was performed during the 

summer at a time when growth of the conifer seedlings had ceased, but that of the 

shrubs, forbs, and grasses was continuing. A guide often was used to ensure uniform 

coverage of the herbicide and to avoid overlapping and skipped areas. 

Manual release—

This direct form of plantation release usually was accomplished with grubbing tools, 

and, in a few instances, with shears or a chainsaw. Manual release differed even 

more than herbicides as radii from 1 to 6 ft were grubbed as well as the whole plot. 

In addition, some plots were treated from 1 to 10 times. Sometimes a manual slashing would take place followed by an herbicide application.

3

 In this paper, “free to grow” means almost no competition.

The plantation release 

treatments included 

the five classical 

direct methods of (1) 

herbicides, (2) manual 

release, (3) mulches, 

(4) mechanical (large 

machines), and (5) 

grazing animals, 

as well as genetic 

enhancement and two 

indirect techniques 

(shade and organic 

material).
9

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Mulching—

In this research and application program, mulching was aimed primarily at conifer 

seedling growth; survival was secondary. Seedling survival had been the goal in 

most prior studies (McDonald and Helgerson 1990), but we wanted to broaden the 

application of this method. All mulches in our studies were manufactured mats 

of specific materials and sizes as opposed to applications of wood chips, sand, 

or sawdust that were areawide. Mulches differed greatly, and early trials (Votaw) 

were with kraft paper, black and clear plastic, reinforced paper/asphalt sandwiches, 

pressed peatmoss, and woven and nonwoven polypropylene. Later tests were with 

various sizes and thicknesses of polyester felt and woven polypropylene. Sizes 

varied from 1 to 10 ft square. 

Mechanical release—

Mechanical release was accomplished with large machines that resembled lawn 

mowers but with free-swinging rotary cutting heads. These machines were powerful, stable, and maneuverable. They easily severed the stems of large, tall shrubs 

and small hardwood trees, and adroitly dodged rocks and stumps in the planted 

area. Cutting widths were 4 to 8 ft. The Hydro-Ax is an example. It cuts vegetation 

close to the ground and leaves the chopped stems and other organic material on site. 

It is efficient on slopes of up to 40 percent, and with a skilled operator is capable of 

treating 3 to 4 acres per day. 

Grazing animals—

Cattle and sheep were the grazing animals in this study. In general, the cattle 

grazed mostly on grass and the sheep browsed mostly on shrubs, but both kinds of 

animals did both. Hence for simplicity, we use the more inclusive term “grazing” 

to describe this direct-release category. The use of cattle to manipulate competing 

vegetation was planned in three studies. Unfortunately, two of these studies were 

abandoned because of problems with permittees and fencing. The one study that we 

had was titled “Cattle II” and was located on the Forest Hill Ranger District of the 

U.S. Forest Service’s Tahoe National Forest. Measurements took place from 1988 

through 1997. The herd consisted of 20 to 100 mature cows with calves that roamed 

through the study area from June through September. Grazing took place in 5 years 

of the 10-year study. The animals were familiar with mountain topography and 

wildland grazing.

We had two studies with sheep (Sheep I and II). The bands consisted of 600 to 

1,500 animals, mostly nonlactating ewes. In one study, the sheep were experienced 

with wildland conditions and the plant species in the area; in the other study, they 

were trucked to an area near the study site, unloaded, and allowed to browse for 
10

GENERAL TECHNICAL REPORT PSW-GTR-231

a week to become accustomed to the wildland environment. A herder and dogs 

guided the sheep in and out of the allotments (which included the study areas) 

during the grazing season, which was from June through September. The animals 

bedded down each night near the study areas where there was water. The animals 

passed through the study areas about 10 times each year, and, in each pass, grazed 

no longer than a week. 

Genetic enhancement—

Genetically improved conifer seedlings, the product of years of selection, progeny 

testing, and breeding, have potential value for lessening the negative impact of 

undesirable vegetation in young conifer plantations. If genetically improved seedlings outgrow competing shrubs and grasses, especially during the first 3 years 

after planting, the plantations may not need to be released, or released a second 

time. Eliminating release treatments could save millions of dollars in California 

and perhaps elsewhere, given full implementation of genetically improved planting 

stock. 

Although more were planned, we ended with only one study on genetic 

enhancement (Chaix). It was with ponderosa pine seedlings. Three classes of 

enhancement, each with and without competition, were tested for survival and stem 

growth: (1) nursery run, (2) wind-pollinated, and (3) control-pollinated. Each class 

was considered a treatment. Nursery-run seedlings were typical seedlings planted 

in conventional plantations throughout California; wind-pollinated seedlings were 

from genetically superior females selected from progeny tests with seed collections 

from natural stands; and control-pollinated seedlings had both males and females 

selected, based on progeny tests, with the seed collected from a breeding orchard. 

Control-pollinated seedlings were the best planting stock that would be available for 

large-scale operations in California for the next two decades. Nursery-run seedlings 

were from seed collected from several stands in the seed zone corresponding to the 

elevation of the study. Wind-pollinated seedlings were from four wind-pollinated 

families. Control-pollinated seedlings were from four full-sib families. All families 

were in the appropriate or adjacent seed zone. 

Seed from each genetic class was sown in spring 1989 in a seedbed at the 

Forest Service nursery at Placerville, California, with alternating rows of each class 

perpendicular to the long axis of the bed. To avoid possible bias, sowing rate and 

seedbed density were similar for each genetic class. All seedlings were transplanted 

from the nursery to the study site after one season in the nursery and planted by the 

same workers the same day (March 20, 1990). Each worker planted a different class 

If genetically improved 

seedlings outgrow 

competing shrubs and 

grasses, especially 

during the first 3 years 

after planting, the 

plantations may not 

need to be released, 

or released a second 

time.
11

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

of seedling in each block (replication) to avoid planting bias. Seedling spacing was 

12 by 12 ft in rows with planting spots prepared by augers. Rows were perpendicular to the slope.

Indirect methods—

The indirect strategy of vegetation management is based on modifying the environment to the disadvantage of the weeds. The key to its application is use of natural 

elements such as organic matter, shade, overstory, and adjacent vegetation to 

decrease or weaken normally robust and aggressive competing plant species. In 

an indirect strategy, the environment becomes the treatment. 

The use of organic matter to retard germination of dormant seeds in the soil 

has been reported many times by forest observers and documented in one of our 

studies on the Plumas National Forest (Lee’s Summit). Likewise, the effect of shade 

has been observed to dramatically slow the growth of aggressive shrub species as 

well as conifer seedlings. But which is affected the most? A long-term study with 

group selection on the Challenge Experimental Forest in north-central California 

provided data that documented this effect. Another study on group-selection cutting 

on the Boggs Mountain State Demonstration Forest in central California quantified 

the effect of both shade and the roots of edge trees on conifer seedling growth. 

Additional analyses documented the relationship of opening size and distance from 

opening edge to height and diameter growth of both naturally seeded and planted 

ponderosa pine seedlings. 

For all studies, the experimental design was completely randomized with three 

to six replications, called plots, in each study. Plots were rectangular and ranged 

from 0.l to 0.25 acre with one or two rows of buffer (seedlings receiving similar 

treatment) located around sample seedlings. Statistical analysis was predominately 

by one- or two-way analysis of variance, with regression, simple t-tests, Bonferroni 

tests, mixed-model analysis, and Tukey tests also used.

Conifer seedlings selected for sampling had to have good potential of becoming 

harvestable trees, and chlorotic and misshapen seedlings that would be removed in 

the first precommercial thinning were excluded. Sampling intensity in almost all 

studies ranged from 25 to 35 seedlings per plot, occasionally from 15 to 40 seedlings. Stem diameter (measured at 12 in above mean groundline) and height were 

measured on each seedling. Needle length and foliar cover (the sum of shadows that 

would be cast by leaves and stems of individual species expressed as a percentage 

of the land surface [Daubenmire 1968]) often were quantified as well. The seedlings were periodically checked for injury from herbicides, animals, and insects. 
12

GENERAL TECHNICAL REPORT PSW-GTR-231

Plant species richness (Perry 1994) was quantified when each study began, 

when it ended, and whenever the study area was visited. Woody and herbaceous 

vegetation in all studies was measured in seedling-centered, square subplots, 

each containing 1 milacre (0.001 acre or 43.56 ft2) of area, with all four corners 

permanently marked (fig. 3). Sampling intensity almost always was five subplots 

per plot. Categories of species (forbs, graminoids for instance) and very abundant 

species were evaluated for density, foliar cover, and average dominant height 

(average of the three tallest stems measured from mean groundline to bud). More 

specifically, the number of plants in each subplot was counted, foliar cover was 

visually estimated to the nearest square foot, and height was measured with a 

graduated pole. The estimate of foliar cover was checked often by a foot-square 

wire frame and a graduated tape. 

Figure 3—Milacre frame used to sample lesser vegetation in our studies. 

The costs that we report for installing and maintaining the treatments were 

those of an experienced Forest Service crew that ranged from two to five workers. 

Costs were collected during the life of the study. No allowance for travel, equipment, or overhead was included. The cost of installing each treatment, both initially 

and for retreating, was based on hourly records and the prevailing wage rate for a 

WG-1 laborer established by the U.S. Department of Labor as of June 1993. Hence, 

the costs that we report have value for comparative purposes, but undoubtedly are 

low by current standards. 
13

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Internal moisture stress of ponderosa pine and Douglas-fir seedlings, as well as 

several hardwoods and shrubs, was quantified to help explain growth differences 

that became apparent among treatments in our study program. Xylem sap tension 

was measured with a Scholander pressure chamber (Scholander and others 1965) in 

five early studies from August 8 to August 25, in 1982 and 1984. In these studies, 

the conifer seedlings were about 5 years old and had been growing for 3 years after 

treatment.

Sampling of xylem sap tension began just before dawn and continued through 

late afternoon. Time and distance considerations mandated that only one replication 

per treatment be sampled. For this reason and because of the ever-changing amount 

of tension in the plants, no statistical analysis of differences among treatments was 

attempted. Sampling intensity was three randomly selected seedlings and shrubs 

of each species in each treatment. Consequently, three twigs of each species were 

tested in the pressure chamber at five measurement times throughout the day. Each 

twig was placed in a sealed plastic bag to minimize moisture change while en 

route to the pressure chamber. The elapsed time between cutting and placement in 

the chamber did not exceed 3 minutes. Every third sample was measured twice in 

the chamber as a check on technique and working order of the equipment. “Plant 

moisture stress” is expressed in atmospheres (Waring and Cleary 1967) because it 

is a direct reading and is familiar to foresters and biologists. 

This paper has three distinct, but interacting, components: plant community 

development, biological influences, and operational techniques. The goal is to 

describe what plant species are present in young conifer plantations, how they and 

the communities that they represent develop, and how they can be manipulated to 

achieve vegetation management objectives. These interacting components can be 

considered as the “results” part of the paper. 

Plant Community Development

After disturbance, such as from fire or site preparation, the composition of the plant 

community tends to be highly variable. Some species arise from root crowns or 

rhizomes, some from windblown seeds, and still others from dormant seeds in the 

soil. The distribution of plant species ranges from clumpy for some species to random for others. Number of species depends on several factors, including the kind 

and degree of disturbance, time since disturbance, availability of propagules, and 

rapidity of colonization by early invaders. Development of plants, at least for the 

first 10 years, is governed primarily by quality of microsite, freedom from intense 

competition, inherent growth rate, and herbivory.
14

GENERAL TECHNICAL REPORT PSW-GTR-231

This section documents (1) the presence and development of every plant species 

each year for 5 years (1976–1980) in a single study on the Challenge Experimental 

Forest, (2) changes in species richness within several categories of vegetation before 

and after 10 years in 21 study areas, and (3) development of vegetation in three 

differing plant communities given a wide selection of treatments.

From 1976 through 1980, every species of vegetation that was present in a small 

clearcutting on the Challenge Experimental Forest in north-central California was 

recorded and its density, foliar cover, and height were ascertained (fig. 4). Here, 

site quality is high and the dominant species, ponderosa pine, will average about 

95 ft in height in 50 years (Powers and Oliver 1978). Site preparation was windrow 

and burn, which left a mostly bare mineral soil seedbed, free of vegetation aboveground. No sampling took place in or near burned windrows or in riparian zones 

within the sampled area. 

Figure 4—Careful searching on this circular milacre subplot has identified 53 species

of plants. 

The first species to appear in the study area were sprouts of hardwoods 

and shrubs from root crowns and rhizomes as well as patches of bracken fern 

(Pteridium spp.). In late fall, conifer seeds, many seeds of annuals, and those of 

a few perennials blew into the area. Several thousand germinated. 

After two growing seasons, thousands of plants from every regeneration strategy (Grime 1979) had become present, and those already there had grown much 
15

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

larger. Many grasses and forbs (annuals and perennials) also produced seeds, and 

species richness and population size increased greatly. Animal populations followed 

a similar trend, and browsing of several hardwoods, shrubs, and forbs by deer was 

documented. Of note, was that several of these species had new, tender, palatable 

foliage at this stage of development that became unpalatable the following year. 

More species, rapid development, and intensifying competition characterized the 

next three growing seasons. 

In total, 71 plant species were found during the 5-year study, although no 

more than 62 were present during a given year (McDonald 1999). Species richness increased from 47 after 1 year to 62 after 5 years. After 5 years, average 

dominant height of plants in each category of vegetation ranged from 2.4 ft for 

graminoids (grasses and sedges) to 0.8 ft for perennials (fig. 5). Of course, this 

measure of height is lowered by the infusion of inherently shorter recruits. If only 

the tallest plants in each category were noted, then the tallest category would have 

been shrubs followed by conifers, graminoids, annuals, ferns, and perennials. The 

density of all species was 24,860 plants per acre after one growing season and 

increased erratically to 112,408 per acre after five seasons. Foliar cover increased 

each year from 950 ft2 per acre to 17,433 ft2 per acre after five seasons. Proportional changes in density (fig. 6) and foliar cover (fig. 7) by category each year 

show the developing nature of the plant community.

Figure 5—Height of five	categories	of vegetation, Challenge	Experimental	Forest, 1976–

1980. Standard errors for shrubs ranged from 0.1 to 0.3 ft in a given year; for annual forbs, 

from 0.1 to 0.2; for perennial forbs, from 0.1 to 0.2; and for graminoids, from 0.2 to 0.4 ft. 

2.5

2.0

1.5

1.0

0.5

0

Height (ft)

Year

1976 1977 1978 1979 1980

Shrubs Annuals Perennials

Graminoids Ferns
16

GENERAL TECHNICAL REPORT PSW-GTR-231

Figure 6—Proportional plant density (number per acre) for several categories of vegetation, Challenge Experimental Forest, 1976–1980.

Shrubs

Annuals

Perennials

Graminoids Ferns

Hardwoods

Shrubs

Annuals

Perennials

Graminoids

Ferns

Conifers Hardwoods

Shrubs

Annuals

Ferns Shrubs

Annuals

Ferns

Conifers Conifers

Shrubs

Annuals

1976 1977

1978 1979

1980

Perennials

Graminoids

Perennials

Graminoids

Ferns

Perennials

Graminoids
17

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Figure 7—Proportional foliar cover (square feet per acre) for several categories of vegetation, Challenge Experimental Forest, 1976–1980.

1976 1977

1978 1979

Shrubs

Annuals

Perennials

Graminoids

Ferns

Hardwoods

Shrubs

Annuals

Perennials

Graminoids

Ferns

Hardwoods

Shrubs

Ferns

Shrubs

Annuals

Ferns

Hardwoods

Shrubs

Annuals

1980

Perennials

Graminoids

Annuals

Hardwoods

Perennials

Graminoids

Perennials

Graminoids

Ferns Hardwoods
18

GENERAL TECHNICAL REPORT PSW-GTR-231

Results from 21 study areas after 10 years portray the plant species present in 

young conifer plantations, how long they are present, and whether they increase or 

decline (McDonald and Fiddler 2006). The results also identified the plant species 

and genera that are most common in young plantations. Almost all of the study 

areas were located in conventional plantations in northern and central California 

whose location, site characteristics, and treatments were described in table 1. 

We found 237 plant species in six categories (conifers, hardwoods, shrubs, 

forbs, graminoids, and ferns): 194 at the beginning of the studies and 213 at the end. 

The average number of species in each plantation was 23 at the beginning and 28 

after 10 years. Most of the increase was in the forb category. In 12 plantations, the 

total number of species increased; in 8 areas, the number decreased; and in 1 area 

the number of species remained the same. For those areas with a species gain, the 

increase over 10 years was 53 percent; for those with a species loss, the decrease 

was 11 percent. No statistically significant difference was found between the beginning and ending plant community ( p = 0.05). The range was 13 to 61 species in 

each plantation, with high variability and no trend of species richness related to site 

productivity (above to below average) or age of vegetation (5 and 10 years). 

Were some core species and genera (defined as being in at least 50 percent of 

our study areas), common to young conifer plantations? We identified three species 

and ten genera. The species were greenleaf manzanita (Arctostaphylos patula E. 

Greene), Sierra gooseberry (Ribes roezlii Regel), and bull thistle (Cirsium vulgare

[Savi] Ten.); the genera were Quercus, Arctostaphylos, Ceanothus, Prunus, Ribes, 

Rubus, Symphoricarpos, Cirsium, Achnatherum, and Bromus. Only one nonnative 

species in one study area (Snag) was found. It was yellow star-thistle (Centaurea 

solstitialis L.) that probably was brought into the area on a hunter’s truck. Although 

it persevered for 10 years, it had peaked in density and foliar cover and was declining at the end of the study (table 2). 

Table 2—Density, foliar cover, and height of yellow star-thistle on 15 

1-milacre plots in a study in the northern Sierra Nevada, 1983–1992

Density Foliar cover Height

Year Mean Range Mean Range Mean Range

 Number of plants/acre - - - Percent - - - - - - - - Feet - - - - -

1983 438 0–2,250 1 Ta–7 1.5 0.6–2.3

1988 10,138 250–30,250 4 T–12 1.9 0.9–3.0

1992 1,850 0–7,125 T T 1.6 1.3–2.1

a

 T = trace.
19

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

A representative plantation in three widely differing plant communities was 

chosen to portray the major changes in vegetation composition over time and the 

magnitude of competition to conifer seedlings. These communities consisted primarily of (1) mixed shrubs; (2) mixed shrubs and graminoids; and (3) mixed shrubs, 

forbs, graminoids, and ferns. The number of treatments in each ranged from three 

to six and included herbicides, grazing, and manual release. 

