United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

General Technical

Report

PNW-GTR-718

August 2007

A Synthesis of the Literature

on the Biology, Ecology, and

Management of Western

Hemlock Dwarf Mistletoe

John A. Muir and Paul E. Hennon
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Authors

John A. Muir is a forest pathologist and owner, John Muir Consulting, 2031 Casa

Marcia Crescent, Victoria, BC V8N 2X5; and Paul E. Hennon is a research plant

pathologist, U.S. Department of Agriculture, Forest Service, Forest Health Protection, State and Private Forestry, and Pacific Northwest Research Station, 2770

Sherwood Lane, Suite 2A, Juneau, AK 99801-8545.

Cover photo

Dead western hemlock tree with numerous hemlock dwarf mistletoe infections.

Tree death resulted in a canopy gap.
Abstract

Muir, John A.; Hennon, Paul E. 2007. A synthesis of the literature on the

biology, ecology, and management of western hemlock dwarf mistletoe. Gen.

Tech. Rep. PNW-GTR-718. Portland, OR: U.S. Department of Agriculture,

Forest Service, Pacific Northwest Research Station. 142 p.

Hemlock dwarf mistletoe (Arceuthobium tsugense [Rosendahl] G.N. Jones) is a

small, inconspicuous parasite that has significant effects on tree growth and stand

structure in coastal forest ecosystems of western North America. Most previous

research focused on the effects of hemlock dwarf mistletoe on timber production.

Previous clearcut harvesting of large areas that removed virtually all infected trees

and forestry practices that established even-aged stands of trees effectively prevented

or minimized future hemlock dwarf mistletoe impacts. Under this regime, further

research on hemlock dwarf mistletoe was considered unnecessary. However, current

forestry practices that restrict clearcut harvesting to small openings and retain live

trees to preserve attributes of old-growth forests create conditions that appear highly

favorable for enhanced seed production by hemlock dwarf mistletoe, early spread of

the mistletoe to infect young trees, and, consequently, increased growth impacts to

residual trees over time. More information is needed on the biology and impacts of

hemlock dwarf mistletoe in coastal western hemlock retention-harvested forests in

the United States of America and Canada. Further work is recommended to develop

sampling and monitoring procedures to determine hemlock dwarf mistletoe spread

and impacts. We also need to investigate several unusual aspects of hemlock dwarf

mistletoe biology and development such as long-distance seed dispersal and persistence in old-growth forests. Detailed tree, stand, and forest-level models are needed

to monitor and project hemlock dwarf mistletoe effects over a wide range of

ecological conditions and management regimes in coastal forests.

Keywords: Disease management, Arceuthobium, selection harvest, retention

harvest, disease impact.
Contents

1 Introduction

4 Previous Studies

5 Objectives

6 Hemlock Dwarf Mistletoe Biology

6 Overview

6 Taxonomy, Subspecies and Host Tree Susceptibilities

10 Geographic Range

14 Aerial Shoots and Flowering

17 Pollination and Fruiting

18 Seed Dispersal

18 Seed Retention and Germination

19 Penetration and Infection

19 Symptoms

24 Rate of the Reproductive Cycle

24 Factors Affecting Biology and Seed Dispersal

24 Local Seed Dispersal

27 Long-Distance Seed Dispersal

29 Mortality Factors and Use of Biological Control

30 Host Resistance to Infection

31 Effects of Dwarf Mistletoe on Individual Trees

31 Tree Physiology and Form

35 Tree Growth

37 Tree Mortality

39 Wood Quality

40 Wood Decay

40 Tree Hazards

42 Effects on Trees, Stands, and Forests

42 Infection Rating Systems

44 Effects on Growth of Trees and Stands

55 Forest-Level Effects on Growth

57 Ecology

57 Ecological Factors and Geographic Distribution
62 Disturbance Regimes

63 Stand Structure and Species Composition

67 Effects of Stand Development

77 Wildlife Habitat and Biodiversity

80 Management of Hemlock Dwarf Mistletoe-Infested Western Hemlock Forests

81 Clearcut Harvesting and Even-Aged Stands

93 Selection or Retention Harvesting and Uneven-Aged Stands

106 Needs for Further Research

108 Diverse Opinions on Hemlock Dwarf Mistletoe Impacts

111 Conclusions

112 Acknowledgments

113 English Equivalents

114 Literature Cited

139 Appendix 1: Common and Scientific Names

142 Appendix 2: Hemlock Dwarf Mistletoe Research Topics or Needs for

Further Study
This Page Left Blank Intentionally
1

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Introduction

In coastal forests of western North America from northern California to southern

Alaska, hemlock dwarf mistletoe (Arceuthobium tsugense (Rosendahl) G.N. Jones)1

is a common but inconspicuous parasite of western hemlock and occasionally

other tree species (Geils et al. 2002, Hawksworth and Wiens 1996). Arceuthobium

tsugense consists of four subspecies that occur on and are named after their principal host tree species: western hemlock, mountain hemlock, shore pine, and Pacific

silver fir (Mathiasen and Daughtery 2007, Wass and Mathiasen 2003). The western

hemlock subspecies that predominates in most western hemlock coastal forests has

been the subject of most of the literature on hemlock dwarf mistletoe and is the

major subspecies of concern for forest management. For convenience in this review, “hemlock dwarf mistletoe” is used to designate the western hemlock subspecies unless specified otherwise, and A. tsugense is used to designate the whole

species.

Arceuthobium tsugense is one of approximately 30 species of dwarf mistletoe

in North America (Geils et al. 2002, Hawksworth and Wiens 1996, Nickrent et al.

2004). Dwarf mistletoes are flowering plants that live as perennial parasites in

living tree tissue. Although they produce small aerial shoots with some chlorophyll,

they photosynthesize at a very low rate (Hull and Leonard 1964a, 1964b; Leonard

and Hull 1965; Miller and Tocher 1975; Tocher et al. 1984) and lack phloem elements to distribute any photosynthates (Kuijt 1960; Srivastava and Esau 1961a,

1961b). Dwarf mistletoes are believed to derive most of their nutrients from parasitism of their coniferous hosts (Hawksworth and Wiens 1996; Hull and Leonard

1964a, 1964b). Dwarf mistletoes spread within trees and from tree to tree by

explosively discharged seeds that are covered with a natural glue-like substance

called viscin. Most seeds that hit foliage and lodge on tree bark germinate, producing a small root-like structure that penetrates into the bark. Microscopic threads

of the mistletoe grow through the inner bark and sapwood. The infection produces

a swelling of an infected tree branch or bole. Aerial shoots and flowers are produced from the swollen bark 2 to 3 years after infection. Berries with a single

seed mature a year after pollination and forcibly discharge seed in autumn. Over

periods of several decades to centuries, hemlock dwarf mistletoe can proliferate on

1

 Hereafter see appendix 1 for common and scientific names of species.

Arceuthobium

tsugense is one of

approximately 30

species of dwarf

mistletoe in North

America.
2

GENERAL TECHNICAL REPORT PNW-GTR-718

and between trees, developing large swellings and fan-shaped masses of swollen

branches called witches’ brooms. As trees become older and more severely infected, they experience growth reductions in annual growth of bole diameter and

height of 30 to 40 percent or more.

Hemlock dwarf mistletoe is an ecologically important component of western

hemlock forests from southern Oregon to southeastern Alaska. After disturbances

by windstorms or wildfires, hemlock dwarf mistletoe that survives on live residual

trees can spread to infect nearby regenerated trees. Patches of infected trees expand, and after several decades to centuries, severely infected trees begin to die,

are windthrown, and/or break off. This creates gaps in the forest canopy that allow

new trees to establish and grow, and hemlock dwarf mistletoe spreads to infect

these new trees. These processes and their effects affect the growth of other tree

species and the biology of associated birds, animals, and other organisms. Many of

the effects of dwarf mistletoes on trees and forest stand structures are beneficial for

wildlife habitat and populations (Geils et al. 2002, Hawksworth and Wiens 1996).

These kinds of effects for A. tsugense have received limited study but could be

important for some areas and wildlife species.

In contrast to several reports of herbal uses and possible medicinal properties

for leafy mistletoes in Europe, dwarf mistletoes including hemlock dwarf mistletoe, have few herbal uses or medicinal properties (Hawksworth and Wiens 1996).

Smith (1928) reported that First Nation people near Bella Coola, British Columbia,

used the shoots of hemlock dwarf mistletoe to treat hemorrhages and tuberculosis.

However, reports of medicinal properties or therapeutic uses of leafy and dwarf

mistletoes have not been substantiated by clinical trials (National Cancer Institute

2006).

Arceuthobium tsugense has been a long-term component of the extensive western hemlock forests of western North America. Hawksworth (1978) concluded

from fossil evidence that dwarf mistletoe species and western conifers in North

America have coevolved since at least the Miocene period, or about 25 million

years. Hemlock dwarf mistletoe pollen deposited over the last several thousand to

30,000 years ago has been reported from several coastal regions (Greenwald and

Brubaker 2001; Hansen and Easterbrook 1974; Hebda 1983; Heusser 1973a, 1973b,

1974, 1977, 1978, 1983; Mathewes 1973, Mathewes and Rouse 1975; Petersen et

al. 1983). Thus, A. tsugense has been an integral component of forest ecosystems in

western coastal North America.

Despite the widespread occurrence and significant effects of A. tsugense, most

descriptions of west coast forests and ecological processes, e.g., Franklin et al.
3

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

(2002) and MacKinnon (2005), include very little or no mention of A. tsugense as

a significant ecological component. An analysis of forest succession based on eight

old-growth forest chronosequence installations on southern Vancouver Island included a report (Garbutt and Allen 1998) that five of the eight installations in

which western hemlock predominated had hemlock dwarf mistletoe in the immature to old-growth stages. However, the effects of hemlock dwarf mistletoe on

succession and ecology of coastal forests were not described. Franklin et al. (2002)

noted that wind, insects, and diseases were causes of tree mortality that result in

canopy gaps and creation of old-growth conditions. However, the ecological role

of hemlock dwarf mistletoe was only briefly described (Franklin et al. 2002) as

contributing to development of decadence by creating top breakage, wood decay,

and wounds in living western hemlock trees.