After 10 years, the mixed-shrub community had a smaller number of species 

than the other plant communities, and a range that was characterized by a lower 

number of plants per acre, many times more foliar cover, and taller plants (table 3). 

If graminoids are present, as in the mixed shrub/graminoid community, the range of 

plant density is huge, but the range of foliar cover and height is generally lower. If 

many more species are present and they are a mixture of shrubs, forbs, graminoids, 

and ferns, the range in density is similar to that in the mixed shrub/graminoid community, but foliar cover is much reduced, and the range in height is greater. 

Table 3—Development of vegetation after a wide assortment of treatments in 

three differing plant communities after 10 years

Number Foliar cover Height

Community of species Density range range range

Number of plants/acre Square feet/acre Feet

Mixed shrubs 21 467–10,745 0–18,428 0.6–3.2

Mixed shrubs/graminoids 32 37,950–64,400 523–4,356 0.3–1.2

Shrubs, forbs, graminoids, 46 36,338–62,727 370–976 1.3–1.9

and ferns

Biological Influences

Biological influences are essential for understanding the operational relationships 

and technologies that have been attempted and why some have been effective and 

some have not. This section includes information on several fundamental processes 

such as plant moisture stress, competition, adaptation, regeneration strategies, 

successional pathways, biological control, and past stand history—all of which 

affect the survival and growth of young conifer seedlings. The section also includes 

material on some complex insect-vegetation-animal interactions and on plant 

species replacements and exclusions—biological processes that could alleviate the 

need for one or more operational treatments and could be thought of as treatments 

themselves. 

Biological influences 

are essential for 

understanding 

the operational 

relationships and 

technologies that have 

been attempted and 

why some have been 

effective and some 

have not.
20

GENERAL TECHNICAL REPORT PSW-GTR-231

Plant Moisture Stress

This form of internal stress was studied on ponderosa pine seedlings in two studies. 

In one study (Logan; McDonald and Fiddler 1995), a typical diurnal (daily) trend of 

internal plant moisture stress on a hot summer day indicated that seedlings treated 

by grubbing the entire plot had low predawn minimum stress, lower maximum 

stress, and lower late afternoon stress than counterparts in the other grubbing 

treatments (fig. 8). Conversely, seedlings in plots manually grubbed to a 4-ft radius 

and those in the control had to endure more and longer periods of stress. Internal 

moisture stress in the other study of ponderosa pine (Elk Springs; McDonald and 

Fiddler 1990), indicated a similar trend with seedlings in the herbicide treatment 

having the lowest stress and those in the control having the highest stress. Lowest 

to highest diurnal values for the most effective treatment in both studies ranged 

from 5.7 to 14.8 atmospheres, and for the least effective treatment from 8.8 to 

19.3 atmospheres. 

Figure 8—Trend of internal plant moisture stress of ponderosa pine seedlings in three 

release treatments on the Lassen National Forest, California, August 17, 1982.

25

20

15

10

5

Plant moisture stress (atmospheres)

Pacific standard time

2 4 6 8 10 Noon 2 4 6 8

4-ft radius

Entire plot

Control
21

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

For the three studies with Douglas-fir seedlings (Henry Bell, Rocky, Signal; 

McDonald and Fiddler 1986a, 1996, 1999b), the lowest internal moisture stress 

values generally occurred in seedlings in treatments that effectively targeted 

the most vigorous competing species. Seedlings in plots treated with 2,4-D, cut 

and spray with Garlon 3A, and grubbing the entire plot had the lowest internal 

moisture stress. The highest stress occurred in seedlings on plots that had a 2-ft 

grubbed radius or in the untreated control. Lowest to highest diurnal values for 

the most biologically effective treatment in the three studies ranged from 4.5 to 22 

atmospheres, and for the least effective treatment from 13 to 28 atmospheres. 

Internal moisture stress of root-crown sprouts of tanoak (Lithocarpus densiflorus [Hook. & Arn.] Rehd.) was evaluated in two studies in northwestern California (Rocky, Signal; McDonald and Fiddler 1996, 1999b), and, as for Douglas-fir 

seedlings, was found to be lower in treatments that targeted the species that were 

the most competitive to them. Tanoak sprouts in plots treated with Garlon 4 or a 

tank mix of Garlon 4 and 2,4-D had the lowest values, and those in the control and 

in plots grubbed to a 5-ft radius one time had the highest values. Lowest to highest 

diurnal values for tanoak in the most effective treatment (herbicides) in the two 

studies ranged from 4 to 8 atmospheres, and for the least effective treatments 

(grubbing, control) from 7 to 18 atmospheres. 

Internal moisture stress was evaluated in two species of Ceanothus: deerbrush 

(C. integerrimus Hook. & Arn.) and snowbrush (C. velutinus Hook. var. velutinus). 

For deerbrush, it was lowest in plots where the entire area was manually released 

and highest in plots that were manually grubbed to a 4-ft radius. Lowest to highest 

values for the most effective treatment were 13.5 to 32.5 atmospheres, and for the 

least effective treatment from 32.5 to 42.7 atmospheres (Henry Bell; McDonald and 

Fiddler 1986a). For snowbrush, internal moisture stress was lowest in plots where 

the entire area was grubbed and highest in those grubbed to a 4-ft radius (Logan; 

McDonald and Fiddler 1995). Lowest to highest values ranged from 6.3 to 16.3 

atmospheres in plots where the entire area was grubbed to 8.0 to 18.7 atmospheres 

in plots grubbed to a 4-ft radius.

We also quantified internal moisture stress in two species of manzanita: 

greenleaf manzanita and hairy manzanita (A. columbiana Piper). For greenleaf 

manzanita, internal moisture stress was lowest in plots where the entire area was 

manually grubbed and highest in plots grubbed to a 4-ft radius. Stress values 

ranged from 6.4 to 17.0 atmospheres in entire-area-treated plots and from 10.2 to 

23.8 atmospheres in radius-grubbed plots (Logan; McDonald and Fiddler 1995). 

For hairy manzanita, internal moisture stress was lowest in plots treated with 2,4-D 
22

GENERAL TECHNICAL REPORT PSW-GTR-231

where values ranged from 3.5 to 9.2 atmospheres, and highest in plots treated by 

a 5-ft radius one time (Signal; McDonald and Fiddler 1999b). Here values ranged 

from 7.5 to 22 atmospheres. 

Competition

Competition is a complex ecological process with many biological, environmental, 

and proximity factors that interact when plants grow together over time. Competition for critical environmental resources such as light, water, and nutrients, which 

are necessary for photosynthesis and respiration, is primarily responsible for the 

negative interaction between conifer seedlings and other vegetation. Over 200 

studies in the United States have demonstrated that small increases in the density or 

biomass of competing vegetation significantly lower conifer seedling survival and 

growth (Stewart and others 1984).

Given the long hot summers typical of most of California, it stands to reason 

that competition to conifer seedlings involves adequate soil moisture, and this 

means that the seat of early competition is belowground at the fine-root level. 

A likely scenario follows: In the absence of competition, the roots of young 

conifer seedlings extend vertically at the maximum rate possible. They increase 

in width and length, number of root tips, and in moisture absorption capacity. 

By increasing the volume of soil exploited, they increase the amount of water 

and nutrients available for rapid growth. The resources stored in or acquired 

by the root system lead to production of more aboveground biomass and more 

carbohydrates. This in turn fuels additional growth both above and belowground 

in a manner that accelerates each year. But competing shrubs and grasses, if 

present, characteristically begin root expansion and soil exploitation earlier in the 

growing season and in greater amounts than the conifer seedlings and capture 

the bulk of available site resources. The contrast between new pines and grass 

is striking. Based on measurements of several thousand seedlings from federal, 

state, and private nurseries, a typical 1-year-old ponderosa pine seedling had a total 

root length of about 78 in (McDonald and Fiddler 1989). A single wild oat plant 

excavated 80 days after emergence had developed a root system that totaled over 

50 mi in length (Radosevich and Holt 1984). The lack of initial resources stresses 

the conifer seedling by causing decreased root expansion, less resource collection, 

poor growth, and, in many instances, death. Even if the seedling survives, losses 

in growth are seldom made up. 

Competition is a 

complex ecological 

process with 

many biological, 

environmental, and 

proximity factors that 

interact when plants 

grow together over 

time. Competition for 

critical environmental 

resources such as 

light, water, and 

nutrients, which 

are necessary for 

photosynthesis and 

respiration, is primarily 

responsible for the 

negative interaction 

between conifer 

seedlings and other 

vegetation.
23

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Plainly, it is not just competition but early competition that has the most impact 

on the survival and growth of planted conifer seedlings. In a study with California 

white fir seedlings near Mount Shasta, California (Stephens Pass A), treatments 

were applied at different times and lasted for different durations in an effort to demonstrate the effect of early release (McDonald and Fiddler 2001A). When seedlings 

were released each year 4 to 6 years after planting, they had statistically smaller 

average diameters than if released each year for only the first 3 years. Furthermore, 

the mean diameter of seedlings with the delayed release did not differ statistically 

(p < 0.05) from seedlings in the untreated control. 

In southwest Oregon, Douglas-fir seedlings were planted in treated and 

untreated plots (control) where competing vegetation was primarily rapidly growing 

sprouts of canyon live oak (Quercus chrysolepis Liebm.) and greenleaf manzanita. 

After just one growing season, the negative effect of the resprouting shrubs could 

be seen. After three seasons, the growth of Douglas-fir seedlings planted in treated 

(slashed) 3-year-old sprouts was just as poor as those planted in the untreated 

control (Tesch and Hobbs 1989), indicating the power of the shrubs to recover and 

capture site resources. After five growing seasons, excavation showed that seedlings in the control and slashed areas had produced virtually no new roots and had 

retained the same shape of root system as when planted. But root biomass of the 

essentially free-to-grow seedlings was 9 times that of seedlings planted after slashing and 22 times that of seedlings in the control (Tesch 1991).

In a study in northern California, biomass accumulation of 1-year-old greenleaf 

manzanita plants on good site was almost 60 times that of ponderosa pine seedlings 

(Radosevich 1984). After the third growing season, reductions of 80 to 90 percent 

in pine growth were noted from shrub proportions of 50 percent or more. Reducing 

manzanita density by 75 percent did not free pine seedlings for adequate growth 

because rapid regrowth by the remaining 25 percent soon equaled the competitive 

effect of that removed.

On the lower slopes of Mount Shasta in northern California, a long-term study 

on a poor site exemplified the effect of competition. Here, ponderosa pine seedling 

growth was quantified in replicated plots containing various densities of aggressive 

woody shrubs that ranged from none to heavy (McDonald and Abbott 1997). From 

1966 through 1992, the regressed value of pines without competition was 25.0 ft 

tall; with light competition, 17.5 ft tall; with medium competition, 12.5 ft tall; and 

with heavy competition, 7.0 ft tall. Best-fit regression of these data is shown (fig. 9). 

Furthermore, the slope of the curves indicated that the differences in growth were 

accelerating. Similar findings were found for average foliar cover (fig. 10) and 

crown volume as well.

It is not just 

competition but 

early competition 

that has the most 

impact on the 

survival and growth 

of planted conifer 

seedlings. 
24

GENERAL TECHNICAL REPORT PSW-GTR-231

Figure 9—Relationship of ponderosa pine height to shrub density category, Mount Shasta 

brushfields, 1966–1992.

Figure 10—Relationship of ponderosa pine foliar cover to shrub density category, Mount 

Shasta brushfields, 1966–1992.

35

30

25

20

15

10

5

0

Height (ft)

Year

1965 1970 1975 1980 1985 1990 1995

No brush

Light brush

Medium brush

Heavy brush

35

30

25

20

15

10

5

0

Cover (thousand square feet/acre)

Year

1965 1970 1975 1980 1985 1990 1995

No brush

Light brush

Medium brush

Heavy brush
25

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Adaptation, Regeneration Strategies, and 

Successional Pathways

The one “truth” that approaches absolute in vegetation management is that Mother 

Nature abhors bare ground. Moreover, almost all plant species in summer-dry 

California have a strategy for taking advantage of disturbance with rapid early 

growth. Success in completing the life cycle accrues to those plants that can get to, 

and stay in, a zone of adequate soil moisture. Even after good site preparation, the 

number of pioneering species and the number of plants are impressive. Densities of 

10,000 plants per acre, 1 year after site preparation are conservative. A few years 

later, a density of more than 100,000 plants per acre is common if grasses and forbs 

constitute the majority of invaders. Adaptation, regeneration strategies, and successional pathways are processes that plant species employ to practically guarantee 

that bare ground does not stay bare for long.

Adaptation—

Through natural selection over millions of years, competing plant species tend to 

be well adapted to the harsh environment of the new plantation, particularly in the 

summer, which often is characterized by low precipitation, high temperatures, low 

relative humidity, and searing winds. Many shrubs, for example, have a host of 

morphological and physiological adaptations that allow them to prosper in a broad 

range of microsites, some of which are environmentally harsh for establishing 

conifer species. And the harsher the site, the better adapted are the shrubs relative 

to the conifers. Indeed, “shrubs exemplify more than any other kinds of plants, the 

great plasticity that has been largely responsible for the outstanding evolutionary 

success achieved by flowering plants” (Stebbins 1972).

Sunken stomates, thick cell walls, large root systems, crown architecture and 

smooth bark to facilitate stem flow, and capability to fix carbon at relatively high 

levels of moisture stress are but a few of the adaptations characteristic of many 

shrub species. Capability to limit the loss of moisture and ability to use that which 

is available efficiently (McDonald 1982) is an extremely valuable adaptation. Ability to limit respiration when internal moisture levels are low and temperatures are 

high is an outstanding adaptation that permits use of nearly all available moisture 

without penalty of wilting or depletion of energy reserves (Mooney and Dunn 

1970). Another physiological adaptation of many shrubs and some trees is the 

ability to consume most of the available soil moisture early in the spring to attain 

maximum photosynthesis at the expense of reduced photosynthesis and growth 

later in the season. This hoarding of moisture also limits the survival and growth 

of competing species. 

Through natural 

selection over millions 

of years, competing 

plant species tend to 

be well adapted to the 

harsh environment of 

the new plantation, 

particularly in the 

summer, which often 

is characterized by 

low precipitation, high 

temperatures, low 

relative humidity, 

and searing winds.
26

GENERAL TECHNICAL REPORT PSW-GTR-231

Two widespread shrub species that have many of the above adaptations are 

deerbrush and greenleaf manzanita, but which of them best uses these adaptations 

to gain an early growth advantage? Such information is worthwhile in its own right 

and also is useful for comparison to the growth rates of conifer seedlings. Growth 

of deerbrush and greenleaf manzanita was compared in a replicated side-by-side 

study in northern California (McDonald and others 1998). Seed was gathered from 

a local source, grown in a nursery for several months, and outplanted in spring 

1992 on a cleared site of high quality. The shrub seedlings were 2 to 4 in tall when 

outplanted. Number of stems per plant, crown diameter, height of tallest stem, and 

phenology (leaf development, onset and duration of flowering, seed production and 

dissemination, leaf fall) were recorded at various times throughout the year from 

1992 through 1995. 

The development of deerbrush and greenleaf manzanita was similar in some 

respects and different in others. Both species sustained moderate first-year 

mortality (17 versus 11 percent, respectively), no subsequent mortality, multiple 

stems on most plants (5.6 versus 3.6 stems per plant), maintained the initial 

proportion of plants with multiple stems (89 versus 76 percent), and grew rapidly. 

For differences between species, deerbrush developed a clump of stems the first 

growing season and greenleaf manzanita the second. By the end of the second 

growing season, deerbrush had produced statistically more stems per plant, was 

taller, and had wider crowns than greenleaf manzanita (table 4). Deerbrush also 

began to produce flowers and seed during the third growing season, but manzanita 

did not do so during the 4-year study. 

Table 4—Comparison of average height and crown width between deerbrush and 

greenleaf manzanita plants, Mount Shasta Ranger District, 1992–1995

Species Year Height Probabilitya Crown width Probability

Inches Inches

Deerbrush 1992 5.4 5.6

Manzanita 4.9 0.3700 4.3 0.0403

Deerbrush 1993 18.8 29.4

Manzanita 12.2 .0001 10.9 .0001

Deerbrush 1994 32.0 42.9

Manzanita 18.5 .0001 18.8 .0001

Deerbrush 1995 40.4 51.5

Manzanita 24.9 .0001 27.0 .0001

a

 Based on a two-way analysis of variance.
27

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

These differences suggest that deerbrush is the more competitive species, at 

least in early years. More stems per clump means a wider clump, and this, plus 

greater height, allows occupation of a larger area sooner than rival greenleaf 

manzanita. And being able to produce seed soon after disturbance enables at least 

some new plants to occupy favorable microsites that quickly become unavailable. 

Grasses are equally well adapted (Logan 1982), especially belowground where huge 

masses of fine roots efficiently capture soil moisture (Trappe 1981). Forbs also can 

furnish strong competition to young conifer seedlings because they often consist of 

many species that together occupy all available niches above and belowground for 

an extended period (McDonald and others 1994a).

Regeneration strategies—

Conifer plantations characteristically occupy areas that have been severely disturbed by timber harvest and site preparation. Here, slash has been windrowed and 

perhaps burned, and most remaining vegetation has been uprooted and removed. 

Soil surfaces in these areas often are almost bare and temporarily devoid of most 

plants and animals. While harsh aboveground, the environment belowground is 

benign. In the spring following disturbance, available resources are high because 

large quantities of organic material have been incorporated into the soil and large 

quantities of moisture are absorbed by it. Warm temperatures and plentiful moisture 

cause a rapid buildup of micro-organisms that decompose organic material and 

liberate nutrients. In this rapidly changing and somewhat unstable environment, 

plant species compete effectively by using different strategies of reproduction and 

development.

Regeneration strategies fall into two main groups: those that involve the 

reproductive potential of plants, and those that affect plant growth. Strategies that 

affect reproductive potential include windblown seeds, seed banks, and banks of 

persistent seedlings (Grime 1979). Strategies that affect growth potential include 

vegetative expansion aboveground and vegetative elongation belowground.

The relatively bare ground of new conifer plantations serves as a near-perfect 

medium for colonization by wind-dispersed seeds from species having small seeds 

with large wings, plumes, or pappus. Both woody and herbaceous species regenerate with this strategy. They often produce huge amounts of seed that are carried 

long distances by wind and effectively saturate an area. Herbaceous species of the 

family Compositae are common windblown invaders, as are plant species from 

the genera Cirsium, Madia, Epilobium, Stephanomeria, Hypochoeris, and Senecio. 

Small seeds often mean meager reserves of stored food, short viability timespans, 

and low competitive capability of the seedlings. Species with wind-dispersed seeds 
28

GENERAL TECHNICAL REPORT PSW-GTR-231

often circumvent these shortcomings with biennial cycles. Their seeds blow in on 

the wind in fall, germinate after the fall rains, punch a taproot into the soil for a few 

inches, and overwinter as short, inconspicuous plants. They then resume growth 

in the spring when soil temperatures are still cold enough to inhibit root growth of 

conifer seedlings. Fueled by the underutilized resources in the plantation area, and 

with their photosynthetic apparatus already in place, these plants bolt, grow vigorously, and complete their life cycles. A good example is bull thistle, where in a new 

plantation on a good site in the northern Sierra Nevada of California, peak density 

was 34,000 seedlings per acre (McDonald and Tappeiner 1986). In another study 

on the Tahoe National Forest in northern California (Snag), the thistle population 

neared 60,000 plants per acre and the thistledown (winged seeds) was so great that 

it covered the plantation in places to the point that lower plants could not be seen. 

In both studies, pine survival was lowered to the point that the studies were jeopardized. Populations of windblown species usually peak after 2 to 4 years, but while 

present, constitute a formidable drain on site resources. 

Plant species with the seed-bank reproductive strategy generally produce seeds 

that are incorporated in the litter and soil in a dormant condition for at least 1 year. 

These seeds almost always are small and have dormant embryos and hard seed 

coats that prevent imbibition of water. Length of time in the soil may range from a 

few years to centuries. Families of plants in young plantations include Compositae, 

Cruciferae, Ericaceae, Gramineae, Polygonaceae, Ranunculaceae, Rosaceae, and 

Scrophulariaceae. The number of seeds in a persistent seed bank may differ greatly 

for a given species, but, in general, is largest after a bumper seed crop. A general 

trend is for seed density and viability to decrease with soil depth. Because nature 

does not like to risk the entire seed crop in the soil, polymorphism in germination requirements is an attribute of the seed of most species with this reproductive 

strategy. Thus only some seeds germinate at a given time and a reservoir of seeds 

remains intact. 

A study at the base of Mount Shasta on the Shasta-Trinity National Forest in a 

converted brushfield (McBride) was notable for the endless supply of new greenleaf 

manzanita seedlings from the seed bank in the soil. Thousands of plants per acre 

were present initially, with manual grubbing facilitating a new crop of seedlings 

each year for 3 successive years (McDonald and Fiddler 2001a). 

Plants of shade-tolerant species, which often are lower in stature than vegetative 

associates, are found in the understory in the seedling-bank reproductive strategy 

and accumulate for many years. Shoot growth is slow or even negative because of 

dieback, but root systems expand slowly and may develop rudimentary burls that 

serve as food-storage organs (McDonald and Tappeiner 1986). Surviving for long 
29

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

periods under unfavorable circumstances ensures that the potential for regeneration 

is maintained until gaps in the overstory are created, or until the entire overstory is 

removed by senescence, insects, disease, windfall, fires, or harvest. After release, 

the seedlings take advantage of the additional site resources, especially light, and 

grow at a faster rate. 

Tanoak provides a good example of the seedling-bank regenerative strategy. 

This species regenerates from seed, root-crown sprouts, and seedling sprouts. Seedlings from acorns almost always die back to the root crown, then sprout and become 

seedling sprouts (stems arising from stumps less than 1 inch in diameter at ground 

line). Capability to sprout had been observed to occur within 7 days of emergence 

on hundreds of tanoak seedlings in a large plantation (McDonald 1978). Detailed 

examination of growth rings above and below the burl, which form at about age 3, 

revealed that tanoak seedling sprouts may be up to 62 years old and less than 85 in 

tall. Sprouts have died back and sprouted several times by this age, and stem age 

just below the burl is much greater than sprout age above the burl (Tappeiner and 

McDonald 1984). The number of sprouts per stump ranges from 2 to 14, and the 

number of seedling sprouts may exceed 10,000 per acre. After release by cutting 

or burning, the seedling sprouts die back, sprout, and grow vigorously, fueled by 

intake from the root system and augmented by food reserves stored in the burl. 

Seedling sprouts of several hardwoods and shade-tolerant shrubs of the genera 

Prunus and Chrysolepis were present in several of our studies. Their characteristically slow development left them vulnerable to rapidly growing root-crown 

sprouts of their own and other species, and most were relegated once again to the 

understory.

The above regeneration strategies concern seed and equally importantly, its 

dissemination. Wind and the feet, feathers, fur, and digestive tracts of animals 

are important for spreading the species to new, sometimes more advantageous, 

locations. The huge seed crops typical of many shrubs, forbs, graminoids, and 

hardwoods in young plantations not only sustain the species, but also the animal 

disseminators. Grinnell (1936) said it well: “It is not extravagance, but good investment for the oaks to provide subsistence for a continuing population of animal 

associates.”

Another regeneration strategy of plants in recently disturbed areas is rapid 

expansion aboveground. It is accomplished almost entirely by root-crown sprouts 

of hardwood trees and woody shrubs that originate from dormant buds on burls 

located at or just belowground line. Many species of many genera have this 

strategy including Arbutus, Arctostaphylos, Ceanothus, Lithocarpus, Quercus, 

and Rhamnus. These species occupy an environment where wildfire is common 
30

GENERAL TECHNICAL REPORT PSW-GTR-231

and where the conifer trees are killed outright, but the hardwood trees and woody 

shrubs are killed only to ground line. 

Root-crown sprouts are often thought of as being “instant reproduction” 

because of their abundance and rapid growth. Number of sprouts per stump can 

differ from several score to 1,400 on one large tanoak stump (Tappeiner and others 

1990). Enriched by the food reserves and extensive root system of parent plants, 

root-crown sprouts grow as much as 5 ft in height in one growing season. After 

three growing seasons in northwestern California, sprouts of bigleaf maple (Acer 

macrophyllum Pursh) were 12.8 ft tall, those of Pacific madrone (Arbutus menziesii 

Pursh) were 10.1 ft tall, and sprouts of tanoak were 6.8 ft tall (Roy 1955). Rapid 

expansion continues. On a good site in north-central California, average 10-year 

clump height and width in a clearcutting were Pacific madrone 20 ft tall, 10-ft 

wide; tanoak 19 ft tall, 10 ft wide; and California black oak 20 ft tall, and 10 ft 

wide (McDonald and Tappeiner 2002). 

Woody shrubs also have capability for sustained rapid growth. In our study 

near the Pacific Ocean on a good site (Signal), snowbrush (Ceanothus velutinus

Hook. var. hookeri M. Johnston) and blue blossom (Ceanothus thyrsiflorus 

Eschsch.) were over 30 ft tall after 10 years of growth (McDonald and Fiddler 

1999b). A much less vigorous component of the rapid expansion above-ground 

strategy is from trailing stems that root freely. Such plant species as mahala mat 

(Ceanothus prostratus Benth.) and creeping snowberry (Symphoricarpos mollis 

Nutt.) were found in our study on the Plumas National Forest in the northern Sierra 

Nevada (Deans) (McDonald and Fiddler 2007). 

An outstanding reproductive strategy, particularly on poor sites, is vegetative 

expansion belowground by rhizomes and root sprouts. The below-ground location 

and attachment to a food source practically guarantees a low risk of mortality and 

successful establishment of the offspring. Areas with already established vegetation, deep litter layers, or where establishment from seed is difficult because of 

chronic destruction or consumption are other conditions where this strategy serves 

its species well. Representative species in many of our study areas were from the 

genera Apocynum, Aster, Convolvulus, Gaultheria, Nama, Pteridium, Rubus, and 

Vaccinium. Species from these genera often form dense patches and suppress the 

regeneration and growth of other species. 

Species that propagate from root sprouts were not common in our study areas 

with one exception—Sierra plum (Prunus subcordata Benth.). This shrub also 

produces intermittent crops of large seeds in nutritious fruits, and may occasionally 

reproduce from rootcrowns. In the long-term study on the lower slopes of Mount 

Shasta, not a single new seedling of Sierra plum was found (McDonald and Abbott 

An outstanding 

reproductive 

strategy, particularly 

on poor sites, is 

vegetative expansion 

belowground by 

rhizomes and root 

sprouts. The belowground location and 

attachment to a food 

source practically 

guarantees a low 

risk of mortality 

and successful 

establishment of 

the offspring. 
31

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

1997). Apparently all seed was consumed by herbivores. Over the 1966–1992 study, 

sprout density, foliar cover, and volume generally declined and height increased 

marginally. Height growth was beset by periods of dieback and resprouting and in 

general was less than other shrub species.

Another species with multiple regenerative strategies is bearclover (Chamaebatia foliolosa Benth.). It sprouts from the rootcrown, produces rhizomes, forms 

scores of flowers, and, at least in one instance (McDonald and Fiddler 1999c), sends 

up root sprouts from deep in the soil. In a new plantation in north-central California, bearclover produced more than 224,000 stems and 24,800 ft2

 of foliar cover 

per acre when less than 1 ft tall. Unlike Sierra plum, which often resides in the 

understory of taller trees and shrubs, bearclover dominates over thousands of acres 

of open and semiopen forest land in northern California, and has withstood a wide 

variety of treatments aimed at its control (McDonald and others 2004). 

Successional pathways—

Of the two classical types of plant succession, it is secondary succession that 

is operative in young conifer plantations because it involves revegetation after 

disturbance. Secondary succession is a complex phenomenon that involves the 

regenerative strategies and the dispersal efficiencies of plants and their capability 

to progressively modify the soil and microenvironment. Odom (1969) suggested 

that succession is an orderly process of community development that is reasonably 

directional and therefore predictable. Knowledge of succession is important because 

vegetation management often sets an initial point (bare ground) through site 

preparation, and imposes a narrow range of environmental conditions that lead 

to a particular (often desired) plant community. 

Connell and Slatyer (1977) described three mechanisms that govern succession 

in the plant community. Species with a “facilitation” mechanism require earlier species to modify the environment for their entry and growth. Facilitators can colonize 

only after suitable changes in the microenvironment. Species having the “tolerance” 

mechanism can become established in locales having inhibiting chemicals or lower 

levels of site resources. These species generate a predictable successional sequence 

in which later colonizers invade and grow to maturity in the presence of those that 

preceded them. The “inhibition” mechanism, finally, operates where all species, 

even the earliest, resist invasion by competitors. The first occupants preempt space 

and exclude or inhibit later colonists. Later colonizers become established and reach 

maturity only after the first become damaged or die. Morris and Wood (1989) found 

that pioneer species can both facilitate and inhibit subsequent invaders.

Knowledge of 

succession is important 

because vegetation 

management often 

sets an initial point 

(bare ground) through 

site preparation, and 

imposes a narrow 

range of environmental 

conditions that 

lead to a particular 

(often desired) plant 

community. 
32

GENERAL TECHNICAL REPORT PSW-GTR-231

Plant succession in the vast majority of the study areas in our National Study 

on Vegetation Management in California followed a general pattern. The first species to appear were those that escaped death from site preparation and arose from 

rhizomes or root crown sprouts. The next colonizers came from buried seeds in the 

soil followed by seed from a few species that blew in on the wind or were carried 

into the area by animals. Within a year or two, almost all species were producing at 

least some seed, and together with new arrivals from the soil and wind, constituted 

a dynamic and rapidly expanding population dedicated to capturing site resources 

and completing life cycles.

Biological Control and Exclusion

If a desired plant species naturally replaces an undesired plant species, the desired 

species in effect becomes a treatment and the cost of manipulation and control are 

either eliminated or much reduced. This process often is termed biological control 

and is defined as the use of living organisms to lower plant pest populations to the 

point where the pests are no longer management or economic problems. Besides 

animals and micro-organisms, biological control agents can be other plants that are 

either parasitic, competitive, or allelopathic (Rosenthal and others 1985). Not only 

does biological control have the advantage of cost, it also has a social attribute. In 

sensitive areas, this means of weed control may be the only method applicable.

Short, shallow-rooted forbs, that consume only a small amount of soil moisture 

are ideal candidates for biological control if they limit more aggressive shrubs. In a 

study on the Lassen National Forest on a good site (Logan) (McDonald and Fiddler 

1995) and another on the Tahoe National Forest on a poor site (McDonald 2003), 

wooly nama (Nama lobbii A. Gray), which is a low, shallow-rooted forb, quickly 

became established on plots where most of the competing vegetation had been 

eliminated by site preparation. This species prefers bright sunlight and declines in 

shade. It becomes present soon after site preparation, spreads rapidly, and forms 

dense masses of vegetation. This forb suppresses germination of dormant shrub 

seeds in the soil for at least 5 years or until shaded out by rapidly growing pines. 

Smooth cat’s ear (Hypochaeris glabra L.), hairy cat’s ear (H. radicata L.), and 

prickly lettuce (Lactuca serriola L.) are other shallow-rooted forbs that invade 

recently disturbed areas in large numbers and preempt site resources (McDonald 

and Fiddler 1993a).

Another example of plant species exclusion took place in a converted brushfield 

on a medium-poor site in the southern Cascade Mountains northeast of McCloud, 

California. One of the three dominant shrub species was bitter cherry (Prunus 

emarginata [Hook.] Walp.) that was 4 to 5 ft tall before the site was prepared by 

Biological control is 

defined as the use 

of living organisms 

to lower plant pest 

populations to the 

point where the 

pests are no longer 

management or 

economic problems.
33

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

windrowing and burning (McDonald and Fiddler 1997b). Following site preparation, the density of bitter cherry decreased steadily from 487 plants per acre after 1 

year to zero plants per acre after 5 years. Number of species increased from 9 to 17 

during the 10-year study, and three other shrubs had large increases in density and 

foliar cover. Competition-induced drought is the likely cause for the exclusion of 

bitter cherry. 

The long-term study on the lower slopes of Mount Shasta (McDonald and 

Abbott 1997) provides a good example of a shrub-pine seedling-graminoid competitive interaction and biological control. Five years after planting of the pines, 

needlegrass (Achnatherum nelsonii [Scribner] Barkworth ssp. dorei [Barkworth & 

J. Maze ] Barkworth) began to invade the area. Needlegrass density was inversely 

related to shrub density: the more the shrubs, the less the grass. Of importance was 

that on the plots where the shrubs were eliminated, grass density was high, and 

no new shrubs were found in spite of the virtual certainty that more seeds were 

buried in the soil and more were carried in from nearby areas by birds and animals. 

Interference by the needlegrass prevented germination of shrub seeds and the grass excluded the shrubs. 

Because the grass became established after the pines, 

and the grass roots were not as deep as those of the 

pines, it constituted only minor competition to them. 

Eventually, the pines almost eliminated the grass 

(table 5).

Another aspect of replacement vegetation for 

biological control of undesirable vegetation involves 

the introduction of beneficial plant species that are 

not native to the area. These species often have the 

attribute of being preferred by domestic grazing 

animals like cattle and sheep. A concern is that they replace native plant species, 

especially smaller forbs and grasses. In one of our studies on the Tahoe National 

Forest (Cattle II), we hand seeded Potomac orchardgrass (Dactylis glomerata L.) 

and pubescent wheatgrass (Agropyron trichophorum [Link] Richter) and quantified 

their density, foliar cover and height along with that of manzanita (Arctostaphylos

spp.), other shrubs, and grasses in a ponderosa pine plantation grazed by cattle 

(McDonald and Fiddler 1999a, 2002).

In general, the introduced grasses, along with the native needlegrass, became 

established in a wide range of niches including sun and shade and grazed and 

ungrazed, and grew well irrespective of the kind and amount of other vegetation. 

They also benefited from grazing and had higher density in grazed plots than 

Table 5—Relation of needlegrass 

density to shrub-density class 18 and 

31 years after planting in a ponderosa 

pine plantation near Mount Shasta, 

California

Shrub Age (years)

density class 18 31

Number of plants/acre 

None 50,000 540

Light 17,600 0

Medium 8,200 0

Heavy 533 0
34

GENERAL TECHNICAL REPORT PSW-GTR-231

in ungrazed plots. As a whole, the effect of the introduced grasses was minor, 

although slightly beneficial because of the weight gain of the cattle. They did not 

appear to replace native plant species nor were they excluded by them. Indeed, a 

total of 34 plant species was found in the study area over an 11-year period. Other 

investigators (Eissenstat and Mitchell 1983, Krueger 1983) have found similar 

results in grazing studies in the Western United States. 

Vegetation managers occasionally try to biologically control a vigorous native 

species by introducing a nonnative species. On the Stanislaus National Forest, a 

study (Grapevine) was designed to specifically test the effect of nonnative annual 

ryegrass (Lolium multiflorum L.) on the regrowth of native bearclover. The study 

area burned in a wildfire in late summer 1987 and was seeded to annual ryegrass 

that fall. A vigorous stand of grass became established in 1988, and a myriad of 

seed was produced. Each of these seeds produced a plant in 1989, and sampling 

revealed a density of over 10 million per acre by that fall. Thus bearclover had to 

sprout and regrow in an environment characterized by a very high level of established competition.

Results from the study plots in fall 1992 showed a dramatic decrease in 

number of stems of both bearclover and ryegrass (table 6). Although both species 

maintained their initial height values, the density of grass actually fell below that 

of bearclover in 1992, indicating that the grass population was being stressed by 

the bearclover. Subsequent visits to the study site indicated the total domination 

of bearclover and its virtual immunity to competition from this vigorous grass. 

Unfortunately the study area was burned by a forest fire in 1996.

Not only can individual species of replacement vegetation influence future 

plant community composition and dynamics, but the sheer amount of vegetation 

from other categories may constitute replacement as well. In one of our studies 

(Harrison Gulch) on the Shasta-Trinity National Forest, for example, the number 

of forb, fern, and grass species and their density increased very rapidly in all 

Table 6—Average stem density and height of bearclover and annual ryegrass 

(Lolium multiflorum L.), Stanislaus National Forest, 1989 and 1992

Bearclover Ryegrass

Density Height Density Height

Year Mean SEa Mean SE Mean SE Mean SE

 Number of stems/acre - - - Feet - - - Number of stems/acre - - Feet - -

1989 339,900 12,200 0.8 0.1 10,489,000 805,000 1.6 0.1

1992 9,867 1,555 .9 .1 9,133 1,711 1.6 .1

a SE = standard error.
35

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

treatments and in the untreated control (McDonald and Fiddler 1997). After 10 

years, species numbered 46 and average density 217,800 plants per acre in the 

control and 177,105 per acre in the most effective treatment (5-ft radius, grubbed 

three times). It is likely that competition from the forbs, ferns, and grasses 

interfered with the establishment of highly competitive shrubs from seeds in 

the soil.