Arceuthobium tsugense is the only western mistletoe in mild, wet coastal western hemlock forests and has several unique and interesting biological features compared to other dwarf mistletoes. It has one of the most extensive latitudinal ranges

of any dwarf mistletoe species (Hawksworth and Wiens 1996). Most other dwarf

mistletoe species grow inland in relatively open or low-density stands of pines or

other tree species in regions with continental climates. In these inland forests, soil

moisture usually is a limiting factor for tree growth and dwarf mistletoe development. In extremely dense and shaded western hemlock coastal forests, light rather

than rainfall is a limiting factor, particularly for growth of aerial shoots and seed

production of hemlock dwarf mistletoe (Shaw and Weiss 2000). Arceuthobium

tsugense is also notable in that its four subspecies occur in very different forest

ecosystems. The western hemlock subspecies occurs in western hemlock forests in

humid to hyperhumid coastal maritime climates; the subspecies in shore pine stands

occurs in “rainshadow,” low-rainfall coastal climates; and the mountain hemlock

and pacific silver fir subspecies, in high-elevation, subalpine climates.

Most of the previous research on hemlock dwarf mistletoe focused on the

effects this parasite has on tree growth and timber production, and was limited to

infection of relatively small trees in clearcut areas in only a few localities. Previous

clearcut harvesting removed virtually all infected and uninfected trees, and forestry

practices maintained dense, even-aged stands. Under these conditions, hemlock

dwarf mistletoe incidence was substantially reduced in second-growth stands, effects on tree growth were effectively prevented or minimized, and further research

on hemlock dwarf mistletoe appeared unnecessary.

Recently, forestry practices have changed, often drastically. In Washington and

Oregon, harvesting of old-growth western hemlock forests on federal government
4

GENERAL TECHNICAL REPORT PNW-GTR-718

lands is virtually prohibited with only limited selection harvesting. In other regions

such as British Columbia and southeastern Alaska, these new practices include

restriction of clearcut harvesting to small openings (in some instances prohibition

of clearcut harvesting), reservation of large blocks of old-growth forest, and

selection and retention harvesting and silvicultural systems. These measures are

intended to preserve attributes of the residual forest for wildlife and biodiversity

conservation, but they often result in retention of mistletoe-infected trees. These

conditions appear highly favorable for increased and earlier spread of hemlock

dwarf mistletoe to young trees and ultimately, increased growth impacts. In some

areas, these new forestry practices are being implemented apparently with little or

no consideration of the potential effects on hemlock dwarf mistletoe. Hemlock

dwarf mistletoe effects in small openings and in retention-harvested forests have

not yet been determined.

Further, in coastal western North America, there is a wide range of forest conditions, and in many of these areas, we know relatively little about potential spread

and impacts of A. tsugense and the effects of using these new practices. There is a

wide diversity of views and opinions among forestry professionals about existing

and future impacts of hemlock dwarf mistletoe under these new scenarios. The

changing management needs, the relative lack of information on hemlock dwarf

mistletoe behavior, and the apparently wide variability of hemlock dwarf mistletoe

effects over the extensive geographic range of hemlock dwarf mistletoe prompted

us to review the scientific literature and other information for hemlock dwarf

mistletoe.

Previous Studies

For this review, we identified and cited approximately 300 reports that included

information directly or closely related to aspects of hemlock dwarf mistletoe ecology and management. Geils et al. (2002), Hawksworth and Wiens (1996), and

Kuijt (1955) previously reviewed dwarf mistletoe literature, but a large number

of published reports on hemlock dwarf mistletoe were not included or were published subsequently. Hennon et al. (2001) and Muir et al. (2004b) summarized

information on hemlock dwarf mistletoe with emphasis on preventing or reducing impacts by forest management during harvesting and silvicultural treatments.

Literature on hemlock dwarf mistletoe has a lengthy history beginning with early

studies by Drake (1915), Rosendahl (1903), Weir (1915, 1916, 1918), and others.

Hildebrand (1995) reviewed hemlock dwarf mistletoe status in old-growth forests

of Washington and Oregon. At the Wind River Canopy Crane Research Facility in

There is a wide

diversity of views

and opinions among

forestry professionals about existing

and future impacts

of hemlock dwarf

mistletoe under

these new

scenarios.
5

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

southern Washington, Shaw and several colleagues (Mathiasen and Shaw 1998;

Meinzer et al. 2004; Shaw and Weiss 2000; Shaw et al. 2000, 2005) studied hemlock dwarf mistletoe in the upper canopy of an old-growth forest. Shaw et al.

(2004) compared Australian mistletoes and western North American dwarf mistletoes, supporting a proposal by Watson (2001) that mistletoes are keystone species.

A shorter report tailored to hemlock dwarf mistletoe and forest management in

British Columbia (Muir et al. 2007) was based on this review.

Objectives

This paper reviews information about A. tsugense and particularly hemlock dwarf

mistletoe from available scientific literature, unpublished data, and observations

by the authors. The information is integrated here with an emphasis on predicting

the behavior of the parasite and its impact in coastal forests of northwest North

America, particularly under selection or retention harvesting. In this report, we

suggest how hemlock dwarf mistletoe will behave and could be managed under

these new forestry management regimes. Another major objective is to review

how hemlock dwarf mistletoe contributes to and is important for stand structure,

biodiversity, and wildlife habitat. Recently, Shaw et al. (2004) suggested that dwarf

mistletoes represent keystone species based on their similar association with many

birds and other species as previously proposed for leafy mistletoes (Cooney et al.

2006, Watson 2001). Several reports reviewed here suggest that hemlock dwarf

mistletoe might have a similar ecological function. We review how natural disturbances and forest management affect hemlock dwarf mistletoe populations, apparently in predictable ways that can be adapted for forest management. We identify

knowledge gaps and research needs for hemlock dwarf mistletoe, particularly

related to biology, ecology, and growth effects of hemlock dwarf mistletoe, and

management of hemlock dwarf mistletoe under the new selection and retention

harvesting regimes.

Forestry professionals will often find that the distribution and local incidences

of hemlock dwarf mistletoe are highly variable, and at times, technical information

and management guidelines are contradictory or controversial. We have attempted

in this review to present the various results and opinions and suggest how they can

be rationalized and possibly reconciled. We hope our approach provides information and perspectives that are valuable for interested people, researchers, and

forestry professionals.

Our objectives are to (1) compile and review recent information and literature

on hemlock dwarf mistletoe, (2) highlight the diversity of data and opinion on

We review how

natural disturbances

and forest management affect hemlock

dwarf mistletoe

populations, apparently in predictable

ways that can be

adapted for forest

management.
6

GENERAL TECHNICAL REPORT PNW-GTR-718

hemlock dwarf mistletoe biology and management, and (3) identify topics that

require further research or study to effectively manage coastal forests with hemlock

dwarf mistletoe.

Hemlock Dwarf Mistletoe Biology

Overview

In North America, several dwarf mistletoes and a few leafy mistletoes

(Phoradendron spp.) are parasites of coniferous trees (Geils et al. 2002). Dwarf

mistletoes are perennial, parasitic flowering plants that live in the cortex and

sapwood of trees. In contrast to the leafy mistletoes, dwarf mistletoes produce

aerial shoots with leaves that are reduced to small squamate scales. Shoots have

a small amount of chlorophyll, but dwarf mistletoes derive most of their carbohydrates, water, and mineral nutrients directly from their coniferous hosts

(Hawksworth 1978). The perennial absorptive structure of dwarf mistletoes is

called the endophytic system. It is embedded in the host cortex and sapwood.

Dwarf mistletoes do not have conducting tissue (phloem) that could transport

nutrients from their aerial shoots to their endophytic system.

Dwarf mistletoe flowers and berries are produced on aerial shoots that develop

from the endophytic system. Berries explosively discharge a single sticky seed to

distances of up to 15 m. Dwarf mistletoe seeds land on the foliage of a suitable

tree, lodge on the bark, and germinate in the spring. If infection occurs, a swelling,

aerial shoots, and flowers develop in 3 to 5 years. Over a period of several decades,

hemlock dwarf mistletoe infection produces a pronounced swelling of the infected

tissues and often an extensive growth of tree branches termed witches’ brooms

(fig. 1). Depending on tree species, ecological conditions, and disturbances, in

some instances—often over a period of several decades to centuries—dwarf mistletoes can spread extensively within tree crowns and from tree to tree. Numerous

and/or large dwarf mistletoe infections can substantially reduce the growth of

infected trees. In some instances, mistletoe spread is slow or limited and effects

on trees appear minimal.

Taxonomy, Subspecies and Host Tree Susceptibilities

Hawksworth and Wiens (1996) reviewed the taxonomy and nomenclature of dwarf

mistletoes in detail. Rosendahl (1903) first described hemlock dwarf mistletoe as a

species from a collection on Vancouver Island, British Columbia. It was named
7

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Razoumofskya tsugensis, then briefly Arceuthobium campylopodum forma

tsugensis (Rosendahl) Gill (Gill 1935). Jones (1936) gave it species status again as

Arceuthobium tsugensis (Rosendahl) G.N. Jones, the name that is still recognized.

The specific epithet was changed to “tsugense” to agree with the gender of the

genus as required by the Botanical Rules of the International Code of Botanical

Nomenclature (Hawksworth and Wiens 1996).

The taxonomy and nomenclature of hemlock dwarf mistletoe have been controversial. Recently, Douglas et al. (2000) grouped it and the dwarf mistletoes on

true fir (A. abietinum Engelm. ex Munz) and western larch (A. laricis (Piper) St.

John) as “A. campylopodium [sic] Engelmann” on the basis that there were “ ...no

reliable morphological differences to distinguish them.” However, we prefer the

taxonomy and nomenclature of hemlock dwarf mistletoe as a distinct species and

subspecies as outlined by Hawksworth and Wiens (1996) and Wass and Mathiasen

(2003). We believe that the subspecies designations have substantial utility for

ecological studies and management of A. tsugense and hemlock dwarf mistletoe.

Several subspecies of A. tsugense have been described (Hawksworth and Wiens

1996, Hawksworth et al. 1992, Wass and Mathiasen 2003) based on small but significant differences in morphological features, phenology of flowering and seed

Figure 1—Western hemlock subspecies of

Arceuthobium tsugense caused infections

(witches’ brooms) throughout the crown of this

mature western hemlock tree.
8

GENERAL TECHNICAL REPORT PNW-GTR-718

dispersal, and differences in susceptibility of their hosts to infection. The currently

described subspecies include (1) western hemlock dwarf mistletoe (fig. 2), (2)

shore pine dwarf mistletoe (fig. 3), and (3) mountain hemlock dwarf mistletoe (fig.

4) (Wass and Mathiasen 2003). Another subspecies on Pacific silver fir in a limited

distribution in central Oregon was recently described by Mathiasen and Daugherty

(2007).

Arceuthobium tsugense infects several host tree species (Hawksworth and Wiens

1996, Mathiasen 1994, Mathiasen and Daugherty 2005, 2007, Wass and Mathiasen

2003). The susceptibilities of common tree species in western coastal forests

associated with the three currently described subspecies of A. tsugense are shown in

table 1.

Recent studies of susceptibility of several host tree species to hemlock dwarf

mistletoe indicated that A. tsugense subspecies have a narrow or limited host range

(Mathiasen and Daugherty 2005, Mathiasen and Hawksworth 1988, Wass and

Mathiasen 2003). Mathiasen and Daugherty (2005) concluded that in stands where

a high proportion of western hemlock trees are infected with the western hemlock

subspecies, “...any tree species other than western hemlock can be retained without

expecting severe infection to occur.” Further work is needed to confirm this conclusion for other subspecies and regions, but results of experimental inoculations

(Smith 1971; Smith and Wass 1976, 1979; Wass and Mathiasen 2003) indicate that

this is true for all of the hemlock dwarf mistletoe subspecies.

Figure 2—Western hemlock subspecies of Arceuthobium

tsugense.

Robert Armstrong
9

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Figure 3—Shore pine subspecies of Arceuthobium tsugense.

R.L. Mathiasen

Figure 4—Mountain hemlock subspecies of Arceuthobium tsugense.

R.L. Mathiasen
10

GENERAL TECHNICAL REPORT PNW-GTR-718

Table 1—Susceptibilities of common western conifers to the subspecies of Arceuthobium tsugense

Susceptibilityb to A. tsugense subspecies

Common coastal Western Mountain

tree speciesa hemlockc Shore pined hemlocke Pacific silver firf

Western hemlock Principal Occasional Rare Occasional

Shore pine (var. contorta) Rare Principal ND ND

Lodgepole pine (var. latifolia) ND Occasionalg Rare ND

Lodgepole pine (var. murrayana) Immuneb ND Immune ND

Mountain hemlock Occasional ND Principalc Principalf

Pacific silver fir Occasional ND Principalc Principalf

Grand fir Rare ND Rare Rare

Subalpine fir Rare ND Principalc Secondaryf

Noble fir Occasional ND Principalc Principalf

Sierra white fir ND ND ND Occasional

Sitka spruce Rare ND ND ND

Engelmann spruce Rareg ND ND ND

Douglas-fir Rare Immune Immune ND

Western white pine Raref Rare Occasionalh Raref

Whitebark pine ND ND Secondaryb NDf

a

 Tree species commonly occurring within the geographic range of A. tsugense. Totally immune species such as western

redcedar are not listed. For scientific names, see appendix 1.

b

 Based on susceptibility classification by Hawksworth and Wiens (1972) of proportion of trees infected within 6 m of a

severely infected host tree: principal host is 90 or more percent infected; secondary, 50 to 90 percent; occasional, 5 to 50

percent; rare, 1 to 5 percent; immune, 0 percent. ND indicates no data or unknown.

c

 Mathiasen and Daugherty (2005)

d

 Wass and Mathiasen (2003)

e Hawksworth and Wiens (1996)

f Mathiasen and Daugherty (2007)

g

 Mathiasen (1994)

h

 Kuijt (1955), Hawksworth et al. (1992)

Geographic Range

Hemlock dwarf mistletoe has a natural geographic range from northern California,

through Oregon, Washington, and British Columbia to southeastern Alaska (fig.

5). The most southerly occurrence is in California, at approximately 38 degrees

latitude (Hawksworth and Wiens 1996, Mathiasen and Marshall 1999) and the

most northerly occurrence near Chilkoot Lake, Alaska, at approximately 59 degrees

(Drummond and Hawksworth 1979). Hemlock dwarf mistletoe occurs farther north

than any other species of Arceuthobium and over one of the largest latitudinal

ranges.

As described by Hawksworth and Wiens (1996) and Wass and Mathiasen

(2003), the western hemlock subspecies extends from a few localities in northern
11

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

California to southeastern Alaska (fig. 6). Only a few western hemlock trees are

infected in California (Mathiasen and Marshall 1999). The shore pine subspecies

occurs from a few locations on the San Juan Islands in northern Washington to

approximately Campbell River, British Columbia (fig. 7) (Wass 1976, Wass and

Mathiasen 2003). The mountain hemlock subspecies occurs in a few localities in

northern California (Mathiasen and Marshall 1999), more commonly in Oregon,

and in only a few locations in Washington and southern British Columbia

(Hawksworth and Wiens 1996) (fig. 8). The recently described Pacific silver

fir subspecies has a small distribution limited to central Oregon (Mathiasen and

Daughtery 2007).

Generally, hemlock dwarf mistletoe is abundant on the coastal mainland and on

all large and most small islands wherever western hemlock is found. In British

Columbia, Alfaro (1985) estimated that 15 percent of western hemlock stands are

infested by hemlock dwarf mistletoe, mostly in a band about 150 km wide along

the coast. It occurs farther east in watersheds of major rivers such as the Columbia

River between Washington and Oregon and the Skeena River in British Columbia.

Approximately 51 percent of inventory sample plots on Vancouver Island, British

Figure 5—General geographic distribution of Arceuthobium tsugense (from

Hawksworth and Wiens 1996) from

northern California to southeast Alaska.
12

GENERAL TECHNICAL REPORT PNW-GTR-718

a b

Figure 6—Geographic distribution of western hemlock subspecies of Arceuthobium tsugense: (a) Alaska and British Columbia;

(b) Washington, Oregon, and California (from Hawksworth and Wiens 1996).
13

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Figure 7—Geographic distribution of shore pine

subspecies of Arceuthobium tsugense (from

Hawksworth and Wiens 1996).

Figure 8—Geographic distribution of mountain hemlock

subspecies of Arceuthobium tsugense (from Hawksworth

and Wiens 1996).
14

GENERAL TECHNICAL REPORT PNW-GTR-718

Columbia, were infested by hemlock dwarf mistletoe (Beale 1985). The distribution of A. tsugense and several other Arceuthobium species in Canada were further

described and mapped by Kuijt (1963), Van Sickle and Smith (1978), and Wood

(1986). In southeast Alaska, vegetation plot data from an extensive 5- by 5-km

grid indicated a widespread occurrence of hemlock dwarf mistletoe in the southeast but not north of Haines or farther to the northwest along the Gulf of Alaska

even though western hemlock grows in these areas (fig. 9).

Aerial Shoots and Flowering

In 1 to 2 years after infection of the host branch or bole, aerial shoots of dwarf

mistletoe develop from the endophytic system in a swelling and emerge from the

bark. Aerial shoots are perennial, but most usually live for only 4 to 8 years. Infections without aerial shoots can continue to grow through live host tissues for many

years (Trummer et al. 1998, Weir 1916), producing large swellings and witches’

brooms, and then after a disturbance, can resume production of aerial shoots.

Dwarf mistletoes were called “light-loving plants” by Weir (1916) because

of the tendency for rapid aerial shoot growth when a stand is partially thinned.

In contrast, Wagener (1961) suggested that partial sunlight was more favorable to

the establishment of dwarf mistletoes on their hosts. With another species of dwarf

mistletoe, Knutson (1984) found the greatest number of aerial shoots with the least

amount of shade (i.e., maximum light) in an experiment with seedlings, suggesting

that dwarf mistletoe growth is enhanced by higher light levels. Studying other species of dwarf mistletoe, Scharpf (1972) found that light also enhanced the infection

process. Baranyay (1962) indicated that increased light levels, e.g., from thinning,

encouraged reproduction in previously dormant infections of hemlock dwarf

mistletoe.