Insect-Vegetation-Animal Interactions

Vegetation managers need to be watchful for complex vegetation-insect and insect-vegetation-animal 

interactions that may harm their plantations. One 

interaction took place on the lower slopes of Mount 

Shasta where it relates to shrub density, pine seedling height, and insect damage. The setting was the 

long-term study mentioned earlier involving plots 

manipulated to create the range of no shrubs to 

heavy shrubs (McDonald and Abbott 1997). Insect 

damage to the pines, principally by the gouty pitch 

midge (Cecidoymia piniinopis Osten Sacken) (Bedard 

and others 1989), and specifically death of terminal 

shoots, did not become serious until the pines were 

about 8 years old and continued for another 10 years. 

More than five times as many terminal shoots were 

killed on pines in heavy shrubs as in light shrubs 

during this timespan. The general trend was an 

increasing number of dead terminals with increasing 

shrub density (fig. 11). Apparently the shrub environment is, or perhaps the weaker trees in the shrub 

environment are, favorable to the insects. After the 

trees reached a height of 12 ft, damage to terminals 

lessened appreciably. Damage to lateral shoots by the 

pine needle-sheath miner (Zelleria haimbachi Busch) 

(Stevens 1959) followed the same general trend with 

less damage above 12 ft. By slowing pine height 

growth, the shrubs held the trees below this height. 

The insects then attacked the trees, deformed them, and together with the shrubs, 

continued to hold the trees to a size where they are attacked repeatedly. The insect 

and the shrubs together did more damage to the pines than either alone. 

Figure 11—Insects have killed the terminal shoot of this pine 

seedling many times.
36

GENERAL TECHNICAL REPORT PSW-GTR-231

Another interaction, again on the west-facing slope of Mount Shasta, took place 

in a large brushy compartment that was treated with herbicide to promote broadcast 

burning. Several years after planting ponderosa pine seedlings, copious amounts of 

shrubs and grasses that emerged were again treated with an herbicide. The shrubs 

were affected greatly, the grasses were not. Pocket gophers were attracted to the 

grassy environment, and a wingless grasshopper (Bradynotes obesa opima Scudder) 

(Furniss and Carolin 1977) soon became epidemic. The shift from shrubs to grass 

and corresponding increase in pocket gophers and grasshoppers caused the loss of 

most of the pine seedlings (McDonald and Tappeiner 1986). 

Past Stand History

An enduring theme in vegetation management is the study of human alteration of 

the forest. This alteration often has the goal of modifying the forest to purposeful 

ends, but sometimes it is not achieved because of events that happened in the past. 

Past stand events are a major factor governing which species occur at what locations in the landscape. If greenleaf manzanita, for example, was present before it 

was replaced by conifer trees, its seed remains in the soil, and it will be present 

again in large numbers after disturbance. A good example of this occurred in 

northern California where the brushy site was prepared by windrow and burn. One 

year later, greenleaf manzanita seedlings numbered 123,500 per acre and after 10 

years, 44,450 per acre (McDonald and Fiddler 2001a). Reinvasion of this species 

was expected, but not to that extent.

A classic example of the consequences of not knowing the past occurred in a 

clearcut and broadcast-burned compartment on the Challenge Experimental Forest 

in north-central California. Before harvest and burning, the site consisted of a dense 

stand of conifers with a deep layer of organic material on the forest floor. Not even 

a skeleton of a shrub species was present. After burning, seeds of deerbrush germinated by the thousands, and much to the consternation of the forest manager, the 

area turned “green” almost overnight (McDonald 1983). Subsequent natural generation of desired conifers was highly variable and deficient in places.

As expected in lands having a mediterranean climate with hot, dry summers, 

wildfire and its frequency is important. A good example concerns pines and shrubs 

in the southern Cascades (Skinner and Taylor 2006). Older stands that have endured 

many fires have less shrub seed stored in the soil than young stands that have just 

emerged from a brushfield or stands that experienced fewer fires. A long interval 

between burns that promotes dense conifer stands can lead to the absence of such 

shrubs as bush chinquapin (Chrysolepis sempervirens [Kellogg] Hjelmq.) and 

Past stand events are a 

major factor governing 

which species occur at 

what locations in the 

landscape. 
37

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

huckleberry oak (Quercus vaccinifolia Kellogg) that do not develop seed banks 

in the soil. Another example is the more common phenomenon of California white 

fir replacing ponderosa pine with continued absence of burning. Here, more shadeenduring shrubs replace those that are prone to burning and, hence, develop better 

in full sunlight. 

On a broader scale, continued absence from burning or logging often leads to 

a dense forest and a more shady environment that promotes a shift from annual 

plant species to perennial species after disturbance as in western Oregon (Antos 

and Halpern 1997) and in the north-central Sierra Nevada (McDonald and Reynolds 

1999).

Operational Techniques

When this research program began, vegetation management specialists were doing 

a little work on manual release and various kinds of mulch materials, but most 

attention was being given to phenoxy herbicides, especially 2,4-D and 2,4,5-T. 

These herbicides proved to be biologically worthwhile and cost effective for 

achieving acceptable survival and growth rates of conifer seedlings. They were 

so successful that little need was present to have alternative treatments or to learn 

about their effect on plant communities or plant succession. Consequently, sideby-side vegetation management operational comparisons were rare and information on their effectiveness, cost, and ecological consequences was scarce. 

This paper compares most of the vegetation management treatments used 

for releasing young conifer seedlings in plantations in the Western United States 

during the study period (McDonald and Fiddler 1989, Walstad and Kuch 1987). 

These include direct treatments and genetic enhancement, as well as a new concept 

called “indirect treatments” (McDonald and Fiske 2000). Although this category 

of treatment is not found in contemporary textbooks as a vegetation management 

technique, we believe that it is a needed adjunct to the art of vegetation manipulation 

and worthy of inclusion here.

Direct Treatments

Directly manipulating competing vegetation and reducing its potential to kill 

or seriously inhibit the growth of young conifer seedlings included the use of 

herbicides, manual release, mulching, large machines, and grazing animals. 

We evaluated 21 variations of these treatments and 96 trials in this part of our 

administrative study (table 7). 
38

GENERAL TECHNICAL REPORT PSW-GTR-231

Results from the direct treatments are portrayed in a table showing each release 

technique relative to the control. They were chosen to represent a range of biological responses in each treatment. Data were extracted directly from published tables 

and figures. Of course, release techniques other than those displayed in the tables 

were present in each study. For example, the table on herbicides might be from a 

study that also had two manual release treatments and a control.

Table 7—Kind and number of plantation-release treatments and variations, 

northern and central California, 1980–2005

Treatment Variation Number of trials

Herbicides 3-ft radius 1

5-ft radius 10

Whole plot 15 

Cut/spray 4

Manual release 1-, 2-, 3-, 4-, 5-, 6-ft radius, 1 time 20

2-ft radius, 3 times 1

5-ft radius, 3 times 1

Mulching Whole plot, 1 time 4

Whole plot, 3 times 9

Whole plot, 5 times 1

Whole plot, 10 times 3

1-ft radius 1

1.5-ft radius 1

2-ft radius 1

3-ft radius 1

4-ft radius 6

5-ft radius 4

Mechanical Mechanical only 2

Mechanical + herbicide 2

Grazing Cattle 1

Sheep 2

Genetic enhancement Nursery run 1

Wind pollinated 1

Control pollinated 1

Indirect control Organic material 2

Control Shade 1

 To tal 96
39

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Herbicides—

As noted in the three specific studies involving herbicides, at least one chemical 

in each (fig. 12) gave conifer seedlings a statistically significant growth advantage 

over counterparts in the control after 10 years (table 8). Sometimes the significant 

variables were conifer diameter, height, and foliar cover (McDonald and Fiddler 

1996, 1999a), or even height, diameter, and crown diameter (McDonald and others 

1999a). A general advantage with herbicides is that a single application usually 

is effective, which tends to lower cost. Exceptions occur, however, with specific 

shrubs like bearclover (McDonald and others 2004), and situations where large 

numbers of plants from large seed banks in the soil repeatedly emerge. Here, a 

second application often is necessary. 

Figure 12—Garlon 3A herbicide is being applied to cut shrubs as a means of 

vegetation control.

We have noticed that soil-active Velpar persists in the surface soil for about 3 

years, which aids conifer seedling growth during the critical early years by almost 

eliminating competing species. After 3 years, the chemical has moved downward 

enough to allow shallow-rooted forbs and graminoids to become established. However, deeper rooted shrubs, perennial forbs, and graminoids still cannot establish. 

By the time the Velpar has moved even deeper in the soil, the conifer seedlings are 

well established and growing vigorously. 

A general advantage 

with herbicides is that 

a single application 

usually is effective, 

which tends to lower 

cost.
40

GENERAL TECHNICAL REPORT PSW-GTR-231

One minor variation related to herbicides was with Aluma-Gel (see footnote 

2)—a mixture of aluminum dust and jellied petroleum that generates high heat 

when burned. Although not an herbicide in the traditional sense, it is considered 

a chemical, as are herbicides, hence its inclusion here. It was used to treat sprouts 

from the root crowns of large tanoak stumps. These root crowns constitute regenerative platforms for scores, if not hundreds, of vigorous sprouts, which average over 

20 ft tall and 10 ft wide in 10 years (McDonald and Tappeiner 2002). They are so 

vigorous that they are hard to control, and thus worthy of innovative treatment. On 

the Plumas National Forest in northern California, shallow trenches that exposed 

tanoak root crowns were dug around more than 40 stumps, Aluma-Gel was poured 

in the trenches, and ignited (fig. 13 A, B). The heat, and possibly steam, were 

supposed to kill the sprout buds (Fiddler and McDonald 1987). Observation several 

months later showed a large reduction in sprout density, but very few stumps with 

no sprouts. Hence the treatment was judged unsatisfactory. The reason for lack of 

success was that the sprout buds were located in protected cracks and crannies on 

the root burl, and some were deeper than previously thought. These escaped the 

heat and fire and soon constituted vigorous competition for site resources.

The cost of the traditional herbicide treatment tends to differ with the size of 

area, method of application, number of applications, and type of chemical. In our 

studies, the cost ranged from $85 to $137 per acre, not including the chemical.

Table 8—Average diameter, foliar cover, and height of conifer seedlings 10 years 

after treatment in three study areas treated with herbicides, northern and central 

California

Treatment Species Diameter Foliar cover Height 

Inches Square feet/acre Feet

Cattle II:

Velpar Ponderosa pine 6.59 a 40,900 a 16.51 a 

Control 4.58 b 27,850 b 10.94 b 

Rocky:

	 2,4-D	 Douglas-fir	 3.55 ab	 26,400 ab	 18.18 ab

 2,4-D + Garlon 4 4.50 a 35,895 a 19.91 ab 

Garlon 4 4.05 a 37,750 a 17.94 ab 

 Cut + Garlon 3A 4.64 a 39,850 a 21.55 a 

Control 1.96 b 10,950 c 12.08 b

Latour State Forest:

Velpar Ponderosa pine 5.08 a 37,267 a 12.92 a

Garlon 4.12 ab 31,867 b 10.71 b

 E scort 3.62 ab 19,200 c 9.89 b

Control 2.80 b 10,133 c 8.23 b

Note: For each study area, treatment means in each column followed by the same letter do not differ statistically 

at the 0.05 level.
41

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Manual release—

For manual release (fig. 14), a common theme among the three representative study 

areas was that larger plots, treated most often, yielded a statistical advantage in 

conifer growth over those in the control (table 9) (McDonald and Fiddler 1995, 

1997, data on file4). Kind and size of vegetation are important and can affect cost. 

In two other studies, for example, manually grubbing a 5-ft radius one time around 

conifer seedlings in a primarily forb and grass community cost $210 per acre, and 

for chain sawing and shearing vegetation in plots of the same size in a primarily 

shrub community cost $315 per acre (McDonald and Fiddler 1997, 1999b).

Figure 13—(A) Flaming Aluma-Gel around a tanoak stump; (B) tanoak sprouts that escaped the heat.

A B

Figure 14—Manual release often is accomplished with hand tools. 

4

 Data from unreported study on file with Philip McDonald, Pacific Southwest Research 

Station, 3644 Avtech Parkway, Redding, CA 96002. 
42

GENERAL TECHNICAL REPORT PSW-GTR-231

Duration of growth also is important. In another study in the northern Sierra 

Nevada, the mixed shrub and herbaceous plant community was manually grubbed 

each year for the first 3 years after planting, for the first 6 years, and each year for 

years 4 through 6 (T-second-3). The treated area was a 5-ft radius around 30 to 35 

pine seedlings in each of three plots. The total cost for each treatment period was:

Treatment Time Cost per acre

Hours Dollars

T-first-3 27.0 197

T-first-6 65.5 477

T-second-3 82.0 598

It took 60 person-hours to grub the first year of the T-second-3 treatment 

(McDonald and Fiddler 2007).

The high cost of the T-second-3 treatment was caused not only by the high 

density of the vegetation but also its development belowground. Some root masses 

of shrubs had to be split before they could be removed, and others had coalesced 

for a radius of 2.5 ft or more and had to be grubbed in sections. Still others with 

deep, rhizomatous root systems, required much work and even then were almost 

Table 9—Average diameter and height of conifer seedlings 10 

years after treatment in three study areas treated by manual 

release (grubbing), northern and central California

Treatment Species Diameter Height 

Inches Feet 

Logan:

 4-ft radius, 1 time Ponderosa pine 3.67 b 8.77 b 

 4-ft radius, 2 times 4.04 b 9.86 b

 4-ft radius, 3 times 4.38 b 10.16 b 

 Whole plot, 2 times 6.33 a 14.00 a

Control 2.92 b 8.66 b

Harrison Gulch:

 2-ft radius, 1 time Ponderosa pine 3.04 b 8.45 b

 2-ft radius, 3 times 3.32 b 9.19 b

 5-ft radius, 1 time 4.18 b 11.52 a 

 5-ft radius, 3 times 4.68 a 14.27 a

Control 3.12 b 9.64 b

Snag:

 5-ft radius, 1 time Ponderosa pine 6.61 ab 19.87 ab

 5-ft radius, 2 times 6.62 ab 19.89 ab

 Whole plot, 1 time 7.18 a 21.80 a

Control 5.68 b 18.53 ab

Note: For each study area, treatment means in each column followed by the same letter do 

not differ statistically at the 0.05 level.
43

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Figure 15—A 10- by 10-ft polyester mulch mat on a hillside with all sides tucked in. 

impossible to remove completely. And, based on repeated sampling, several forbs 

seemed to have an inexhaustible seed supply, whereby grubbing stimulated more 

plants than were removed. Even the grasses (Achnatherum spp.) and brome (Bromus

spp.) required loosening around the edges before the main clump could be removed.

For the manual release treatment as a whole, costs were quite variable and 

ranged from $150 to $315 per acre.

Mulching—

Our mulching studies were concerned with widening the application of this method 

by testing mats that were larger and more durable than those normally used in 

forestry, and comparing them to the more commonly used smaller, temporary 

mulches. Results from the three representative study areas for this direct treatment 

showed that the mulch mats need to be long-lasting, of large size, and in contact 

with the soil (fig. 15). Soil contact is necessary to transmit water to the soil and 

prevent it from draining away from the seedling inside the mulch. The large squares 

of durable polyester and polypropylene created enough competition-free area 

around the conifer seedlings to promote statistically significant diameter growth 

and sometimes height growth over counterparts in the control (table 10). More 

specifically, the larger size of mulch allowed the conifer seedling root system to 

develop well before the roots of other vegetation could compete with it under the 

mats. The smaller and short-lived mats did not provide this window of growth 

(McDonald and Fiddler 1997).

Mulch mats need to be 

long-lasting, of large 

size, and in contact 

with the soil.
44

GENERAL TECHNICAL REPORT PSW-GTR-231

The total cost of mulching with mats has three components: material, installation, and maintenance. Because mats smaller than 10- by 10-ft square and those 

made of materials other than polyester and polypropylene proved to be ineffective 

in a wide variety of plant communities, we present cost information only for the 

larger, durable ones. Installation generally is routine, except on steep slopes. In 

one study on a slope of 30 percent and initially bare soil (Big Collars), the upper 

edge of the mats had to be placed in a shallow trench and backfilled to keep them 

from sliding downhill in the winter. This of course, increased the cost. Maintenance typically involves visiting each mat at least once each year and tying down 

windblown corners with pins or stakes. The total cost over a 5-year period, was 

$1,984 per acre (McDonald and others 1994b). 

At Beartrap Ranch, the plant community was composed of forbs, ferns, and 

graminoids on a slope of 50 percent. Because of strong incessant winds, each 

10- by 10-ft square mulch of light-weight polypropylene was placed over the 

existing vegetation and held down by thirteen 9-in metal pins. The combination 

of expensive material, steep slope, and strong wind increased the total cost, 

especially the installation and maintenance costs. Altogether, the total cost 

was $2,059 per acre (McDonald and others 1994a). 

In another study (Mulches), we wished to extend the mulching technique to 

control sprouting shrubs—a plant community rarely tested with mats. Here the 

mats were 10-ft squares of heavy polyester felt applied around planted Douglas-fir 

Table 10—Average diameter and height of conifer seedlings 5 years 

after treatment in three study areas treated by mulching (mats), 

northern and central California

Treatment Species Diameter Height 

Inches Feet 

Big Collars:

 4-ft square, sandwich Ponderosa pine 0.89 b 2.39 b 

 10-ft square, thin polyester 1.34 a 3.48 a

10-ft square, thick polyester 1.44 a 3.67 a 

Control 0.76 b 2.30 b 

Harrison Gulch:

 3-ft square, sandwich Ponderosa pine 1.08 b 3.20 b

Control 1.36 b 4.04 b

Beartrap Ranch:

	 10-ft square,	polypropylene	 Douglas-fir	 1.61	a	 6.16	b

Control 1.26 b 6.25 b

Note: for each study area, treatment means in each column followed by the same letter do not 

differ statistically at the 0.05 level.
45

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

seedlings and over burned stubs of shrub tanoak (Lithocarpus densiflorus [Hook. 