For hemlock dwarf mistletoe, growth and persistence of aerial shoots in oldgrowth trees is correlated with position in the tree crown (Shaw and Weiss 2000,

Smith 1969). Most of the hemlock dwarf mistletoe infections with aerial shoots

are situated in the upper third of the crown (Smith 1969). In dense coastal western

hemlock forests, light can be a limiting factor and shoot abundance is correlated

with the intensity of incident light. At the Wind River Crane Research Facility in

the state of Washington, Parker (1997) divided an old-growth Douglas-fir/western

hemlock forest canopy of 65 m height into three vertical light regimes based on the

proportion of incoming light. These were (1) bright zone above 40 m height with

90 percent or more available radiation, (2) transition zone where available light

decreases rapidly from 90 percent at 40 m height to 10 percent at 8 m height, and

Dwarf mistletoes

were called “lightloving plants” by

Weir (1916) because

of the tendency for

rapid aerial shoot

growth when a stand

is partially thinned.
15

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Figure 9—Occurrence of hemlock dwarf mistletoe in Alaska based on 5-km grid Forest Inventory and Analysis

vegetation plots (wilderness areas not sampled).
16

GENERAL TECHNICAL REPORT PNW-GTR-718

(3) dim zone below 8 m with less than 10 percent of available light. Shaw and

Weiss (2000) determined that all dwarf mistletoe infections at heights of 50 to 55

m had shoots ≥1 cm height and infections below 30 m height had no shoots. At 40

m height where 50 percent of the infections had shoots, yearly insolation was 2,200

MJ m-2 based on hemispherical photographs taken above each infection. Shaw and

Weiss (2000) noted that their results were contrary to findings from several previous studies of aerial shoot production on seedlings with hemlock dwarf mistletoe

and other dwarf mistletoes, which indicated that some light was required for shoot

production. Shaw and Weiss (2000) suggested that in the mid- to upper crowns of

old-growth hemlock trees, the higher temperatures, photosynthetic capacity, and

nitrogen content of foliage reported by several previous authors could enhance

mistletoe shoot production. Light requirements for mistletoe shoot production on

seedlings in experimental environments might also be anomalous compared to those

for old-growth trees.

Dwarf mistletoes, including hemlock dwarf mistletoe, are dioecious, i.e., either

staminate (male) or pistillate (female) flowers on separate plants (fig. 10). Staminate shoots are usually light yellow-green, and pistillate shoots are yellow-green to

dark purple. In some hemlock dwarf mistletoe populations, the number of pistillate

and staminate plants was reported to be unequal (Wiens et al. 1996), but this was

probably attributed to the sampling method. Wiens et al. (1996) sampled plants that

were within 3 m of the ground as these could easily be reached. Pistillate plants

tend to survive (remain on the infected branches) longer than male plants in low

light conditions, so near the ground more female infections have aerial shoots and

thus are identifiable.2

 Results of a more recent study that sampled hemlock dwarf

mistletoe infections over the whole tree height indicated that the sex ratio was 1:1

(Mathiasen and Shaw 1998). Male flowers are 2 to 3 mm in diameter and have

sessile anthers, each containing 1,000 to 4,000 pollen grains (Hawksworth 1978).

After staminate flowers open they remain open, but anthers open and close in response to changes in temperature and relative humidity (Gilbert and Punter 1990).

Pollen is dispersed in clumps of 20 to 100 pollen grains. Pistillate flowers are

small, approximately 1 mm in diameter, and very inconspicuous. Both staminate

and pistillate flowers have nectaries, which are attractive to insects. Several studies

indicate that the larger staminate flowers have more visitations by insects. Flowering occurs from June to August (Carpenter et al. 1979, Smith 1971).

2

 Mathiasen, R.L. 2006. Personal communication, Professor of Forest Ecosystem Health,

School of Forestry, Northern University Arizona, Flagstaff, AZ 86011.
17

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Pollination and Fruiting

The pollination process is poorly understood in hemlock dwarf mistletoe and other

Arceuthobium species, and probably involves wind and/or insects. Flowers have

characteristics suggesting both wind and insects are responsible for transferring

pollen from staminate flowers to pistillate flowers. In one study of the pollination

biology of lodgepole pine dwarf mistletoe Gilbert and Punter (1984) concluded

that both wind and insects transported pollen. Other studies (Gregor et al. 1979,

Penfield et al. 1976, Player 1979) of other dwarf mistletoes were inconclusive but

described several insects that might be involved.

Following pollination, fruit maturation takes approximately 15 months. Berries

contain a single seed. Up to 688 berries have been found on single infections of

hemlock dwarf mistletoe that developed from inoculated seeds (Smith 1971), and

several thousand berries have been observed on naturally occurring plants. During

summer, pistillate plants often have both flowers and ripening fruits that will

discharge seed in a few months.

Figure 10—Pistillate (top left, purple-brown color) and staminate (bottom left and right, yellow)

flowers of hemlock dwarf mistletoe.
18

GENERAL TECHNICAL REPORT PNW-GTR-718

Seed Dispersal

With the exception of one species, all dwarf mistletoes have a fascinating mechanism of seed dissemination—explosive discharge. At maturity, a single seed is

forcibly ejected from each berry at an initial velocity of approximately 100 km

per hour, but seeds tumble in flight and velocity decreases rapidly (Hinds and

Hawksworth 1965). Hemlock dwarf mistletoe shoots and berries are oriented so

that seeds are shot off toward light sources such as patches of open sky (Thomson

1979). Thus, hemlock dwarf mistletoe seeds are mostly discharged in a trajectory

upward and outward from the source tree (Bloomberg et al. 1980).

Most hemlock dwarf mistletoe seeds are dispersed in a period of approximately

3 weeks from early September to late October (Hawksworth and Wiens 1996,

Smith 1973) depending on elevation and probably other factors. The number of

hemlock dwarf mistletoe seeds dispersed during daylight hours is approximately

three to five times the number dispersed at night (Smith 1973).

Seed Retention and Germination

Dwarf mistletoe seeds are coated with a sticky, hygroscopic substance known as

viscin (Paquet et al. 1986), which accounts for 26 to 61 percent of the seed weight

(Knutson 1984) and allows seeds to adhere to objects where they land. Viscin

initially absorbs water during rain or dew and becomes slippery. If the seeds land

on needles, they often slide down to lodge at the base of the needle on the branch.

This is the most effective location for infection. Soluble pectins in the viscin layer

wash away or are absorbed by the bark, and remaining cellulose strands in the layer

anchor the seed to the bark (Paquet et al. 1986).

The endosperm of dwarf mistletoe seeds is chlorophyllous and capable of

small amounts of photosynthesis (Muir 1975, Tocher et al. 1984). In coastal western hemlock forests between seed dissemination in October and germination of

seeds next spring, the winter is typically cold with low light intensity. Under these

conditions, photosynthesis over winter, before germination, is likely minimal.

However, in warmer spring weather during germination, photosynthesis of the

endosperm might facilitate radicle growth and penetration.

In British Columbia, Smith (1974) hand-placed seeds of hemlock dwarf mistletoe on western hemlock foliage and followed their fate. He found that in almost

snow-free winters, 96 percent of seeds were retained over one winter. In southeast

Alaska, Shaw and Loopstra (1991) reported that 65 percent of hand-placed seeds

remained on western hemlock foliage over one winter. Seeds placed on needles

Dwarf mistletoes

have a fascinating

mechanism of seed

dissemination—

explosive discharge

seed is forcibly

ejected from each

berry at an initial

velocity of approximately 100 km per

hour.
19

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

with a downward orientation had only 13 percent survival; most presumably were

removed by rain or snow. Carpenter et al. (1979) found that 67 percent of seeds

survived one winter on foliage in Oregon, with more seeds placed on bark retained

than seeds placed on foliage.

Hemlock dwarf mistletoe seeds germinate in late winter or early spring (Smith

1971, 1985). In British Columbia, Smith (1974) found that 62 percent of handplaced mistletoe seeds germinated in the spring. Smith reported a somewhat lower

germination rate when hemlock dwarf mistletoe seeds were placed on other tree

species. Carpenter et al. (1979) found that in Oregon, 45 percent of seeds germinated. In Alaska, Shaw and Loopstra (1991) found that 33 percent of mistletoe

seeds germinated.

Penetration and Infection

Germinating hemlock dwarf mistletoe seeds produce a short radicle, then a holdfast, and finally a penetration peg that mechanically penetrates into the cortex of

live twigs (Hunt et al. 1996, Knutson 1984) (fig. 11). After penetration of the host

cortex by the radicle, the absorptive, root-like endophytic system develops (Alosi

and Calvin 1984, 1985; Srivastava and Esau 1961a, 1961b). Lateral strands of

dwarf mistletoe grow in the cortex and sinkers develop in the sapwood. Sinkers

develop when filaments of mistletoe contact the cambium, induce ray cells, and

become embedded in extra-wide wood rays in the annual rings of sapwood.

In British Columbia, Smith (1971) found that 22 percent of seeds caused

infection. Infection rate was only 8 percent in Oregon (Carpenter et al. 1979). In

Alaska, 20 percent of retained seeds caused infection (Shaw and Loopstra 1991).

Some infections from these artificial inoculations occurred on tree species that have

never been reported as infected in nature. In inoculation experiments, infection

success was enhanced with increased light levels (Knutson 1984).

By dissecting infections in young hemlock trees around residual mistletoeinfected trees at four sites in southeast Alaska, Shaw (1982) determined that hemlock tissues were between 1 and 14 years old when first infected. Fifty percent of

the infections occurred on branches that were 4 years of age or younger. Only 1 to

10 percent of infections occurred on branch and bole tissues older than 10 years.

Symptoms

The first symptom or indication of dwarf mistletoe infection of host tissue is a

small swelling (fig. 12) at the point of infection (Hawksworth and Wiens 1972).
20

GENERAL TECHNICAL REPORT PNW-GTR-718

Hemlock dwarf mistletoe initially produces a pronouncedly swollen sapwood

growth ring 1 to 2 years after infection as illustrated by Smith (1971), thus allowing determination of the approximate age of infection. As the parasite endophytic

system grows within tree tissues, swellings enlarge and become fusiform-shaped as

they age. Infections caused by the shore pine subspecies are typically more globose.

These differences in shape are distinctive for the subspecies on other host tree

Figure 11—Hemlock dwarf mistletoe seed with radicle (arrow) and holdfast.

R.B. Smith

Figure 12—Small hemlock dwarf mistletoe infection with aerial shoots.
21

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

species and are useful for identifying hemlock mistletoe subspecies in stands with

several tree species (Smith 1971, Wass 1976, Wass and Mathiasen 2003). Many

hemlock dwarf mistletoe swellings stimulate production and/or elongation of

buds on swollen tissues that often develop into a profuse, dense mass of distorted

branches called a “witches’ broom” (fig. 13) that continue to grow into large

structures (fig. 14).

Hemlock dwarf mistletoe occasionally infects the leader of a tree, resulting in

a flat broom at the top (fig. 15). With the loss of apical dominance in the broomed

top, no new uninfected branch becomes a leader, and such trees do not grow any

higher.

When infections occur on the bole, i.e., on the main stem or trunk of the tree,

or when infections occur on branches close to the tree bole, the endophytic system

grows into bole tissues resulting in a “bole infection” (see Baranyay et al. [1971]

for discussion of definitions). These often become large, fusiform swellings, up

to 1 m long, growing on one side of, or around, the bole. Bole infections have

important detrimental effects on the health and commercial value of trees, but

they appear much less important for the actual tree-to-tree spread of hemlock

dwarf mistletoe than branch infections (Smith 1973). Several types or variations

in bole infections occur (fig. 16). Pronounced bole infections are infrequent or

absent from mistletoe-infected overstory old-growth trees in some areas such as at

the Wind River Canopy Crane research area (Shaw et al. 2005).

Figure 13—Young hemlock dwarf mistletoe witches’ broom.
22

GENERAL TECHNICAL REPORT PNW-GTR-718

Figure 14—Hemlock dwarf mistletoe brooms

close to the tree bole.

Figure 15—Hemlock dwarf mistletoe-caused

flat treetops.
23

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

a b

Figure 16—Hemlock dwarf mistletoe bole infections: (a) globose-shaped bole

infection; (b) burl-like bole swelling with few or no shoots commonly found in

infested stands.

From one to several years after a swelling develops, aerial shoots of hemlock

dwarf mistletoe develop from the endophytic system and emerge from the swollen

branch or bole. Aerial shoots of A. tsugense are confined to the basal swollen portion of branches of a witch’s broom. This is termed a localized infection (Kuijt

1960). Other dwarf mistletoes such as lodgepole pine and Douglas-fir dwarf mistletoe form witches’ brooms with shoots emerging in a regular pattern from nonswollen branches termed a systemic infection (Kuijt 1960).

Flowers develop 1 to 2 years after shoots emerge, with berries produced and

seed dispersed in the following year. Flowering and seed dispersal usually continue

on shoots for several years. Individual aerial shoots of hemlock dwarf mistletoe

generally persist for 2 or 3 years with some surviving as long as 5 years (Smith

1971). The perennial endophytic system can produce repeated crops of reproductive shoots, but infections frequently do not continue to produce reproductive aerial

shoots.

The endophytic system remains alive in an infection for as long as the tree,

bole, or branch lives (Smith 1985), i.e., for centuries. Large hemlock dwarf

mistletoe witches’ brooms are often older than 100 years (Trummer et al. 1998).

Large hemlock dwarf

mistletoe witches’

brooms are often

older than 100 years.
24

GENERAL TECHNICAL REPORT PNW-GTR-718

Rate of the Reproductive Cycle

The rate of the reproductive cycle of hemlock dwarf mistletoe, i.e., time from seed

dispersal to seed dispersal from new plants, is one of the factors that appear to influence hemlock dwarf mistletoe occurrence and long-term success as a parasite.

The reproductive cycle of hemlock dwarf mistletoe is completed at a more rapid

rate in Oregon (Carpenter et al. 1979) and British Columbia (Smith 1971) than to

the north in Alaska (Shaw and Loopstra 1991). For example, in British Columbia,

some hemlock dwarf mistletoe swellings are visible on branches during the first

year of infection, and shoots often occur during the second year. Mature fruit is

produced in the fourth or fifth year after infection. In contrast, Shaw and Loopstra

(1991) found that swellings in Alaska were usually produced about 3 years after

inoculation, and generally 3.5 to 4 years elapsed before shoots were produced.

However, these infections died from shading before fruit production occurred; thus,

the duration from infection to fruit and seed production in southeast Alaska has not

been determined conclusively.

Factors Affecting Biology and Seed Dispersal

Local Seed Dispersal

As described below, a detailed knowledge of seed dispersal is important for predicting hemlock dwarf mistletoe spread and impacts after selection or retention harvesting. Several of these factors are included in a recent epidemiological model

developed for hemlock dwarf mistletoe and other dwarf mistletoes (Robinson and

Geils 2006). Factors affecting seed dispersal distances were presumed to be height

in the tree of the source plant, angle of seed discharge, wind velocity, and stand

density (Hawksworth 1978). Boyce (1948) suggested that hemlock dwarf mistletoe

seeds disperse to a maximum distance of 12 m. In British Columbia, Smith (1966)

found that over 95 percent of hemlock dwarf mistletoe seeds fell within 9 m of a

severely infected residual western hemlock. Smith (1973, 1985) determined that

although hemlock dwarf mistletoe seeds were dispersed up to 16 m from the bole

of a source tree, most seeds landed within 2 to 4 m of the tree. Bloomberg et al.

(1980) measured horizontal seed dispersal from infections positioned at three

different heights. Height above the ground did not appreciably affect maximum

horizontal dispersal where wind was not a factor.

In other dwarf mistletoes, wind can influence dispersal distances with the

greatest distances being in the direction of prevailing winds (Scharpf and Parmeter 1971). In many coastal areas, prevailing winds during seed dispersal, i.e.,
25

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

September through October, are generally from the south and southeast; thus,

greatest dispersal distances of hemlock dwarf mistletoe would be expected to the

north and northwest of mistletoe-infected trees. However, Smith (1977) determined

that on his study tree, most hemlock dwarf mistletoe infections and berries were on

the south side of the tree, and most hemlock dwarf mistletoe seeds were dispersed

to the south. Winds during seed dispersal in this area were usually from the south

and did not significantly increase seed dispersal distance.

The structure of the forest canopy presumably affects dispersal of hemlock

dwarf mistletoe. Western hemlock forests with dense canopies probably reduce

hemlock dwarf mistletoe dispersal distances and spread rates because seeds are

frequently intercepted before they reach their maximum dispersal distance. This

often results in seeds not escaping from the source tree to infect neighboring trees.

Thus, actual dispersal distances are probably far less on average than the potential

maximum distances. Shea and Stewart (1972) estimated that the spread of hemlock

dwarf mistletoe is relatively slow in dense, even-aged forests, probably about 0.5 m

per year. In British Columbia, Bloomberg et al. (1980) measured the mistletoe

seeds that escaped from western hemlock source trees relative to the position of infections in the crown (distance to shoot tip). A maximum of 70 percent of hemlock

dwarf mistletoe seeds escaped when they were produced near branch tips, i.e., at

the periphery of the tree crown. However, seed escape fell to 50 percent just 40 cm

from branch tips and continued to drop off to a low of about 30 percent when seeds

were ejected from infections situated farther in the crown interior. When seeds

escape from a source tree, they very rarely pass through an adjacent tree crown to

hit the next tree, i.e., “leapfrogging” a tree (Bloomberg et al. 1980), unless the seed

source is higher than the top of the adjacent tree. Thus, in a high-density, uniformstructured, even-aged western hemlock forest there is probably very slow tree-totree spread of hemlock dwarf mistletoe in the absence of an overhead source of

hemlock dwarf mistletoe seeds, i.e., a large hemlock dwarf mistletoe-infected

residual tree.

As described below in the section on stand development, vertical spread of

dwarf mistletoe appears important for colonization by the parasite and eventual

impacts on tree growth. For immature western hemlock stands in British Columbia,

Richardson and van der Kamp (1972) estimated an upward advance of hemlock

dwarf mistletoe of 30 and 65 cm per year in stands that were relatively dense or

open, respectively. Hemlock dwarf mistletoe spreads vertically at about 20 cm per

year in thinned young-growth stands in southeast Alaska (Shaw and Hennon 1991).

These estimates were consistent with measurements made by Bloomberg et al.

Western hemlock

forests with dense

canopies probably

reduce hemlock

dwarf mistletoe

dispersal distances

and spread rates

because seeds are

frequently intercepted before they

reach their maximum

dispersal distance.
26

GENERAL TECHNICAL REPORT PNW-GTR-718

(1980) on the upward trajectory of hemlock dwarf mistletoe seeds, which suggested that most infections occur less than 1 m above the source infection.

In some areas, height of hemlock dwarf mistletoe infections above ground level

appears to be an important factor. Smith (1969) and Shaw and Weiss (2000) determined that in dense stands of mature trees, hemlock dwarf mistletoe infections with

shoots and berries predominate in the upper one-third of the tree crown where light

conditions are favorable. Generally, in retention- or selection-harvested areas, it is

expected that hemlock dwarf mistletoe shoot growth and seed production will increase in the lower crown of hemlock dwarf mistletoe-infected trees. However,

some well-exposed western hemlock trees appear to have very little hemlock dwarf

mistletoe shoot development, possibly because of host conditions (Bickford et al.

2005) or some other factors.

Local differences in air temperature with tree height could also be important.

As reported by Parker (1995) and Shaw and Weiss (2000), higher air temperatures

occur in the middle to upper crown of a forest canopy. One example of a pronounced vertical temperature gradient was observed in October 2001 at the Wind

River Canopy Crane Research Facility where severe frost was observed at ground

level, although air temperature was well above freezing in the upper crowns of the

50- to 55-m hemlock trees at the site. In the upper crowns, hemlock dwarf mistletoe infections were dispersing seeds. Apparently in some areas, hemlock dwarf

mistletoe seed production and dispersal could be limited at or near ground level by

frost damage (Baranyay and Smith 1974, Kuijt 1955) but remain unaffected in the

upper tree crowns of nearby trees. This could be an important factor at the upper

elevation limits of hemlock dwarf mistletoe, particularly for the mountain hemlock

subspecies, in mountainous areas where cold-air drainage often occurs in valleys

and frost damage occurs in topographic depressions (“frost pockets”). Cold air

drainage and frost damage might be important factors in these areas and where

hemlock dwarf mistletoe occurs in riparian zones (Muir 2004, Swanson et al.