& Arn.] Rehder var. echinoides [R.Br. Campst.] Abrams). Hence the mulches never 

touched the ground. The slope was 40 percent, the aspect northeast, and the soil 

deep. In spite of the large treated area, growth of the Douglas-fir seedlings was 

almost nil and mortality was higher than expected. Further investigation showed 

a miniature desert beneath the mulches. Apparently, water traveled through the 

mulches internally, wicked off in the air at the lower end, and never reached the 

soil near the seedlings. Shrub tanoak is a vigorous sprouter after disturbance and 

it soon created mounds of sprouts under the mats. These tended to overheat, and 

after 4 years, almost all of the sprouts were dead. However, the mounds necessitated 

additional maintenance because they tended to pull up the hold-down pins on 

the mat edges. Over a 4-year period, the cost was more than $2,600 per acre 

(McDonald and others 1989). 

Mechanical release—

In the two mechanical release studies in our research program (Fiddler and 

McDonald 1997, Fiddler and others 2000), cutting 12- and 16-year-old shrubs 

with large machines (fig. 16) did not statistically enhance diameter and height 

growth of pine seedlings over counterparts in the control at the 0.05 level 

(table 11). Rapid resprouting of the vigorous shrubs was the reason. The cost 

was $225 and $218 per acre, respectively.

Figure 16—A Trac Mac at work in an older plantation. Note the layer of shredded 

vegetation in the foreground. 
46

GENERAL TECHNICAL REPORT PSW-GTR-231

Because the primary competition to the conifer seedlings in both studies was 

from well-developed sprouting shrubs, an additional treatment was called for. 

This was 2,4-D, applied when the resprouting shrubs were 1 and 2 years old. With 

combined mechanical and chemical treatment, crown cover of ponderosa and 

Jeffrey pine saplings at Third Water was significantly increased over counterparts 

in the control after 11 growing seasons. At Big Tunnel, average ponderosa pine 

diameter, crown cover, and height were statistically increased over pines in the 

control after 11 years. The herbicide added $49.00 at Third Water and $48.00 at Big 

Tunnel for total costs of $273 and $266 per acre. The role of the grasses relative to 

treatment is interesting. At Third Water, the number of grass plants was described 

as few and the size as small during the entire study. At Big Tunnel the density of the 

grass (mostly Achnatherum spp.) in the three treatments was mechanical, 0 plants 

per acre; mechanical and chemical, 1,246,667 per acre; and control, 44,000 plants 

per acre at the end of the study. Diary records noted “grass in these treatments is 

virtually the only grass in the entire plantation.” 

Grazing animals—

Much evidence shows that to be effective, release must take place when the conifer seedlings are young and the competing vegetation also is young and not yet 

capturing many site resources. Grazing cattle (fig. 17) in young plantations often 

is regarded as being too risky—the conifer seedlings are trampled, have their tops 

nipped, or are pulled from the ground. In this study, cattle grazed the plantation 

only 3 months after planting with no damage to the seedlings (McDonald and 

Fiddler 1999a). In spite of this good start, grazing by cattle never significantly 

enhanced the growth of the ponderosa pine seedlings over the 10-year study 

(table 12).

Table 11—Average diameter and height of conifer seedlings 10 

years after treatment in two study areas treated by mechanical 

release (machines), northern and central California

Treatment Species Diameter Height 

Inches Feet 

Third Water:

Trac Mac Ponderosa and Jeffrey pine 4.76 b 15.08 b

Control 4.40 b 14.95 b

Big Tunnel:

Hydro-Ax Ponderosa pine 7.56 b 18.99 b 

Control 6.49 b 16.68 b

Note: For each stud area, treatment means in each column followed by the same letter do 

not differ statistically at the 0.05 level.

Grazing by cattle 

never significantly 

enhanced the growth 

of the ponderosa pine 

seedlings over the 

10-year study.
47

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Figure 17—The cow in the center of this figure is	one	of many utilizing the vegetation in

this ponderosa pine plantation. 

Table 12—Average diameter, foliar cover, and height of conifer seedlings 

10 years after treatment in three study areas treated by grazing animals, 

northern and central California

Treatment Species Diameter Foliar cover Height 

Inches Square feet/acre Feet

Cattle II:

Grazing Ponderosa pine 5.18 b 29,800 b 12.39 b 

Control 4.58 b 27,850 b 10.94 b

Sheep I:

Grazing Ponderosa pine 4.20 b 26,572 b 10.80 b

Control 3.90 b 15,246 b 10.20 b

Sheep II:

Grazing Jeffrey pine 1.90 b 8,276 b 4.70 b

Control 2.20 b 10,890 b 5.20 b

Note: For each study area, treatment means in each column followed by the same letter do not differ 

statistically at the 0.05 level.

Sheep often are the animals of choice for wildland grazing (fig. 18). They are 

well suited to the dry, rough, and often shrub-inhabited terrain typical of conifer 

plantations in the Western United States. But as for cattle, skepticism remains that 

the risk of damage to the conifer seedlings outweighs the gains from utilizing the 

forage, particularly when the seedlings are young. Sharrow (1994) noted: “Forest 
48

GENERAL TECHNICAL REPORT PSW-GTR-231

managers acceptance of sheep as a biological tool is currently limited by lack of 

site-specific data detailing the ecological and economic costs and benefits associated with prescription sheep grazing in forests.”

In both of the sheep studies (McDonald and Fiddler 1993a, McDonald and 

others 1996), at least 30 species of plants were present, and forage in the form of 

shrubs, forbs, and grasses was plentiful. Apparently this forage was preferred, 

and little damage was done to conifer seedlings at any time, even when they were 

young and contained new, succulent foliage. After 10 years, grazing by sheep did 

not significantly increase average diameter, 

foliar cover, or height of pine seedlings 

(table 12). 

For an at-a-glance evaluation, the 

five major direct vegetation manipulation 

treatments are summarized in terms of cost 

(table 13) and application, limitations, and 

effectiveness (table 14).

Table 13–Relative cost of the five 

direct vegetation management 

treatments 

Treatment Cost 

Dollars per acre a

Herbicides 85–137 

Manual release 150–315 

Mulching 1,984–2,600 

Mechanical 218–225

Grazing animals 0 b

a

 U.S. Department of Labor wage rate of 

June 1993.

b

 Income from grazing is assumed to equal 

the cost of administering the allotment.

Figure 18—This large band of sheep	eats	a	lot of	forage,	but not enough to significantly

reduce competition to the conifer seedlings. 

After 10 years, grazing 

by sheep did not 

significantly increase 

average diameter, 

foliar cover, or height 

of pine seedlings.oe
49

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Table 14—Summary of direct treatments and variations: application, limitations, and effectiveness

Treatment and variation Application Limitations Effectiveness

Herbicides:

2,4-D Broad Social pressure, grasses Effective

Garlon 3A Broad Social pressure, grasses Effective

Garlon 4 Broad Social pressure, grasses Effective

Roundup Broad Social pressure, grasses Effective

Velpar Broad Social pressure Effective

Manual release:

 1- to 4-ft radius Narrow Shrubs, hardwoods Ineffective

5-ft radius Broad Cost Effective

Whole plot Broad Cost Effective

Mulching:

 Materials other than polyester Narrow Decay, cost Ineffective

 and polypropylene

 Sizes smaller than 5-ft radius Narrow Treatment size Ineffective

 Polyester and polypropylene Broad Cost, shrubs Moderate

Mechanical:

 Trac Mac and Hydro-Ax Broad Vegetation other than large shrubs, terrain Ineffective without 

 herbicide followup

Grazing animals:

Cattle and sheep Narrow Multiple years, timing of grazing, type Ineffective 

 of vegetation, utilization of forage

Genetic Enhancement

When we began this study, a key question was paramount in our minds: Would 

genetically-enhanced ponderosa pine seedlings outgrow nursery-run seedlings 

at an early age? This was critical. If they would not exhibit rapid juvenile 

growth, then their worth as a vegetation management tool would be limited. 

Shortly after outplanting from the nursery bed in 1989, control-pollinated pine 

seedlings were statistically taller (fig. 19) and had larger diameters just below 

the cotyledon scar than wind-pollinated and nursery-run counterparts at the 

5 percent level (McDonald and others 1994c):

Treatment Height (ft) Diameter (in)

Nursery-run 0.48a 0.16a

Wind-pollinated 0.49a 0.17a

Control-pollinated 0.57b 0.19b

Standard error 0.01 0.01

Values in each column followed by the same letter do not differ statistically 

at the 0.05 level. 

Would geneticallyenhanced ponderosa 

pine seedlings outgrow 

nursery-run seedlings 

at an early age?
50

GENERAL TECHNICAL REPORT PSW-GTR-231

By fall 1991, or after three growing seasons in the field, only the height of 

control-pollinated seedlings differed statistically from nursery-run seedlings with 

competition, which was a vigorous stand of shrubs, hardwood sprouts, forbs, and 

grasses. For free-to-grow seedlings, mean height continued to differ statistically 

among the three genetic classes, but only control-pollinated seedling diameter 

differed significantly from that of nursery-run seedlings. Subsequent analysis of 

growth during the third through sixth growing seasons (1992 to 1995), indicated 

that we had lost the statistical differences for pine height and diameter among 

Figure 19—The rows of control-pollinated seedlings are significantly

taller than wind-pollinated and nursery-run seedlings in this nursery 

bed. 
51

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

genetic classes when growing with competition. For seedlings without competition, 

control-pollinated seedlings continued to be statistically taller than nursery-run 

seedlings (fig. 20); mean values were 9.7 versus 8.4 ft, respectively. All other 

statistical differences had disappeared (McDonald and others 1999b). Plainly, early 

release was needed just as much for genetically improved ponderosa pine seedlings 

as for nursery-run seedlings.

Indirect Treatments

Most forest vegetation managers know that organic matter often contains legions of 

damping-off fungi and other organisms that inhibit the development of tiny radicals 

(roots). They also know that shade inhibits the growth of many, if not most, competing species as well as that of conifer seedlings. Unfortunately, little data have been 

collected on vegetation in these environments and results are mostly anecdotal. 

In one of our northern California studies (Lee’s Summit), the area was broadcast burned in the spring when the slash and logging debris were dry and the soil 

was wet. This resulted in removal of most of the aboveground material and created 

areas where the soil was disturbed by harvest and other areas where it was covered 

with at least a 1-in layer of organic material. Two years after the burn, when the 

organic matter had decomposed, we quantified the density of various species and 

categories of species in both the mineral soil and organic matter areas (table 15). 

Figure 20—Of all the comparisons, only the control-pollinated (CP) seedlings without 

competition differed from nursery-run (NR) counterparts. WP = wind pollinated.

9

8

7

6

5

4

3

2

1

0

Seedling height (ft)

Year

1989 1990 1991 1992 1993 1994 1995

Genetic class

CP without competition

WP without competition

NR without competition

CP with competition

WP with competition

NR with competition

Early release was 

needed just as 

much for genetically 

improved ponderosa 

pine seedlings as for 

nursery-run seedlings.
52

GENERAL TECHNICAL REPORT PSW-GTR-231

Table 15—Plant density 2 years after site preparation at two levels of 

organic matter (as a percentage of ground cover), northern California

Density

Organic matter Deerbrush Manzanita Gr asses Forbs

Percentage - - - - - - - - - - Number of plants/acre - - - - - - - - - -

70–90 1,917 250 417 1,333

10–20 3,750 2,250 14,250 1,250

We found that the area, which once was covered with organic matter, had less than 

one-seventh the amount of deerbrush, greenleaf manzanita, and grass seedlings 

than the area that was mostly bare mineral soil. 

On the Hoopa Valley Reservation in northern California, the Native Americans 

who live there value the tanoak tree for many reasons, including its acorns as a 

favorite source of food. Typical stands tend to be too dense with trees that have 

narrow crowns and produce few acorns. An ongoing goal is to lightly thin such 

stands to widen crowns and provide more sunlight to them. The root-crown sprouts 

that develop from the cut stumps are not wanted and steps must be taken to remove 

them or greatly reduce their vigor. 

Our replicated study involved 120 stumps with three treatments: control, charring the sprouts with a weed burner for 4 consecutive years, and cutting the bole 

twice (double decapitation). The theory of double decapitation is that cytokinins 

and giberellins accumulate at the cut surface of the stump where they prevent 

auxins from inhibiting the lateral buds. Removal of the upper stump after hormone 

translocation occurs may reduce the amount of hormones available to stimulate the 

growth of dormant buds. Two years after the last burning, the number of sprouts 

per stump was significantly larger with burning than with double decapitation or 

the control (Wilkinson and others 1997). Mean height of sprouts after burning was 

significantly lower with burning than in the other two treatments as well. The mobilization of growth hormones to the top of the cut stump apparently did not occur in 

tanoak, rendering double decapitation no different than the control. 

Other unexpected factors influenced sprout height and number. In the double 

decapitation treatment and control, sprouts were infected with mold. Most sprout 

tips eventually died from the infection and a general decrease in vigor of entire 

sprouts was evident. New sprouts from the base of burned sprouts were noticeably 

more tender and of a brighter green color than sprouts in the other two treatments. 

These were heavily browsed by deer and noticeably shortened by them. Thus mold 

and deer browsing became part of the treatments. 
53

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Figure 21—Ten-year-old conifer regeneration in a group-selection opening on the Challenge 

Experimental Forest. Each swizzle stick denotes a seedling.

Another example of a shade-mediated environment is that created by groupselection cutting. This method, which is part of an uneven-age silvicultural system, 

involves the removal of groups of trees to create small openings up to about 2 acres. 

Several of these openings, scattered throughout a stand, usually are harvested 

together on a cyclic basis. The cycle often is 10 or 15 years. There is no rotation, 

and cutting continues in perpetuity, usually removing clusters of mature trees. The 

group-selection method often is perceived as being “gentle” on the land, its vegetation, and its creatures, and hence is applied where the landowner desires to harvest 

some trees periodically, but where logging damage is slight and most ecosystem 

processes are not disrupted. Findings are reported from two studies on groupselection cutting.

In a 1964 study on the Challenge Experimental Forest, 48 openings, 30, 60, and 

90 ft in diameter, were created on a high-quality site (McDonald and Abbott 1994). 

Smaller logging slash was spread out on opening edges with a bulldozer, and larger 

concentrations were burned in adjacent skid roads. After 10 years, more than 9,800 

conifer and 1,445 hardwood seedlings per acre, at least 4 years old, accumulated 

in each opening (fig. 21). In general, tree seedlings and shrubs were tallest in the 

center of the largest openings and shortest in the smaller opening and near opening 

edges. Overall, shrubs had narrow crowns with only one or two stems per clump, 
54

GENERAL TECHNICAL REPORT PSW-GTR-231

tall stems, and a generally weak appearance. Many had died. The density of greenleaf manzanita, for example, was 509 per acre, foliar cover was less than 1 percent, 

and average dominant height was 2.6 ft. In a nearby clearcutting on a similar site, 

greenleaf manzanita averaged 16 times more density, 18 times more foliar cover 

with five to eight stems per clump, and was twice as tall.

After 28 years, the openings remain well stocked with conifer saplings, 

hardwoods, and herbaceous species. Dominance was expressed by the more 

shade-tolerant conifers, especially California white fir (fig. 22) and tanoak. 

Although many shrubs, forbs, and graminoids were present, their foliar cover 

and height were far less than that of the dominant saplings. 

Figure 22—Twenty eight-year-old conifer saplings in a group-selection opening on the 

Challenge Experimental Forest. 

The study on group-selection cutting on the Boggs Mountain State Forest began 

in 1988 and took place in nine openings that ranged from 0.2 to 1.6 acres. After 4 

years, the vegetation was characterized by many species, a huge number of plants, 

and limited foliar cover and height (McDonald and others 1997). However, average 

dominant height of all categories of vegetation (ponderosa pines, shrubs, manzanita, ferns, graminoids, and forbs) was greater near plot centers. 

We were particularly interested in the effect of opening size and distance from 

edge on the growth of naturally seeded and planted ponderosa pines (McDonald 

and others 2009). At Boggs Mountain, the difference between the two regeneration 
55

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

types for both stem diameter and height was dramatic. After nine growing seasons, 

the average height and diameter of planted seedlings was 2.4 and 2.7 times that 

of natural seedlings, respectively. If the seedlings were naturally seeded, growth 

seemed to plod along regardless of opening size and distance from edge. But for 

planted seedlings, distance from edge had a much stronger effect, particularly in 

larger openings. 

Discussion

In the material that follows, results from the plant community, biological influences, and operational techniques are discussed with emphasis on each and how 

the basic tenets in each interact with the others.

Plant Community Development

A major need in forest ecology and vegetation management today is knowledge 

on the plant community that develops after different levels of natural and humancaused disturbance. Sometimes people frown on human disturbance, but overall 

disturbance is necessary and beneficial as Atzet and Martin (1992) noted: “Disturbance brings change that helps maintain compositional, structural, and functional 

diversity, helps to select adapted, resilient individuals, and helps to dampen the 

effects of minor environmental oscillations and extremes. Change is an essential 

ingredient of healthy ecosystems.” Furthermore, Reice (1994) suggested that 

disturbance is ubiquitous and frequent relative to the lifespans of the dominant taxa. 

Thus communities are always recovering from the last disturbance. And, the size 

and extent of the disturbed areas modifies the ensuing disturbances, creating an 

ever-changing environment.

Consequently, vegetation managers need to develop knowledge on changes in 

the plant community over time. The density and growth of individual species and 

their eventual ascendance or decline, is particularly important. Equally important 

are the changes to plant communities brought about by different levels of humancaused disturbance. Such levels, termed treatments, often alter the amount of 

competition to planted conifer seedlings, but seldom have a long-term effect on the 

species composition of the community, at least for the first 10 years.

Unfortunately, knowledge on plant species composition and development 

after disturbance generally is lacking and often tied to a few competitive species 

that limit the survival and growth of economic species. Such is the case for many 

species in almost all major categories of vegetation, some of which are notorious for 

their limiting effect on conifer seedling survival and growth in young plantations. 

After nine growing 

seasons, the average 

height and diameter of 

planted seedlings was 

2.4 and 2.7 times that of 

natural seedlings.

A major need in forest 

ecology and vegetation 

management today 

is knowledge on the 

plant community that 

develops after different 

levels of natural 

and human-caused 

disturbance. 