2006), as well as the northwest limits of the range in Alaska. Retention harvesting

and recent clearcut harvesting create small openings that also could be subject to

more frequent or more severe local frost occurrence and damage to hemlock dwarf

mistletoe.

Hemlock dwarf mistletoe seed dispersal from tall residual trees is another

important but little-known process. Our casual observations suggest that hemlock

dwarf mistletoe infection of small trees is infrequent adjacent to some dense stands

of tall residual trees, similar to previous observations of low incidence of lodgepole

pine dwarf mistletoe adjacent to dense mature stands in Alberta (Muir 1970).
27

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Possibly in some instances very little hemlock dwarf mistletoe seed from the upper

crown of a 50-m-tall tree falls onto adjacent small regenerating hemlock trees.

Another possibility is suggested by Wicker (1967), who found that for lodgepole

pine, most dwarf mistletoe seeds were removed by snow, and consequently, infection of small trees less than 1 to 2 m tall was uncommon adjacent to dense, mature

stands. Similarly, for hemlock dwarf mistletoe, snow loads adjacent to tall mature

trees might be extremely deep, thereby reducing infection of nearby small trees. In

some high-elevation and northern areas, snow makes up 50 percent or more of the

annual precipitation (MacKinnon 2005, Schwartz and Miller 1983), but the actual

amount of snow versus rain is highly dependent on landscape position and elevation. Heavy snow loads in northern forests could be a factor responsible for the low

incidence or absence of hemlock dwarf mistletoe in some areas.

Previous results of Smith (1966, 1973, 1977), Shaw (1982), Shaw and Weiss

(2000), and observations of incidence of hemlock dwarf mistletoe in young hemlock trees, e.g., Edwards (2002) and Muir (2004), indicated that seed production

and dispersal are critical events in the life history and epidemiology of hemlock

dwarf mistletoe. Most of the available information on seed dispersal is derived

from studies of only a few trees in a few widely separated localities. Our unpublished results suggest that there is considerable tree-to-tree variation in hemlock

dwarf mistletoe seed production and dispersal. This requires further study to

determine variation in hemlock dwarf mistletoe seed production, dispersal, and

effects of a wide range of factors including host tree features, weather, and environmental factors. These include, for example, tree height, age, hemlock dwarf

mistletoe severity, position of infections in tree crowns and height above ground

level, ecological zones and subzones (Meidinger and Pojar 1991), riparian zones,

topographic position, latitude, and elevation.

Long-Distance Seed Dispersal

The explosive discharge of dwarf mistletoe seeds effectively spreads the parasite over short distances. Several authors speculate that some other mechanism is

responsible for long-distance dispersal from stand to stand, or in southeast Alaska

and British Columbia, from island to island and across fjords, glaciers, and other

barriers. Little is known about long-distance dispersal of hemlock dwarf mistletoe

seeds, but several studies implicate birds as potential agents (Hawksworth 1978,

Shaw et al. 2005). Most of the research on animal dispersal of dwarf mistletoe

seeds has been conducted in the United States on other dwarf mistletoes

(Hawksworth and Wiens 1996). Here, we summarize several results of these studies
28

GENERAL TECHNICAL REPORT PNW-GTR-718

that might play a role in long-distance dispersal of hemlock dwarf mistletoe in

western forests.

Anderson (1949) speculated that much of the long-range dispersal was the

result of sticky seeds of dwarf mistletoe adhering to birds that were searching for

food and that these seeds could later be transferred to healthy trees as the birds continued to feed. Preening and grooming by birds could deposit seeds on foliage. In

general, most birds do not eat dwarf mistletoe seeds, and seeds that pass through

their digestive track are not viable (Hudler et al. 1979). Hudler (1976) suggested

that pH and temperature in birds’ digestive tracts kill dwarf mistletoe seeds.

Geils et al. (2002) and Hawksworth and Wiens (1996) summarized the role

of birds and mammals in vectoring dwarf mistletoe seeds. Mammals, especially

squirrels, are known to spread dwarf mistletoe, but apparently only play a role in

relatively short-range dispersal because of their small home ranges (Nicholls et al.

1984). Hudler et al. (1974) and Ostry et al. (1983) reported that flying squirrels

carried seeds of eastern dwarf mistletoe in their fur. Northern flying squirrels

might play a similar role in dispersing hemlock dwarf mistletoe seeds. In Alaska,

Bakker and Hastings (2002) found that 54 percent of tagged northern flying

squirrels made dens in mistletoe-infected western hemlock trees. In Washington,

Shaw and Flick (2002) reported that the Douglas squirrel was common in the upper

canopy of an old-growth Douglas-fir and western hemlock forest at the Wind River

Canopy Crane Research Facility during hemlock dwarf mistletoe seed dispersal.

Birds are likely vectors for the long-range dispersal that has allowed hemlock

dwarf mistletoe to become established on all islands of consequential size and

nearly all mainland areas of northern portions of its range. Several studies with

other dwarf mistletoes indicated that the most probable vectors for hemlock dwarf

mistletoe seeds are small insectivorous birds that feed in western hemlock canopies,

particularly in the fall months when dwarf mistletoe seed is dispersed. Based on

previous results that indicated gray jays act as vectors of seeds for some dwarf

mistletoes (Nicholls et al. 1984), Steller’s jay, which is a winter resident, might

be capable of vectoring hemlock dwarf mistletoe seeds. In Washington at the

Wind River Canopy Crane Research Facility, Shaw et al. (2002, 2005) reported

that Steller’s jay, gray jay, and particularly, red crossbills are common and feed

on western hemlock cones during the hemlock dwarf mistletoe seed-dispersal

period. The jays are present in the upper canopy during the seed-dispersal period

but do not commonly feed on seed in cones. However, the red crossbill biological

species feeds exclusively on western hemlock cones and flies in large groups of 10
29

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

to 50 birds from tree to tree, remaining in the upper canopy.3 They actively feed on

hemlock seeds during the period of hemlock dwarf mistletoe seed dispersal (September to late October). These birds would be expected to travel large distances in

search of cone crops and likely be responsible for long-distance spread of hemlock

dwarf mistletoe seed. Shaw et al. (2005) suggested that red crossbills were responsible for establishing new mistletoe infection centers in old-growth western hemlock forests.

Marbled murrelet, which was reported to roost and nest in western hemlock

trees infected by hemlock dwarf mistletoe in Washington (Nelson and Wilson

2002), might be another agent for dispersal of hemlock dwarf mistletoe seed in

coastal forests, although it does not move extensively within forest canopies.

Mortality Factors and Use of Biological Control

Most authors have assumed that hemlock dwarf mistletoe infections on nonfoliated

branches were dead. Bloomberg and Smith (1982) found that the proportion of

such “dead” infections on Vancouver Island, British Columbia, was substantial in

the lower crown position and concluded that these infections were probably dying

because of shading. Data from young western hemlock trees in British Columbia

(Smith 1977, 1985), where light was probably more available, indicated 60 percent

survival for infections up to 12 years of age. Shading and branch senescence are

considered important mortality factors for hemlock dwarf mistletoe. This topic is

explored in more detail in the section on stand development below.

Other significant damaging factors for hemlock dwarf mistletoe include hyperparasitic fungi and early frosts. Several fungi are associated with hemlock dwarf

mistletoe shoots and swellings (Kope and Shamoun 2000). Several that invade and

kill aerial shoots or the bark of swellings appear to be selective, persistent and

effective natural biological controls (Baranyay 1966, Funk and Baranyay 1973,

Funk and Smith 1981, Funk et al. 1973, Shamoun and DeWald 2002, Smith and

Funk 1980). The nutritious hemlock dwarf mistletoe-infected cortical tissues, their

low moisture content (Baranyay 1964) and the presence of dead phloem likely

facilitate the pathogenicity and effects of the associated fungi. Recently, two fungi,

Colletotrichum gloeosporioides and Neonectria neomacrospora, have been studied

to determine their pathogenicity, virulence, and potential as biological control

agents (Askew 2006, Reitman et al. 2005, Shamoun and DeWald 2002).

3

 Shaw, D. 2006. Personal communication, Assistant Professor, Department of Forest

Sciences, Oregon State University, Corvallis, OR 97331.
30

GENERAL TECHNICAL REPORT PNW-GTR-718

Hawksworth et al. (1977) and Hawksworth and Wiens (1996) described fungal

parasites that attack the shoots of the various dwarf mistletoe species in western

North America. Some of these fungi occur at high incidence and appear to limit

or reduce the reproduction of dwarf mistletoes. One of the most common and

damaging fungal species known to occur on hemlock dwarf mistletoe aerial shoots

is C. gloeosporioides (Kope et al. 1997, Hennon, personal observations). Usually,

the incidence of native fungi on dwarf mistletoes is extremely variable from year to

year, and several authors considered that the possibility of their use as biological

control agents was remote (Hawksworth et al. 1977, Smith 1985). However, recent developments in enhancing effectiveness and application of biological control

agents suggested that with further research and development, these fungi could be

developed as effective biological control agents to reduce hemlock dwarf mistletoe

spread in selection- or retention-harvested forests (Shamoun and DeWald 2002).

Stevens and Hawksworth (1984) described insects that feed on shoots or fruits

of dwarf mistletoes. Larvae of Johnson’s hairstreak butterfly, Loranthomitoura

johnsonii, feed on hemlock dwarf mistletoe shoots from California to southern

British Columbia (Carpenter et al. 1979, Guppy and Shepard 2001, McCorkle

1962). Mammals that feed on dwarf mistletoe are discussed below.

Occasionally, early frosts can freeze hemlock dwarf mistletoe fruits and disrupt

the seed-dispersal mechanism (Baranyay and Smith 1974). Hemlock dwarf mistletoe seed dispersal in October or November is vulnerable to frost. Frost also can kill

existing plants or infections (Kuijt 1955), thereby substantially reducing spread and

diminishing populations of the parasite.