Consequently, 

vegetation managers 

need to develop 

knowledge on 

changes in the 

plant community 

over time.
56

GENERAL TECHNICAL REPORT PSW-GTR-231

In the 5-year study on the Challenge Experimental Forest (McDonald 1999), 

the vast majority of species became established early and persisted throughout the 

study period. Several species were present for only a few years and then disappeared, but all categories of vegetation were represented in this decline. Other 

species invaded later in the study, but almost all of these were annual and perennial 

forbs, as well as graminoids. Density of all plant species increased more than 4 

times during the study, and foliar cover more than 18 times. After five growing 

seasons, enough plants were present to cast a solid shadow over about 40 percent of 

the land surface. Thus, space was available for additional horizontal development, 

but although increasing, the rate of increase slowed each year through the second 

to fifth growing seasons. Increasing competition for soil moisture, nutrients, and 

sunlight, and occupancy of the most favorable microsites are probable reasons. 

Of interest, was that 31 percent of the species present during the study never 

had more than 0.5 ft2 of cover. Conversely, after the first year, less than 25 percent 

of the species present contributed over 90 percent of foliar cover. Severe browsing 

of some shrubs by deer early in the study may have aided their development and 

dominance potential. Deerbrush and whiteleaf manzanita (Arctostaphylos viscida 

C. Parry) for example, seemed to have benefited by damage to the main stem. This 

led to the development of multiple stems at an early age, and increased their potential for higher foliar cover. 

We began the analysis of plant species diversity in 21 study areas (McDonald 

and Fiddler 2006) with the idea that, because most plantations were located in 

the general forest zone on land capable of producing an economic crop, a certain 

amount of similarity in species would be present. We tried to relate the number of 

species at the beginning of studies and after 10 years, to six categories of vegetation, to site productivity, to different ages of vegetation, and to possible core species 

with limited success. Plainly, species diversity in young conifer plantations in 

northern and central California is highly variable and related to the interactions 

of more variables than those we evaluated. 

However, knowledge from this study should be helpful in a larger sense. Sometimes opponents of intensive forest management blame the loss of plant species on 

site preparation and release. Vegetation managers often have trouble countering 

this argument. Now managers have some information, at least for environments 

similar to those in this study. They can count on a broad range of plant species, an 

average of about 25 species in six categories, and some core species and genera. 

Together, these species and the planted conifer seedlings protect the site, are useful 

to wildlife, and promote ecological diversity. They provide opportunity for future 

Plainly, species 

diversity in young 

conifer plantations in 

northern and central 

California is highly 

variable and related 

to the interactions of 

more variables than 

those we evaluated. 
57

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

manipulation not only toward a broad array of desired values and commodities, but 

also toward the forest that was present originally. 

The comparison of vegetation development after 10 years in the three differing 

communities manipulated by several release treatments provides two important 

points: (1) an increasing shrub density often causes fewer species in the plantation 

and a broader range in foliar cover and height than in other communities, even if the 

range in number of plants per acre is lower; and (2) whatever the plant community, 

a large range in number of species per acre is present and their range in density is 

large (table 3). The cubic-volume values (foliar cover × height) also could be useful 

to wildlife biologists and fire ecologists because they provide an estimate of the 

amount of browse and fuel. 

Biological Influences

We never fail to be amazed at how well some plant species in the California landscape are adapted. Plants in such genera as Arctostaphylos, Ceanothus, Chamaebatia, Chrysolepis, Pteridium, and Quercus, which arise from both above- and 

belowground propagules, are noteworthy. We have observed that mature plants 

from these genera are rarely killed by insects and disease and even more rarely by 

drought, heat, frost, and other abiotic agents. Several stems in a patch or clump may 

die, but the plant lives on.

The internal moisture stress data help to substantiate how well some plant 

species adapt to the most limiting environmental factor in most of California—soil 

moisture. The predawn minimum moisture stress values for 5-year-old ponderosa 

pine seedlings were very low in all treatments, which indicated that the roots were 

efficient at gathering moisture from the soil even at the end of a typical long, hot 

summer. Predawn stress values also were very low for tanoak and snowbrush. This 

is not surprising, as plants of these species, which were 3 years old at the time of 

sampling, probably were sprouts from root crowns that had access to parent-plant 

root systems. The predawn minimum moisture values for deerbrush and greenleaf 

manzanita, particularly in the least effective treatments where competition was 

fierce, were a classic example of adaptation. Plainly, their roots were unable to 

recharge their systems with water overnight, yet they were able to withstand severe 

internal moisture stress without wilting or dying. Indeed, deerbrush endured high 

stress for the entire day without ill effects.

Maximum internal moisture stress in all the plant species that we tested ranged 

from 18 to more than 42 atmospheres in the least effective treatments with no 

observable wilting. As a whole, these species in these treatments had higher predawn values, earlier peaking, higher maximum values, and the need to endure long 

Whatever the plant 

community, a large 

range in number of 

species per acre is 

present and their range 

in density is large.
58

GENERAL TECHNICAL REPORT PSW-GTR-231

periods of high stress. Capability to limit respiration at high temperatures, ability to 

intake at least a little soil moisture early in the morning, to be able to photosynthesize at relatively high internal moisture stress levels, and to limit transpiration, are 

all valuable adaptations that limited damage to leaves and stems. 

We also have found that plants in the early seral community of young conifer 

plantations in California follow most, if not all, of the five classical regeneration 

strategies of Grime (1979) and one or more of the three traditional successional 

pathways of Connell and Slatyer (1977). Rarely is the vegetation in a plantation 

dominated by a single class of plants or by plants from a single regeneration strategy (McDonald and Radosevich 1992). Consequently, most competitive plant species present after site preparation originate from a variety of structures and employ 

different strategies that impart unique advantages to members. These are “instant” 

site occupancy by burgeoning sprouts and seedlings from thousands of dormant, 

but viable, seeds in the soil, saturation of an area by shade-tolerant seedlings and 

windblown seeds, or rapid underground growth by rhizomes. Where a number of 

species having different regeneration strategies are present, dominance accrues to 

those species that become largest first. All three successional pathways seem to be 

active in new plantations, and together with the regeneration strategies, indicate that 

a large variety of well-adapted species are present, each with unique ability. 

Based on many of our operational studies as well as the study on the Challenge 

Experimental Forest, the mode or process of plant succession is best characterized 

as “modified” environmental succession. The dominant species of the near future 

will be those whose regeneration strategies result in early dominance. Sprouting 

shrubs and hardwoods with rapidly increasing foliar cover and height are good 

examples. As the crowns of trees and shrubs close, and the high density of the 

annuals and graminoids decreases, more shade-tolerant conifers and perennials will become established. Thus succession will occur through environmental 

modification. It is not classical replacement of one stage of species by another 

stage, however, because the first stage will be present for many years. Species of 

random origin, especially those from windblown seeds, will not be a major part of 

the understory plant community. Plants from the seed bank in the soil will not be 

a major part of this community either. However they will have accomplished their 

mission because the storehouse in the soil will have been replenished. 

Longer term, the number of plant species in the plantation likely will decline. A 

good example took place in a mature brushfield on a harsh site near Mount Shasta. 

Before site preparation and planting in 1962, three tall, vigorous shrub species 

dominated in the area. After 5 years, the plant community consisted of 6 shrubs, 

The dominant species 

of the near future 

will be those whose 

regeneration strategies 

result in early 

dominance. 
59

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

10 forbs, and 1 grass (McDonald and Abbott 1997). When the study ended in 1992 

(after 31 years) the community consisted of five shrubs, two forbs, and one grass. 

However, two shrubs, both forbs, and the grass consisted of only a few plants in 

the entire study area. The progression from 3 to 17 and then back to 3 species may 

be anathema to some people. But this gap in species richness probably is only an 

interim phenomenon. The traditional arborescent pioneer species, ponderosa pine, 

is now established on plots with the least amount of competition and 30 years later 

is growing at the potential of the site. This is the first step in plant succession. Eventually shade-tolerant conifers also will become established (or planted) and form the 

mixed-conifer forest that originally occupied the area. It is likely that more species 

of plants will find this forest environment conducive to establishment than if the 

area had remained a brushfield (McDonald and Fiddler 2001b), and plant species 

richness likely will increase again.

Operational Techniques

Over the years, an amazing number of direct control techniques have been 

attempted to control competing vegetation in young conifer plantations in California: dynamite, plastic buckets over spouting stumps, newspaper mulches, copper 

nails, bleach, sheets of plywood, rotary mowers, and blow torches to name a few. 

One early study in California involved a sample of 2,254 stumps of sprouting 

oaks and large shrubs (Bruce 1936). Treated vegetation was live oak (Quercus

spp.), black oak (Quercus spp.), toyon (Heteromeles sp.), mountain mahogany 

(Cercocarpus spp.), prunus (Prunus spp.), coffee berry (Rhamnus spp.), fremontia 

(Fremontodendron spp.) and sprouting manzanita (Arctostaphylos spp.). “The work 

consisted of treating sprouting stumps with various chemicals, under various conditions of concentration, location, season, and mode of preparation of the stump.” 

Treatments included sodium chloride, carbon bisulfide, arsenite, cupric sulfate, 

ferrous sulphate, stove oil, diesel oil, sulfuric acid, calcium oxychloride, pyradine, 

and ammonium thiocyanate. Some chemicals were applied to sprouting stumps, 

desprouted stumps, and even excavated roots of desprouted stumps. Several mixes 

of chemicals, kinds of application, and timing were effective at killing sprouts or 

preventing sprouting. However, the use of most, if not all of these products, would 

be questionable today.

Herbicides were suspended by the USDA Forest Service in March 1984 in 

response to federal court rulings (Click and others 1989). This moratorium was 

lifted in January 1991, but during the 7-year interval, the variety of treatments used 

by vegetation managers increased even more. Some of these treatments included the 

Over the years, an 

amazing number 

of direct control 

techniques have 

been attempted to 

control competing 

vegetation in young 

conifer plantations in 

California: dynamite, 

plastic buckets over 

spouting stumps, 

newspaper mulches, 

copper nails, bleach, 

sheets of plywood, 

rotary mowers, and 

blow torches to name 

a few. 
60

GENERAL TECHNICAL REPORT PSW-GTR-231

use of concentrated fire around stumps using Aluma-Gel, trenching and planting on 

berms, and a host of mulch materials. Most of these were performed “to buy time” 

until the moratorium was lifted or better techniques became available.

Herbicides—

Many factors influence the ability of an herbicide to control undesirable vegetation. 

These include the ability of the chemical to penetrate the leaves, the availability of 

enough water in the leaves and stems to translocate the herbicide to the intended 

plant part, and the capability of the herbicide to effectively damage the plant to the 

point that it becomes moribund. More than in any other treatment, herbicides tend 

to kill the plant, not just its top. Several morphological and physiological factors are 

important and relate to leaf age and plant size. Young leaves have not had time to 

develop thick epidermal layers or oily and waxy coverings and therefore are more 

easily penetrated by the chemical. It stands to reason that small plants are more 

easily affected than large plants because they have less biomass and less sprouting 

potential. Small plants also are developing at a time when internal moisture levels 

and photosynthesis are high, and thus translocation and assimilation of the herbicide 

are increased. It follows that as summer progresses, stomates in the leaves open 

less fully or for shorter intervals, internal moisture stress increases, photosynthesis 

declines, and herbicide intake is much reduced. For these reasons, our studies with 

herbicides took place in young plantations less than 4 years old when the vegetation, whether from seeds or sprouts, was of sufficient size and development to treat 

effectively. 

The three representative studies denoted in table 8 also took place in a wide 

variety of plant communities that ranged from mostly tough, aggressive shrubs 

to those containing a mixture of shrubs, forbs, ferns, and grasses. One study 

also included sprouts from hardwood trees. The studies also were located on site 

qualities that ranged from medium to above average. Thus the results indicate that 

herbicides are effective on a wide range of plant communities on most sites at a 

reasonable cost. 

Manual release—

Of the five classical direct-control vegetation management techniques, manual 

release is probably the easiest to implement. It requires only a hand tool; however, 

the work is hard, dirty, and potentially dangerous. Manual release in California 

was first done in the 1970s by local people with negotiated contracts for relatively 

small areas. This soon changed to competitive bids that were awarded to the lowest 

bidder. Often, local contractors would hire local people and owing to inexperience, 

The results indicate 

that herbicides 

are effective on a 

wide range of plant 

communities on 

most sites at a 

reasonable cost. 
61

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

would default on their contracts. Soon a performance bond was required, which 

hastened conversion to experienced contractors with established crews, many from 

outside the area. Consequently, the practice of manual release evolved from a few 

individuals to groups of people working on larger units for more money (Fiddler 

and McDonald 1990). Over the years, a wide variety of people have organized to 

bid on manual release contracts, including environmentalists, townspeople, loggers, 

students, churchgoers, and Russians. By far the largest group, however, was 

Latinos. 

The three representative studies shown in table 9 took place in plant communities that were composed primarily of shrubs; forbs, ferns, and grasses; and a 

mixture of woody and herbaceous vegetation. The shrubs, although rated as aggressive, were treated when they were young and small. The three representative studies 

and others indicate that manual release is most effective if the vegetation is treated 

when young and when the plant community is mostly forbs and grasses. But even 

then, a second release treatment is almost always necessary. 

Mulches—

We tried to broaden the application of this method by studying many kinds and 

sizes of mulch mats in several plant communities. We also fully quantified costs by 

including maintenance for several years. We learned that mats do have a place in 

the vegetation manager’s handbook provided they are large, durable, and in contact 

with the ground. Mats can even be used effectively in a sprouting shrub community. However, the large, durable mats tend to be costly, and a lack of decomposition 

in the center could eventually strangle the seedling. Mats work best if they simply 

disintegrate in place, or become covered with soil and other organic debris. But if 

they flop around or pieces of them become scattered, then they are aesthetically 

displeasing to people passing by. Gathering them would be yet another cost. The 

total cost could be mitigated if manufacturers would standardize mulches to a given 

material, size, color, and time to degrade. They might even produce camouflaged 

mulch that would blend into the natural environment in visually sensitive areas.

A possible advantage to using mulch mats is the amount and longevity of 

moisture held under them. At Beartrap Ranch, for instance, mulches were applied 

over the existing grass and forb community. Heat from the mulches quickly killed 

these plants and formed another mulch, in effect becoming an organic mulch under 

a mulch (McDonald and others 1994a). This organic material absorbed moisture 

and the soil under it stayed moist well into the growing season. It also temporarily 

blocked at least some upward-moving soil moisture from escaping through the 

pores of the mats. Applying mats early in the spring when the soil is at field 

Manual release is 

most effective if the 

vegetation is treated 

when young and when 

the plant community 

is mostly forbs and 

grasses.

Mats do have a place 

in the vegetation 

manager’s handbook 

provided they are large, 

durable, and in contact 

with the ground.
62

GENERAL TECHNICAL REPORT PSW-GTR-231

capacity is suggested because it would lock in soil moisture under them. This, of 

course, applies to mats in general, whether installed over existing vegetation or 

bare ground. 

Mechanical release—

For monetary, political, or accessibility reasons, plantation release often cannot be 

done in time, and the returning vegetation, often of sprouting shrubs, grows wide 

and tall. Consequently, untreated plantations that are 10 years or older are present in 

the West. Conifers in many of these plantations are penalized by heavy competition 

and a trend of decreasing annual height increments. Controlling vegetation of this 

size and level of development often is both difficult and expensive with commonly 

applied methods such as manual release or grazing. Most herbicides do not work 

well on tall shrubs and hardwoods either. Another factor to consider is the conifer 

seedlings. They are approaching sapling or small-pole size and, although taller than 

the shrubs, have thin bark that causes broadcast burning to be impractical as well. 

Being taller than the shrubs is no guarantee that the conifer seedlings will grow at 

the potential of the site (McDonald and Abbott 1997). 

Results from our two studies on mechanical release reinforce the need for an 

herbicide application soon after treatment with the large machines. Not to do so 

renders the work of the machines ineffective. 

The surge of grasses in the combined mechanical and herbicide treatment at 

Big Tunnel follows the trend noted earlier in the biological control and exclusion 

section. It also is similar to findings for the most effective herbicide treatments in 

some of our other studies (McDonald and Everest 1996, McDonald and Fiddler 

1999c, McDonald and others 1996). Cheat grass (Bromus tectorum L.) and other 

annual grasses were common in these studies and tended to increase in density 3 

to 5 years after the conifer seedlings were planted. Becoming established well after 

the conifers were planted, being shorter than the conifers, and having roots that did 

not grow as deep as the conifers, meant that the grasses were less competitive than 

most invading shrubs and hardwoods. 

Grazing animals—

Managers often think of plantations as being all cost and no income for many years, 

or at least until commercial thinning. But some plantations can provide immediate 

income in terms of nonwood values. Plantations may provide a pleasant view to 

the passing motorist, and provide sustenance to several species of wildlife as well 

as cattle and sheep. For grazing animals, the key is that at least some species in the 

plant community be palatable. As noted earlier, most plant communities in northern 

and central California plantations have many categories of vegetation and contain 

The need is for an 

herbicide application 

soon after treatment 

with large machines. 

Not to do so renders 

the work of the 

machines ineffective. 
63

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

many species, several of which are palatable. If the grazing animals concentrate on 

this vegetation, it could mean less damage to conifer seedlings.

Because the study on cattle and both studies on sheep did not demonstrate a 

significant gain in pine seedling growth, the use of grazing animals as a release tool 

for the vegetation manager is questionable. Current practices, at least as practiced 

in northern California, have several weaknesses. To begin with, the animals are 

seldom used the first season after logging or burning—the perception is that not 

enough forage is present and that damage to the conifer seedlings would be severe. 

In addition, when used in subsequent growing seasons, the timing is poor. Traditionally, the cattle and sheep feed at lower elevations until the forage is gone, then 

they are trucked to higher elevations in the forest zone. Usually, this is too late. 

Most of the early, tender, and more nutritious growth has become hard and mature, 

or eaten by other herbivores. And even when the animals are present for wildland 

grazing, the tendency is to herd them to the most nutritious forage, and remove 

them before secondary species and plant parts are all that remain. The end result 

is that grazing is too little, too late, and utilization is never high enough to weaken 

the competing vegetation to the point that the conifer seedlings have a significant 

growth advantage over counterparts in the control. 

Several steps can be taken to enhance grazing as a viable vegetation management tool. Truck the animals to the selected area the first growing season after 

planting and as soon as possible in the spring to utilize the vegetation while it is 

tender and nutritious. Once there, the animals should be held to graze as much 

vegetation as possible even if it means consuming less than prime forage. If these 

conditions cannot be met, the range deteriorates, or damage to the conifer seedlings 

becomes intolerable, then this method will not be viable from the standpoint of 

seedling survival and growth. In our studies the pines did not benefit, but the cattle 

and sheep did, and the owners were pleased with the low cost and good weight 

gains of their animals. In a broader sense, the return from the land of wool, meat, 

and lambs not only helps the landowner, but also the world, which is ever in need 

of more food and fiber. 

Availability, cost, ease of application, effectiveness, and speedy results are 

factors that can be used to evaluate each direct treatment. When they are packaged 

and compared as a whole, the composite with the highest rating is herbicides. 

Genetic enhancement— 

At this point, we need to mention that genetic enhancement of conifer trees constitutes a broad program for improving conifer tree growth, stem form, and resistance 

to insects and diseases. Our study was concerned with a very narrow part of 

Availability, cost, 

ease of application, 

effectiveness, and 

speedy results are 

factors that can be 

used to evaluate each 

direct treatment. When 

they are packaged and 

compared as a whole, 

the composite with 

the highest rating is 

herbicides. 

Because the study 

on cattle and both 

studies on sheep did 

not demonstrate a 

significant gain in pine 

seedling growth, the 

use of grazing animals 

as a release tool for the 

vegetation manager is 

questionable.
64

GENERAL TECHNICAL REPORT PSW-GTR-231

genetic enhancement. The early finding that genetically enhanced ponderosa pine 

seedlings growing with competition significantly outgrew nursery-run seedlings 

seemed to open a promising and meaningful door in vegetation management. 

However, enhanced seedlings grew no better than the nursery-run seedlings after a 

few years.

This leads to the larger question: “Do genetically improved seedlings have a 

place in forest vegetation management?” In this study, and in a much larger trial, 

the outcome is clouded, but as noted later, more research is needed. The larger trial 

was with slash pine (Pinus elliottii Engelm.) and loblolly pine (Pinus taeda L.) in 

the Southern United States. Early significance in growth faded with time so that 

after 9 years, vegetation control and genetic enhancement had positive and additive 

effects on several loblolly pine variables (Pienaar and others 1997). But by far, the 

major effect was due to vegetation control alone. 

Indirect methods—

As noted earlier, mid-May to October often are rainless, and soil moisture is the 

limiting environmental factor in most of California. Rapid growth of conifer seedlings is best when soil moisture is adequate and sunlight is plentiful, and thus most 

plantations are located in clearcuttings and recent burns. Competing plant species 

grow best in this environment as well. Forest vegetation managers have not needed 

to regenerate areas with intact layers of organic matter or those having a shaded 

environment. Consequently, they have not developed strategies or methodologies 

that take advantage of the fact that a layer of organic matter is an inhibitor of seed 

germination (Click and others 1994, Grime 1979) and that many aggressive shrub 

species are intolerant of shade (Minore and others 1988) and develop poorly there. 

For group-selection cutting, knowledge is needed on the species composition, 

density, and development of competing vegetation and conifer regeneration. Crucial 

questions are whether the desired species become established and develop satisfactorily in this environment, and where in it does aggressive competing vegetation 

develop slowly and conifer seedlings grow well. We have demonstrated that the 

former happens, but the latter are constrained as well. The key is to find the best 

opening size/aspect interaction where the environment dampens the growth of competitors more than that of conifer seedlings, particularly those that are tolerant of 

shade. More work on the topic of natural versus planted seedling growth also would 

be helpful. On a high-quality site in central California, Heald (1999) found that the 

height of naturally seeded and planted ponderosa pine seedlings did not differ at 

the 5-percent level after 9 years. The reason for the dissimilar findings between our 

study and his is unknown. 
65

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Finding this reason will not be an easy task for several reasons: (1) The plant 

community in group-selection openings typically tends to be large, variable, and 

of many ages. The more benign environment encourages recruitment of many 

species and a longer recruitment period than in open plantations. (2) Because of this 

longer recruitment period and the addition of new and shorter recruits, the standard 

measures of average foliar cover and height are meaningless. A sample of “widest” 

and “tallest” plants better portrays differences. (3) The more shady environment 

also may govern what conifer species grows best. For example, after 28 years in the 

Challenge study, sugar pine (Pinus lambertiana Dougl.) and Douglas-fir were 33 

percent taller than ponderosa pine (McDonald and Reynolds 1999). (4) Evaluation 

of the group-selection method probably should not be made in a short timespan. 

In the small openings at Challenge, the five species of conifers and four species of 

hardwoods grew at least twice as fast in height the last 18 years as they did the first 

10 years. (5) A very large study with sophisticated analytical techniques will be 

needed. The opening size/distance-from-edge study at Boggs Mountain suggested 

that many more openings of various sizes on a wide range of aspects and slopes are 

needed to evaluate these variables and a host of complicated and probably important 

interactions.

Principles

Nothing is absolute in vegetation management—nature is far too large, complex, 

and variable to allow “truths” to hold for very long. However, some important 

principles have evolved from this extensive study that can serve as guides to 

vegetation managers in the near future:

•	 The period just after site preparation	is	probably	the most critical	because	

it is here that the growth environment for conifer seedlings can deteriorate 

most rapidly. Conifer seedling mortality or a slow growth rate is the consequence of inadequate early control of competing vegetation.

•	 The	establishment	of individual plants depends on their ability to capture site

resources. That means either being present on the site just after disturbance 

or having the ability to pirate scarce resources from neighbors. This process 

is most intense soon after site preparation.

•	 Most	species that invade young conifer plantations after disturbance are

those that are resistant to disturbance or adapted to it. These species often 

are characterized by rapid recovery from damage and a high growth rate. 

The ability to become large quickly, especially belowground, is critical. 

The period just after 

site preparation is 

probably the most 

critical because it is 

here that the growth 

environment for 

conifer seedlings 

can deteriorate most 

rapidly. 
66

GENERAL TECHNICAL REPORT PSW-GTR-231

•	 Biological	control of most undesired vegetation often	can	be achieved if the

undesired species arises from seed. However, if it originates from belowground propagules, like rhizomes, biological control by desirable seeded 

species, like some forbs and graminoids, is ineffective. 

•	 The population of dormant seeds rarely	is	exhausted after one disturbance

because germination requirements often are polymorphic. This suggests that 

more than one treatment application may be necessary for control. 

•	 Results from many of our studies show that graminoids in young conifer

plantations on average to better sites on west-facing slopes of the Cascade 

and Sierra Nevada Mountains have little effect on the growth of ponderosa 

pine seedlings, provided that they invade after the pines have become established (McDonald and Fiddler 1986b). However, on the drier east-facing 

slopes, the graminoids reduce pine growth for scores of years.

Figure 23—This 29-in tall ponderosa pine 

seedling on a good site has survived in deep 

shade for 27 years.

•	 Survival	is	not	a	good descriptor of future

conifer seedling growth. Ponderosa pine 

seedlings can live for 27 years or more with 

40 needles and 29 in of height (fig. 23). 

•	 Survival of conifer seedlings	is	seldom	a	

problem if weed control is applied early and 

is effective, even in drought years.

•	 Stem diameter of conifer	seedlings	is	a	good

measure of competition and correlates strongly 

to it: a small amount of competition equates to 

rapid diameter growth, and a large amount of 

competition equates to slow diameter growth.

•	 Height of young conifer seedlings often	is	a	poor

indicator of future growth. Seedlings grow tall 

for two reasons: (1) because they have plentiful 

site resources, or (2) because they are forced to 

grow upward to stay in sunlight (fig. 24). In the 

latter, the seedlings usually have long, skinny 

stems that tend to bend or break. Inevitably, 

height growth of these seedlings slows or ceases 

until the stem thickens, and the roots and crown 

develop to the point that they are in balance with 

the stem. 

Survival of conifer 

seedlings is seldom a 

problem if weed control 

is applied early and 

is effective, even in 

drought years.
67

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

Figure 24—To stay in sunlight, this Douglas-fir seedling has grown

tall and thin. 

•	 Based	on 132 regressions involving four shrub variables, three timespans,

and three forms of numerical values, foliar cover and crown volume per 

acre of combined shrubs explained the most variation in most pine parameters (McDonald and Abbott 1997). Because foliar cover is easy to estimate 

and relate to, it may be the most useful variable from a practical standpoint. 

Given the choice of average dominant height, density, or foliar cover to evaluate the competitiveness of a plant species, which is best? The height of most 

forbs and shrubs is governed somewhat like that of the conifer seedlings and 

hence is a poor descriptor of competitiveness. Density or number of plants 

per acre is equally poor because it can vary from one to thousands per acre 

Because foliar cover is 

easy to estimate and 

relate to, it may be the 

most useful variable 

from a practical 

standpoint. 
68

GENERAL TECHNICAL REPORT PSW-GTR-231

on a single plot, let alone over many plots in several replications. The variation almost always is too high to achieve statistically significant results.

•	 Because	roots of competing plant species rapidly extend into cleared areas

and capture valuable site resources, the treated area must be large enough 

(preferably a 5-ft radius) for the conifer seedling to establish its root system unencumbered for at least the first year and usually for the first 3 years. 

Early treatment of competing vegetation is extremely important and the most 

cost effective. Obviously, it is much easier and cheaper to treat competing 

vegetation when the plants are young and small.

•	 What constitutes too much competition to conifer seedlings differs with time

since planting, number of plants, and species of plants. The first year after 

planting, one or two perennial woody shrubs with established root systems, 

but only a few inches tall and within 3 ft of a conifer seedling, are too 

much. Three sprigs of bearclover and a few small manzanita or ceanothus 

seedlings within the same distance are too much also. Tappeiner and others 

(1992) noted that for some shrubs (snowbrush as an example), a stocking rate 

of two plants per milacre may be sufficient to result in 100 percent cover 

and maximum leaf area by age 5 or earlier. A general rule of thumb is that 

competition is too much when the foliar cover of undesirable plants exceeds 

10 to 20 percent on poor sites and 20 to 30 percent on good sites.

•	 Minimizing the amount of competing vegetation often creates an unfavorable habitat for animals and insects that cause problems for regenerating 

conifers, and allows seedlings to grow rapidly and avoid damage. 

•	 Conifer	seedlings	stressed for at least	5	years by strong competition are slow

to respond to increased resources brought about by removal of the competition. McHenry and Radosevich (1985) noted, “There is abundant evidence 

from both historical records and research that early competition imposes 

enormous losses in initial conifer seedling survival and growth.” Results also 

suggest that growth increments continue to be reduced well beyond the first 

decade of a tree’s life. The reason is not fully known, but probably relates to 

the seedling having to first develop its root system belowground as well as its 

photosynthetic potential above. Suppressed seedlings have a higher proportion of less photosynthetically efficient foliage than seedlings that are free 

to grow. Shock to shade-adjusted needles after sudden release also may be 

a factor. Younger trees do not have this slow-start phenomenon and release 

quickly and vigorously. This is another reason to release when the conifer 

seedlings are young. 

The treated area 

must be large enough 

(preferably a 5-ft 

radius) for the conifer 

seedling to establish 

its root system 

unencumbered for 

at least the first year 

and usually for the 

first 3 years. Early 

treatment of competing 

vegetation is extremely 

important and the most 

cost effective.

Growth increments 

continue to be reduced 

well beyond the first 

decade of a tree’s life. 
69

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

•	 Statistical	differences are critical for interpreting study results. Most often,

statistical differences in conifer seedling growth occur in 2 to 4 years, particularly if the variable is stem diameter (McDonald and Fiddler 1989) and 

the test is between the control and the most intensive treatment. Sometimes, 

a statistical difference at the 5-percent level is delayed and knowing at least 

three conditions that cause it is important: (1) if the site is of poor quality and 

the vegetation, including conifer seedlings, grows slowly; (2) if the treatment is only moderately effective and competitive species are present in the 

treated plots (A corollary to this is that the longer the conifer seedling has to 

grow with a high level of competition, the longer it takes for a statistically 

significant difference to occur); (3) if two or three rapidly growing conifer 

seedlings are growing in the randomly selected control. 

•	 Evaluating the success of	a	given treatment relates to time since treatment.

Visual observations can be misleading, and occasionally even an early statistical difference can disappear in a year or two. In general, treatment evaluation at age 5 is adequate, but at age 10 is better. But even 10 years may not be 

long enough to evaluate some treatments in some circumstances. In southwestern Oregon, for example, Tesch (1991) noted that Douglas-fir seedlings 

eventually began to grow well 12 to 15 years after release in a mixed shrubhardwood community. Another example is from a 10-year study in the southern Cascade Mountains of northern California, where McDonald and Fiddler 

(1997B) stated that “more time is needed for diameter and height to develop 

and for relationships to be clarified.” Here California red fir seedlings were 

growing on a poor site in a harsh environment. 

•	 Reforestation success in forest ecosystems with	a	mediterranean climate

depends on the seedlings’ ability to successfully compete for soil moisture. 

This is tempered somewhat by the species’ inherent adaptation to withstand 

moisture stress, and to the increased stress brought about by global warming. 

Ponderosa and Jeffrey pines, for example, are well adapted to withstand this 

stress, other conifers less so, with true firs (Abies spp.) the least able.

•	 Short-term seedling-growth goals need to be tempered with longer term

considerations that maintain the plant and animal community. During the 

conifer seedling establishment period, low density and developmental values 

of competing species are desired, but once the conifer seedlings are growing 

well, more of some species may be valuable from a practical and ecological 

viewpoint. Increased density and growth of a given forb or graminoid may 

be desirable to increase a particular yield from the plantation—sustenance 

for an endangered species of wildlife, for example.
70

GENERAL TECHNICAL REPORT PSW-GTR-231

Challenges 

Although much knowledge has been gained, more is needed, and these constitute 

challenges to future vegetation management endeavors. The challenges are a product of needs originating directly from this study, and suggestions from colleagues 

in both research and management positions:

•	 Evaluate individual species or communities of species to determine their

degree of competitiveness to young conifer seedlings and to quantify the 

potential of a species for biological control. The deerbrush versus greenleaf manzanita growth comparison (McDonald and others 1998) is a case in 

point. On a larger scale, the need is to determine which hardwood, shrub, 

forb, or graminoid variable (density, foliar cover, height, volume per acre) has 

the most effect (explains the most variation) in similar pine or fir seedling 

parameters. Such a determination could indicate which species to control as 

well as to suggest treatment effectiveness and timing. It could even suggest 

the “best” species to plant.

•	 Examine the interaction of various species of conifer	seedlings	and major

competing species (Arctostaphylos, Ceanothus for example) on a wide range 

of site qualities.

•	 The hypothesis that an increasing amount of bare ground, created just after

an effective release treatment, increases plant species richness needs to be 

tested. Conversely, is there an occupied-ground threshold beyond which plant 

species richness declines? Given the large amount of variation in site quality, 

species of competing vegetation, and other variables, 10 years is suggested as 

the time to make this test.

•	 Better	predict what competing plant species, and their potential density and

size, will be present in young conifer plantations. We need a better idea of 

past stand history and number and kind of propagules that are likely to colonize recently disturbed areas. 

•	 Learn	more about the belowground ecosystem. Time and	again,	our

statistical analyses and results on conifer seedling/competing plant 

interactions could explain only so much variation with our aboveground 

data. More specifically, the number, location, and development patterns of 

small feeder roots of the most common competing plant species need to be 

studied. Competition models with the feeder roots of these species and those 

of conifer seedlings then could be developed. Internal moisture stress of 

conifer seedlings and competing species would be a valuable adjunct to these 
71

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

studies. Initiation and magnitude of root growth of conifer seedlings relative 

to that of other vegetation and to soil temperature would also be valuable. 

•	 Mathematical modeling to determine competitive species/growth thresholds

is a challenge. The growth simulator “CONIFERS” correlated crown width 

development of several combined shrubs and hardwoods to several conifer 

seedling growth variables, and equations suggesting competitive thresholds 

were determined (Ritchie and Hamann 2006, 2008). More work with additional conifer species and specific competing species would be significant. 

•	 Work	on the worth of genetic improvement as	a	vegetation management tool.

Tests with seedlings of different conifer species, more genetically enhanced 

families, a less aggressive competing species, lower amounts of a competing species, on sites of lower quality need to be done. Conifer seedlings 

bred specifically for rapid juvenile growth is another potentially worthwhile 

approach. 

•	 Emphasize autecology and developmental studies that relate conifer seedlings to broad sclerophyll species such as those in the genera Arctostaphylos, 

Ceanothus, Rhamnus, Lithocarpus, Quercus, and Arbutus, in partially-tofully shaded plantations. Knowledge on density and growth of graminoids in 

partial-shade environments relative to that of conifer seedlings is particularly 

lacking. On a larger scale, begin studies that compare the results of indirect 

and direct vegetative manipulation in a controlled experiment. Results from 

indirect vegetation management probably will take longer to achieve than 

those from direct methods, but the initial (treatment) cost is low. The key is 

to broaden the research base and evaluate new entities. We would never have 

known the effects of shade on tanoak, for example, if we had not installed 

our study.

•	 Rethink	the sometimes negative regard for what are thought of as “weeds.”

Earlier we noted some species that had biological control and exclusionary value. Other recognized values are for preventing erosion, capture 

and recycling of nutrients, and improvement of soil chemical and physical 

properties. However, few species have been evaluated for potential worth 

by several disciplines. For example, the common weed, rubber rabbitbrush 

(Chrysothamnus nauseosus [Pallas] Britton), provides a good example of 

potential values: “It includes forage for livestock and wildlife (11.8 percent 

protein), revegetation of disturbed areas, landscape use, value as potential 

rubber (7 percent) and resin (up to 35 percent), and application of some of the 

products for nematocide and antimalarial activity” (Weber and others 1987). 
72

GENERAL TECHNICAL REPORT PSW-GTR-231

In a similar vein, it is widely recognized that Native Americans valued an 

amazing number of plant species, sizes, and ages for use in a wide variety of 

foods, medicines, clothing, shelters, tools, and ceremonies (Anderson 1990). 

“Going back to the future” could be worthwhile. 

•	 Establish	large, long-term studies that evaluate mixes of conifers and competing species in plantations and determine which sequesters the most carbon. What species of vegetation, how much of it, and in what mixture stores 

the most carbon? For example, do four 20-year rotations of deerbrush and 

manzanita store more or less carbon than one 80-year rotation of a conifer 

species? A corollary involves finding a balance between storing carbon and 

fire hazard. Given the frequency of wildfire in California, storing massive 

amounts of carbon on the surface of the land for too long, could mean the 

loss of most of it in a conflagration. 