Host Resistance to Infection

Smith et al. (1993) reported genetic resistance of western hemlock to infection

by hemlock dwarf mistletoe. Two of 60 seedlings planted around the mistletoe–

infected residual tree studied by Smith (1966, 1973, 1977) were resistant to hemlock dwarf mistletoe infection. Very few hemlock dwarf mistletoe infections developed on the trees as compared to nearby seedlings that were exposed to the same

numbers of hemlock dwarf mistletoe seeds. The occurrence of two resistant trees in

this small planting suggested that genetically resistant trees might occur frequently

in populations of western hemlock trees. Unfortunately, the two resistant trees

were the slowest growing trees in the trial,4

 but the prospects of finding other faster

4

 Wass, E.F. 2006. Personal communication. Pathologist, Canadian Forest Service, Pacific

Forestry Centre, Victoria, BC V8Z 1M5.
31

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

growing, mistletoe-resistant hemlock trees appears promising. Further research is

underway to determine and select genetically resistant western hemlock trees

(Shamoun and DeWald 2002). Cultivation of hemlock dwarf mistletoe in vitro,

which was reported by Deeks et al. (2001), is a potential screening method.

The two resistant trees that Smith et al. (1993) reported indicate that genetic

resistance could be a significant factor in hemlock dwarf mistletoe spread and

impacts. Western hemlock populations from different locations, elevations, latitudes

and/or provenances might exhibit differences in frequency of resistance to hemlock

dwarf mistletoe infection, as reported by Wu et al. (1996) for lodgepole pine

resistance to stem rusts and other pathogens. Incidence of hemlock dwarf mistletoe

could be low in some locations because of genetically resistant populations of

western hemlock trees. Further research appears warranted to determine the frequency, nature, and extent of western hemlock genetic resistance to hemlock dwarf

mistletoe.

Effects of Dwarf Mistletoe on Individual Trees

Tree Physiology and Form

Although hemlock dwarf mistletoe is capable of very small amounts of photosynthesis (Hull and Leonard 1964b, Miller and Tocher 1975), it draws essentially all

of its nutrients from its host. This leads to growth reductions of the host tree and

several physiological and pathological effects described below.

Several reactions by trees to infection by hemlock dwarf mistletoe, i.e., branch

and bole swellings and witches’ brooms, are believed to be induced hormonally.

Dwarf mistletoe appears to alter concentrations of tree growth regulators (Knutson

1979), particularly abscisic acid, indole-3-acetic acid and zeatin riboside

(Livingston et al. 1984). The most substantial evidence of hormone imbalances

comes from research on eastern dwarf mistletoe infecting black spruce in Minnesota. Similar research has not been conducted on hemlock dwarf mistletoe, but it

appears likely that swelling and witches’ brooms are caused by hemlock dwarf

mistletoe modifying concentrations of growth hormones in western hemlock.

Hemlock dwarf mistletoe is probably affected by the physiology of its host

tree, as recently demonstrated for southwestern dwarf mistletoe on ponderosa pine

in Arizona (Bickford et al. 2005). The influential host factors probably include tree

vigor and associated physiological characteristics such as tree age, size (tree height,

diameter), crown position in canopy, crown condition (density of foliage, dieback

of main stem), sapwood to foliage ratio, height to diameter ratio, and others. A

Several reactions

by trees to infection

by hemlock dwarf

mistletoe, i.e.,

branch and bole

swellings and

witches’ brooms,

are believed to be

induced hormonally.
32

GENERAL TECHNICAL REPORT PNW-GTR-718

more detailed knowledge of these features and their possible association with hemlock dwarf mistletoe seed production and infection could facilitate selection of

hemlock dwarf mistletoe-infected trees to be retained or removed to either enhance

or reduce hemlock dwarf mistletoe spread and future growth impacts.

The most important effects of dwarf mistletoe infection are growth abnormalities and their consequences, but there are other physiological effects. Using potted

seedlings, Tocher et al. (1984) determined that aerial shoots of hemlock dwarf

mistletoe transpire at rates approximately twice that of western hemlock tissues.

Hemlock dwarf mistletoe transpired more than its western hemlock host at almost

all conditions of soil moisture stress and darkness. Values as high as 62 times the

transpiration rate of western hemlock tissues were recorded for hemlock dwarf

mistletoe. Thus, dwarf mistletoe infections may contribute to water imbalances in

trees and exacerbate droughty conditions for severely infected western hemlock

trees.

The physiological effects of hemlock dwarf mistletoe on old-growth trees appear quite different. At the Wind River Canopy Crane Research Facility in southern

Washington, Meinzer et al. (2004) measured hydraulic conductance, water and carbon relations on three uninfected and five severely infected 250-year-old, 55-mtall, western hemlock trees. Uninfected trees had approximately 160 live branches,

50 percent more than severely infected trees. Uninfected trees also had 50 percent

more leaf area than infected trees on branches distal to infections that were used to

measure water conductance. These values were almost the reverse of observations

of much younger trees where branches and foliage appear denser on infected than

uninfected western hemlock trees. Daily water use was consistently and significantly greater in uninfected than in severely infected trees. The mean nitrogen

content and photosynthetic capacity of foliage was less in infected than in uninfected trees. The authors concluded that with the water use of infected trees reduced by 40 percent, photosynthesis reduced by 20 percent, and foliage area

reduced 40 percent, carbon accumulation was reduced 60 percent in severely

infected old-growth trees. The authors further suggest that in dry seasons or

weather, severely infected old-growth trees are at a disadvantage compared to

nearby uninfected trees. This water-deficiency stress might explain why old,

severely infected trees die prematurely in comparison to nearby similar lightly or

uninfected trees.

Although the parasite occurs strictly on aboveground portions of western

hemlock trees, root systems can be influenced by the parasite. Smaller root systems
33

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

of trees infected by other dwarf mistletoe species have been reported (Knutson and

Toevs 1972). In a forest setting, these reductions could affect the ability of an infected tree to compete with neighboring trees for soil nutrients, for water, and for

light in the canopy. Debilitating effects of hemlock dwarf mistletoe infection on

root systems might make infected trees more vulnerable to windthrow or more

susceptible to infection by root disease fungi such as Heterobasidion annosum.

Hemlock dwarf mistletoe affects host-tree witches’ broom formation in a manner somewhat differently than other dwarf mistletoes. Hemlock dwarf mistletoe

witches’ brooms are similar to normal branches in species with prolific branching

but differ in that broom branches are oriented horizontally and usually in overlapping layers. Consequently, although hemlock dwarf mistletoe shoots grow upward

in response to light (Thomson 1979), they often must grow through one or more

dense layers of foliage to expose their berries for effective seed dispersal. Hemlock

dwarf mistletoe also appears to have a more pronounced effect on growth of the

distal portion of infected branches than other dwarf mistletoes and markedly increases the diameter of infected branches at the junction of the branch with the bole

(Smith 1969), even on small infected branches.

The physiology of several previously unreported features of hemlock dwarf

mistletoe also need to be investigated. These include physiological aspects and

effects of live hemlock dwarf mistletoe-infected branches low on the bole below

uninfected foliated branches. These low, infected branches have or do not have tree

foliage. The unknown aspects include:

• Longevity of hemlock dwarf mistletoe-infected branches that have or do not

have foliage.

• How live mistletoe-infected branches without foliage obtain their nutrients.

• How foliage on infected branches differs in shade tolerance and other

features from that on uninfected branches.

• How these hemlock dwarf mistletoe-infected branches affect tree growth.

One of the most striking and interesting effects of hemlock dwarf mistletoe infection is retention of live, nonfoliated hemlock dwarf mistletoe-infected branches

(fig. 17). Apparently hemlock dwarf mistletoe infections on nonfoliated branches at

distances of up to 1 m or more from the bole are able to direct or draw water and

nutrients from the bole in sufficient amounts to keep nonfoliated branches alive for

several decades. These infected, nonfoliated branches likely contribute to overall

growth loss and other effects caused by hemlock dwarf mistletoe, but their role or
34

GENERAL TECHNICAL REPORT PNW-GTR-718

Figure 17—Live hemlock dwarf

mistletoe infections on nonfoliated

branches of western hemlock at Harris

Creek plot, Vancouver Island, British

Columbia.

effects have yet to be determined. Van der Kamp5 found that removal of these types

of branches increased radial growth of boles by 5 to 15 percent, but treatment

effects were not statistically significant, apparently because of large tree-to-tree

variation in growth rates. Most authors have assumed that nonfoliated branches are

dead, but this is not the case for many branches that have hemlock dwarf mistletoe

infections.

Another interesting and potentially significant feature of hemlock dwarf mistletoe is its apparent capability to increase the shade tolerance of western hemlock

foliage. Under dense canopies in coastal western hemlock forests, there is usually a

heavily shaded, well-defined lower level where tree branches have no foliage (i.e.,

below the “live crown”). In dense hemlock stands, less than 10 percent of available

light penetrates to this level (Parker 1995, 1997). However, in this extremely

5

 van der Kamp, B. J. 2004, Personal communication. Professor, Department of Forest

Sciences, University of British Columbia, Vancouver, BC V6T IW4.
35

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

shaded layer, hemlock dwarf mistletoe-infected trees occasionally have mistletoeinfected branches that have retained their foliage (fig. 18). These branches occur

on trees that are components of both the upper canopy and understory. Judging by

the sizes of some trees, some of these infected, foliated branches are retained for

several to many decades, and possibly even centuries, after noninfected branches

have been shaded out and died. As with the lower live branches that appear dead,

described above, these lower live foliated branches probably represent a net energy

loss for these trees. The photosynthetic capacity and/or efficiency of foliage on

these hemlock dwarf mistletoe-infected branches needs to be determined particularly in comparison to the foliage on shaded and exposed uninfected branches.