•	 Encourage	related future research, and specifically to provide funding to

make this a truly long-term study by safe-guarding and remeasuring many 

of our study sites at future dates, age 20 for example; and to employ more 

instruments to gather data such as xylem sap tension of more species, and 

foliar nutrients of shrubs and graminoids on a range of sites. The values of 

20 percent cover on poor sites and 30 percent cover on good sites as being 

the thresholds beyond which release treatments should be applied also 

needs to be strengthened with specific studies aimed at doing so. 

Conclusions

In many parts of the Western United States, forests of conifer trees stand at the 

pinnacle of plant succession. These trees and the forest ecosystems that they form 

constitute places where plants and animals flourish, the soil provides valuable nutrients and moisture, and water flows from the land in a gentle and measured way. 

Unfortunately, wildfires and harvesting without adequate reforestation have left 

large tracts of forest land in California with few or no trees, and large areas inhabited by brush and weeds. In these areas, plant and animal diversity is much reduced 

and fewer plant communities and habitats are present. Plainly, these lands are not 

producing the variety of benefits, goods, and services that today’s public demands. 

This paper brings together the results of our study on plantation release in an effort 

to alleviate this problem and to start these lands toward attaining full productivity. 

In places, it also “passes the torch” to scientists and managers by bringing attention 

to new and different approaches to vegetation management.

Given the frequency of 

wildfire in California, 

storing massive 

amounts of carbon 

on the surface of the 

land for too long, could 

mean the loss of most 

of it in a conflagration. 
73

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

This paper and the publications upon which it is based, have both scientists and 

managers as their audience. Some are aimed at one or the other, but in general, a 

scientific finding quickly becomes useful to the manager, and no attempt has been 

made to specify either audience. Similarly, this paper should interest vegetation 

managers, botanists, and ecologists as the primary audience, followed closely by 

range and wildlife managers. Fuel and fire researchers and managers also should 

find the developmental tables on various species of vegetation useful because volumes per acre over various timespans can be calculated. Also as the various species 

of vegetation develop, the amount of carbon that is sequestered can be ascertained 

as well. Because of the large variety of disciplines in the audience, no attempt has 

been made to prioritize the principles or the challenges. The intent is that they or 

pieces of them will be useful to all. 

Results from more than 25 years of research, 30 study areas, and 60 publications have answered many questions on the development of plant communities in 

young conifer plantations, when and where they develop, and how they influence 

young conifer seedlings. But judging from the challenges above, much more needs 

to be done. Indeed, for every question answered, several more arise. The need for 

vegetation management research never ends. This is the first of several overarching 

conclusions. 

Aldo Leopold once said “The first rule of intelligent tinkering in nature is to 

keep all the pieces.” Our goal in this study was to bring a more holistic perspective 

to the art of managing vegetation. Consequently, we present detailed material on the 

plant communities that are present in young conifer plantations and the biological 

influences that shape them. We believe that this material is necessary to understand 

the effectiveness of the operational techniques that we used. In this sense, we kept 

all the pieces. 

Of the direct release techniques that we tested, herbicides were the most effective biologically, largely because they “worked” both above- and belowground. 

They tended to kill undesirable plants and consequently provided additional 

moisture and nutrients to conifer seedlings. A general ranking of our treatments 

from biologically effective to ineffective following herbicides are large mulches and 

large-area manual grubbing, mechanical, grazing, small mulches, small grubbed 

areas, and control (McDonald and Fiddler 1993b). A general trend for cost is low 

for herbicides and grazing, increases with size of area treated by grubbing and 

mulches, decreases with size of area treated with large machines, and is highest 

for grubbing large areas or applying large mulches. 

A general ranking of 

our treatments from 

biologically effective 

to ineffective following 

herbicides are large 

mulches and largearea manual grubbing, 

mechanical, grazing, 

small mulches, small 

grubbed areas, and 

control.
74

GENERAL TECHNICAL REPORT PSW-GTR-231

All of the release treatments that vegetation managers have at their command 

have degrees of application and at least one limitation. No treatment is immune to 

causing too much disturbance to the environment, or being too costly, or not being 

socially acceptable, or simply being limited by the steepness of slope or rocky 

ground. All are subject to some degree of risk, regulation, or creating conditions 

that will be difficult to manage in the future. In almost all instances, however, the 

ecological implications of manipulating the vegetation are more favorable than 

doing nothing (fig. 25). 

Figure 25—This fully stocked and rapidly growing plantation in a clearcutting will soon be 

a forest with many more options for managing the ecosystem than if it had not been treated. 

Good vegetation management fits well into good ecosystem management, and 

many inferences for enhancing ecosystem management have been presented in this 

paper. Two stand out: plant species succession and carbon sequestration. Successful 

ecosystem management includes the ability to predict the plant species, density, 

and developmental trends of future plant communities. As Taylor and others 

(2008) noted “Projecting forest succession has become an increasingly relevant 

component of ecosystem management.” For sequestering carbon, new forests 

begin to accumulate carbon rapidly, both aboveground in crowns and boles, and 

belowground in new root systems. Thus it is important to establish vigorous stands, 

keep them healthy, and growing well. Of course, establishing new, fully stocked, 

Successful ecosystem 

management 

includes the ability 

to predict the plant 

species, density, 

and developmental 

trends of future plant 

communities. 
75

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

rapidly growing stands is a major goal of vegetation management. Indeed, “reduced 

deforestation, fast-growing plantations, and agroforestry could contribute 12 to 15 

percent towards offsetting global carbon emissions. It is good business ecologically 

and may catch the conscience of the public” (Powers 2004). 

Another yield from forest land is becoming critical, and that is water. As noted 

in the Redding Record Searchlight (2009), “With our [changing] weather patterns, 

with climate change, and our population growth, we’ve got to look at how we use 

every drop,” and “Facing a third dry year and record-low reservoirs, the Bureau of 

Reclamation announced major water cuts last month in California.” Grant (2008) 

stated “In the not so distant future, clean water will be the single most important 

commodity produced from national forest lands. It will totally eclipse timber.” 

Some basic research in the form of questions needs to be done: (1) Which herbaceous, hardwood, and conifer species uses less water? (2) Can a conifer plantation 

be planted and managed for both timber production and water yield? (3) Can the 

ensuing forest be cut in such a way as to provide more high-quality water later in 

the spring when it is most needed? (4) How can the conifer seedlings and inevitable 

competing vegetation from this cutting be manipulated to maintain increased 

growth and water yield? 

Results from the direct release methods that we tested remain viable today. 

For example, the herbicides that we studied are still being used. New ones have 

been formulated, but they tend to be variations of those we used, perhaps with 

application to a broader range of competing species or to one or two target 

species. Planting through slash and eliminating at least some site preparation is 

being increasingly used today. It leaves much organic material on the ground that 

inhibits the germination of seeds of competing species. In terms of regeneration, 

this form of indirect vegetation management is a gain: in terms of fire hazard it 

is an added risk.

In their book on forest vegetation management for conifer production, Walstad 

and Kuch (1987) stated that quantitative models, which forecast the long-term 

effect of competing vegetation on stand development, are generally lacking. To 

correct this deficiency, they recommended that treatments should be evaluated for 

their (1) ability to suppress competing vegetation, (2) effect on economic crop, (3) 

cost effectiveness, and (4) protection of site resources and other values. This paper 

contributes at least some knowledge to each of these recommendations.

Given 50 or more years of vegetation management in California, have the 

plant species that vigorously compete with young conifer seedlings been markedly 

affected by all the work that has been done? More specifically, have all the release 

techniques that have been used given vegetation managers an “edge” over the many 
76

GENERAL TECHNICAL REPORT PSW-GTR-231

adaptations and capabilities of these species and the communities in which they 

reside? Based on our extensive observations and sampling throughout northern and 

central California, the best answer is “probably,” but only temporarily. Not all areas 

in a plantation are treated, and islands of competing vegetation remain. In addition, 

buried seeds in the soil and rhizomes from edge vegetation can lead to invasion in 

a matter of months. In the short term, and with vigilance, vegetation mangers have 

an edge, but in the long term, the presence and capability of competing species 

remains unaltered across the landscape.

Occasionally, opponents of intensive forest management want to remove or 

severely limit some of the techniques that vegetation managers use. This seems 

paradoxical. Today, the vegetation manager is bombarded from every direction. 

Forest managers, biologists, environmentalists, economists, and politicians demand 

special consideration. We have shown in this paper that almost all of the treatment 

alternatives that we examined have application, or at least feasibility, in some sense. 

In this ever-more-complex world with its myriad of demands, the manager needs 

more tools and methods, not fewer. A major conclusion drawn from our research 

is that plantations in clearcuttings, herbicides, burning, and grazing are feasible at 

some place, at some time, and for some purpose. They should remain in the vegetation manager’s repertory of available methods and techniques. 

Acknowledgments

The authors thank Richard Fitzgerald, John Fiske, and Bill Merrihew for their 

thoughtful, insightful, and comprehensive reviews. We also are grateful to Todd 

Hamilton for producing the computer-assisted charts and figures. 

Metric Equivalents

When you know: Multiply by: To f ind:

Inches (in) 2.54 Centimeters

Feet (ft) .305 Meters

Miles (mi) 1.609 Kilometers

Acre .405 Hectares

Square feet (ft2

) .0929 Square meters

Square feet per acre (ft2 per acre) .229 Square meters per hectare 

Cubic feet (ft3

) .0283 Cubic meters

Degrees Fahrenheit (°F) .55(°F – 32) Degrees Celsius
77

Twenty-Five Years of Managing Vegetation in Conifer Plantations in Northern and Central California

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on high sites in north-central California. Corvallis, OR: Oregon State University. 

309 p. Ph.D. dissertation. 

McDonald, P.M. 1982. Grasses in young conifer plantations—hindrance and help. 

Northwest Science. 60(4): 271–278.

McDonald, P.M. 1983. Clearcutting and natural regeneration: management 

implications for the northern Sierra Nevada. Gen. Tech. Rep. PSW-70. Berkeley, 

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research

Station. 11 p.

McDonald, P.M. 1999. Diversity, density, and development of early vegetation 

in a small clear-cut environment. Res. Pap. PSW-RP-239. Albany, CA: U.S. 

Department of Agriculture, Forest Service, Pacific Southwest Research Station.

22 p.

McDonald, P.M. 2003. Development of a mixed-shrub–planted ponderosa pine 

community on a poor site after site preparation and release. Res. Pap. PSWRP-248. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific

Southwest Research Station. 28 p.

McDonald, P.M.; Abbott, C.S. 1994. Seedfall, regeneration, and seedling 

development in group-selection openings. Res. Pap. PSW-RP-220. Albany, CA: 

U.S. Department of Agriculture, Forest Service, Pacific Southwest Research

Station. 13 p.

McDonald, P.M.; Abbott, C.S. 1997. Vegetation trends in a 31-year-old ponderosa 

pine plantation: effect of different shrub densities. Res. Pap. PSW-RP-231. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest

Research Station. 35 p.
80

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McDonald, P.M.; Abbott, C.S.; Fiddler, G.O. 1999a. Development of a shrubfern-ponderosa pine community eleven years after site preparation and release. 

Western Journal of Applied Forestry. 14(4): 194–199.

McDonald, P.M.; Anderson, P.J.; Fiddler, G.O. 1997. Vegetation in groupselection openings: early trends. Res. Note PSW-RN-421. Albany, CA: U.S. 

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McDonald, P.M.; Everest, G.A. 1996. Response of young ponderosa pines, 

shrubs, and grasses to two release treatments. Res. Note PSW-RN-419. Albany, 

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research

Station. 7 p.

McDonald, P.M.; Fiddler, G.O. 1986a. Release of Douglas-fir	seedlings:	growth

and treatment costs. Res. Pap. PSW-182. Berkeley, CA: U.S. Department of 

Agriculture, Forest Service, Pacific Southwest Research Station.	9	p.

McDonald, P.M.; Fiddler, G.O. 1986b. Weed treatment strategies to control losses 

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on forest pest management in southwest Oregon. Corvallis, OR: Oregon State 

University: 47–53. 

McDonald, P.M.; Fiddler, G.O. 1989. Competing vegetation in ponderosa pine 

plantations: ecology and control. Gen. Tech. Rep. PSW-113. Berkeley, CA: U.S. 

Department of Agriculture, Forest Service, Pacific Southwest Research Station.

26 p.

McDonald, P.M.; Fiddler, G.O. 1990. Ponderosa pine seedlings and competing 

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10 p.

McDonald, P.M.; Fiddler, G.O. 1993a. Vegetative trends in a young conifer 

plantation after 10 years of grazing by sheep. Res. Pap. PSW-RP-215. Albany, 

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research

Station. 9 p.

McDonald, P.M.; Fiddler, G.O. 1993b. Feasibility of alternatives to herbicides in 

young conifer plantations in California. Canadian Journal of Forest Research. 

23: 2015–2022. 
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McDonald, P.M.; Fiddler, G.O. 1995. Development of a mixed shrub–ponderosa 

pine community in a natural and treated condition. Res. Pap. PSW-224. Albany, 

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research

Station. 19 p.

McDonald, P.M.; Fiddler, G.O. 1996. Development of a mixed shrub–tanoak–

Douglas-fir community in	a	treated and untreated condition. Res. Pap. PSWRP-225. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific

Southwest Research Station. 16 p.

McDonald, P.M.; Fiddler, G.O. 1997b. Treatment duration and time since 

disturbance affect vegetation development in	a	young California red fir

plantation. Res. Pap. PSW-RP-233. Albany, CA: U.S. Department of Agriculture, 

Forest Service, Pacific Southwest Research Station. 14 p.

McDonald, P.M.; Fiddler, G.O. 1997. Vegetation trends in a young ponderosa 

pine plantation treated by manual release and mulching. Res. Pap. PSW-RP-234. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest

Research Station. 15 p.

McDonald, P.M.; Fiddler, G.O. 1999a. Effect of cattle grazing, seeded grass, and 

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Southwest Research Station. 15 p.

McDonald, P.M.; Fiddler, G.O. 1999b. Ecology and development of Douglas-fir

seedlings and associated plant species in a Coast Range plantation. Res. Pap. 

PSW-RP-243. Albany, CA: U.S. Department of Agriculture, Forest Service, 

Pacific Southwest Research Station. 18 p.

McDonald, P.M.; Fiddler, G.O. 1999c. Recovery of a bearclover (Chamaebatia 

foliolosa) plant community after site preparation and planting of ponderosa 

pine seedlings. Res. Note PSW-RN-423. Albany, CA: U.S. Department of 

Agriculture, Forest Service, Pacific Southwest Research Station.	7	p.

McDonald, P.M.; Fiddler, G.O. 2001a. Timing and duration of release affect 

vegetation development in	a	young California white fir plantation. Res. Pap.

PSW-RP-246. Albany, CA: U.S. Department of Agriculture, Forest Service, 

Pacific Southwest Research Station. 15 p.
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McDonald, P.M.; Fiddler, G.O. 2001a. Timing and duration of release affect 

vegetation development in a young ponderosa pine plantation. Res. Pap. PSWRP-245. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific

Southwest Research Station. 14 p.

McDonald, P.M.; Fiddler, G.O. 2001b. Changes in plant communities after 

planting and release of conifer	seedlings:	early findings. In: Barras, S.J. ed.,

Proceedings of the national silvicultural workshop. RMRS-P-19. Fort Collins, 

CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

Station: 26–31.

McDonald, P.M.; Fiddler, G.O. 2002. Relationship of native and introduced 

grasses with and without cattle in a young ponderosa pine plantation. Western 

Journal of Applied Forestry. 17(1): 31–36.

McDonald, P.M.; Fiddler, G.O. 2006. Plant species diversity in young conifer 

plantations in northern and central California. Western Journal of Applied 

Forestry. 21(1): 49–54.

McDonald, P.M.; Fiddler, G.O. 2007. Development of vegetation in a young 

ponderosa pine plantation: effect of treatment duration and time since 

disturbance. Res. Pap. PSW-RP-251. Albany, CA: U.S. Department of 

Agriculture, Forest Service, Pacific Southwest Research Station. 23 p.

McDonald, P.M.; Fiddler, G.O.; Harrison, H.H. 1994a. Mulching to regenerate 

a	harsh site: effect on Douglas-fir	seedlings,	forbs,	grasses,	and ferns. Res. Pap.

PSW-RP-222. Albany, CA: U.S. Department of Agriculture, Forest Service, 

Pacific Southwest Research Station. 10 p.

McDonald, P.M.; Fiddler, G.O.; Henry, W.T. 1994b. Large mulches and manual 

release enhance growth of ponderosa pine seedlings. New Forests. 8: 169–178. 

McDonald, P.M.; Fiddler, G.O.; Kitzmiller, J.H. 1994c. Genetically improved 

ponderosa pine seedlings outgrow nursery-run seedlings with and without 

competition—early findings. Western Journal of Applied Forestry. 9(2): 57–61.

McDonald, P.M.; Fiddler, G.O.; Meyer, P.W. 1996. Vegetation trends in a young 

conifer plantation after grazing, grubbing, and chemical release. Res. Pap. PSWRP-228. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific

Southwest Research Station. 17 p.
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bearclover (Chamaebatia foliolosa) in northern and central California: the status 

of our knowledge. Gen. Tech. Rep. PSW-GTR-190. Albany, CA: U.S. Department 

of Agriculture, Forest Service, Pacific Southwest Research Station. 26 p.

McDonald, P.M.; Fiddler, G.O.; Ritchie, M.W.; Anderson, P.J. 2009. Naturally 

seeded versus planted ponderosa pine seedlings in group-selection openings. 

Western Journal of Applied Forestry. 24(1): 48–54. 

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fail to enhance Douglas-fir seedling survival and	growth.	In:	Proceedings	of

the 10th annual forest vegetation management conference. Redding, CA: Forest 

Vegetation Management Conference: 140–153. 

McDonald, P.M.; Fiske, J.N. 2000. An indirect strategy for controlling 

undesirable vegetation in young conifer stands. In: Proceedings of the 3rd 

annual PNW-Integrated Vegetation Management Association forestry and 

rights-of-way conference. Pullman, WA: Washington State University: 46–51.

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and Oregon forests: past, present, and future. Gen. Tech. Rep. PSW-123. 

Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest

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McDonald, P.M.; Laurie, W.D.; Hill, R. 1998. Early growth characteristics of 

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