Retention of foliage on hemlock dwarf mistletoe-infected understory trees

could enhance survival of hemlock dwarf mistletoe in dense stands by conferring

a competitive advantage to mistletoe-infected understory trees. After a major disturbance to the canopy, such hemlock dwarf mistletoe-infected understory trees

could grow rapidly to reach a dominant position in a regenerating forest. These

trees could become major sources of hemlock dwarf mistletoe seeds for dispersal

to and infection of nearby regenerating trees, similar to the tree studied by Smith

(1966, 1973, 1977). Apparently, this could occur frequently, based on results of

Thomson et al. (1985a, 1985b) that indicated that many of the most severely hemlock dwarf mistletoe-infected trees had an early period of suppressed growth of 60

years or more before growing rapidly after a stand disturbance. Retention of foliage on suppressed, understory hemlock dwarf mistletoe-infected trees also suggests

that despite a low incidence of infection, hemlock dwarf mistletoe could persist and

survive for several decades and perhaps centuries, in some areas until conditions are

favorable for renewed seed production and infection of regenerating trees. Retention of foliage on hemlock dwarf mistletoe-infected trees also appears to be unusual

because hemlock dwarf mistletoe infection of branches commonly results in

chlorosis of foliage distal to the infection. This is apparently due to a pronounced

deficiency in nitrogen associated with hemlock dwarf mistletoe infection (Meinzer

et al. 2004). This type of chlorosis appears to be a frequent and often diagnostic

feature of hemlock dwarf mistletoe infection (fig. 19).

Tree Growth

Perhaps the most important ecological and economic consequence of dwarf mistletoe infection is reduced annual tree growth. In general, annual growth reduction

occurs only in moderately to severely infected trees. Height growth is more seriously affected than diameter growth for a given degree of infection (Hawksworth

Perhaps the most

important ecological

and economic consequence of dwarf

mistletoe infection

is reduced annual

tree growth. In general, annual growth

reduction occurs

only in moderately

to severely infected

trees.
36

GENERAL TECHNICAL REPORT PNW-GTR-718

Figure 18—Hemlock dwarf mistletoeinfected branch with foliage in the

lower heavily shaded canopy.

Figure 19—Chlorosis of western hemlock foliage distal to hemlock dwarf

mistletoe infection.
37

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

1978). In severely mistletoe-infected old-growth, tree top-kill or tree death sometimes results. Shea and Stewart (1972) claimed that excessive top-kill and mortality

can reduce aesthetic values. Hawksworth (1978) reported that cone and seed crops

can be reduced on severely infected trees of some dwarf mistletoes, but no quantitative information exists on these effects for hemlock dwarf mistletoe. Even if

hemlock dwarf mistletoe limited tree reproduction to some degree, western hemlock reproduces so prolifically in most of its range that this probably is not an important issue. Several studies have been undertaken in various regions to quantify

growth loss caused by hemlock dwarf mistletoe as described below.

Tree Mortality

Hemlock dwarf mistletoe is associated with top-kill. Bloomberg and Smith (1982)

suggested that top dieback could limit spread of the mistletoe into young stands.

Buckland and Marples (1952) suggested that hemlock dwarf mistletoe influences

top-kill by diverting nutrients from the tops of trees to witches’ brooms lower in

the crown. Excessive growth of infected tissues drains resources from other parts

of the tree, and carbon allocation can be greatly affected throughout the tree. However, results of Meinzer et al. (2004) did not support this supposition. They found

that the proportion of mistletoe-infested tissue was very low on their study trees.

Hemlock dwarf mistletoe is also associated with tree mortality. Tree mortality

associated with hemlock dwarf mistletoe infection differs between young and old

forests. In young, fast-growing forests, growth loss associated with severe dwarf

mistletoe infection could debilitate trees to the extent that they lose their ability to

compete with lightly or uninfected hemlock trees, or other species. Consequently,

severely infected trees that are relegated to a lower crown position as relatively

small trees can eventually die because of suppression. In older forests, severely

infected trees in codominant and dominant canopy positions can die directly from

dwarf mistletoe infection rather than from intertree competition, and their death

can create canopy gaps.

Mortality of old-growth trees (fig. 20) is often associated with numerous

infections or large witches’ brooms. Older, severely infected trees are weakened

by the physiological effects of hemlock mistletoe infection (Meinzer et al. 2004)

and consequently are likely more vulnerable to secondary insects and/or diseases.

For example, western hemlock trees infected with hemlock dwarf mistletoe in

British Columbia did not survive repeated defoliation during outbreaks of hemlock

looper, but hemlock trees without hemlock dwarf mistletoe survived (Buckland and

Marples 1952). Dead standing trees with numerous witches’ brooms, apparently

In older forests,

severely infected

trees in codominant

and dominant canopy positions can

die directly from

dwarf mistletoe

infection rather than

from intertree competition, and their

death can create

canopy gaps.
38

GENERAL TECHNICAL REPORT PNW-GTR-718

killed by hemlock dwarf mistletoe, are a common feature of old-growth forests in

southeast Alaska, and probably other areas, but the actual cause of death in this

mortality has not been carefully evaluated.

In Washington near the Wind River Canopy Crane Research Facility, several

long-term studies indicated a substantial amount of mortality in a 350-year-old

Douglas-fir and western hemlock forest, with a large proportion of the mortality

associated with hemlock dwarf mistletoe infection (DeBell and Franklin 1987,

King 1961). From 1947 to 1983 gross tree growth was 22 m3

·ha-1·yr-1 with 20

m3

·ha-1·yr-1 mortality, essentially resulting in zero net growth (DeBell and Franklin

1987). Western hemlock accounted for one-half of the growth and 28 percent of the

mortality, most of which was attributed to effects of hemlock dwarf mistletoe.

In British Columbia, Alfaro (1985) summarized data collected from seven 40-

to 100-year-old western hemlock stands infected with hemlock dwarf mistletoe. An

average of 4.7 stems per hectare, 0.6 percent of the total stems per hectare, were

dead due to mistletoe infection. Three stands had no mortality attributed to hemlock dwarf mistletoe. Tree death not resulting from hemlock dwarf mistletoe averaged 46.7 stems per hectare or 4.9 percent of the total.

Figure 20—Tree mortality of western hemlock severely infected by hemlock dwarf

mistletoe.
39

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

New information on tree mortality associated with hemlock dwarf mistletoe in

Alaska indicates a greater annual rate of mortality associated with higher infection

rates (see “Effects on Growth of Trees and Stands”).

Dwarf mistletoes are associated with other mortality agents in some forest

ecosystems (Geils et al. 2002, Hawksworth and Wiens 1996). Bark beetle populations can build in dwarf mistletoe-infected trees and attack other trees. However,

bark beetles are usually not considered to be important in coastal forests and

western hemlock does not have serious bark beetle enemies (Furniss and Carolin

1977).

Wood Quality

Along with causing growth loss, allowing the entry of heart rot fungi, and killing

trees, hemlock dwarf mistletoe affects the timber resource by reducing wood

quality. Limbs that support witches’ brooms form a large knot of very dense

wood, a response that probably helps support the additional weight of the broom.

Branches in the lower and middle portions of the crown of severely infected trees

are approximately 30 percent larger in diameter than those in lightly infected trees

(Smith 1969). Large knots associated with infections reduce wood quality (Shea

and Stewart 1972) and adversely affect yields (Smith 1969).

Another aspect of wood quality is the dense compression wood that forms on

the side of a tree that supports witches’ brooms (Wellwood 1956). Resin canals are

more common in infected wood (Srivastava and Esau 1961b). Infected wood tends

to have a greater specific gravity (Wellwood 1956). However, when the specific

gravity of whole trees was compared by infection severity, the more severely infected trees had lower specific gravity. Thus, these trees produce a more variable

wood density, i.e., some wood in their boles is light and some wood is dense, and

they also produce less pulp because of generally lower specific gravity. In addition,

the wood in boles near witches’ brooms has lower moisture content (Wellwood

1956). Hunt (1971) found higher lignin content and a doubling of concentrations

of alcohol and benzene associated with hemlock dwarf mistletoe bole infections

that make this wood unsuitable for use as pulp unless mixed with large amounts

of uninfected wood. Microscopic examination of the dwarf mistletoe endophytic

system and the cellular structure of infected wood and bark indicated that the

largest effect of infection was an increase in the size of wood rays (Srivastava and

Esau 1961a, 1961b).
40

GENERAL TECHNICAL REPORT PNW-GTR-718

Wood Decay

Bole infections and large witches’ brooms on primary branches near the bole

frequently harbor wood decay or heart rot fungi (fig. 21a). Wood decay is one

of the major losses of commercial wood volume in old-growth western hemlock

forests (Farr et al. 1976, Kimmey 1956). Wood decay fungi are important ecologically because they often are associated with or cause tree death and, consequently,

canopy-gap formation in old-growth forests (Hennon 1995). In Oregon and

Washington, Englerth (1942) estimated that wood decay fungi that entered large

witches’ brooms and bole infections caused 31 percent of the heart rot in western

hemlock. The same decay fungi reported by Englerth were present in southeast

Alaska including Ganoderma applanatum, Phellinus hartigii, and Ganoderma

tsugense (Holsten et al. 1985). Phellinus hartigii had a particularly close association with hemlock dwarf mistletoe in British Columbia and Alaska (Foster et al.

1958, Holsten et al. 1985) as a sapwood parasite (fig. 21b). Trees collapse after 25

percent of the circumference is killed (see footnote 5) creating a distinctive steplike stump as illustrated by Allen et al. (1997). Other heart rot fungi associated

with hemlock dwarf mistletoe include Echinodontium tinctorium (Weir and Hubert

1918), Fomitopsis pinicola (Etheridge 1973), and Pholiota adiposa (Kimmey

1964). Foster et al. (1953) found that 75 percent of hemlock dwarf mistletoe bole

infections were associated with wood decay in British Columbia, but in southeast

Alaska bole infections are commonly but not consistently associated with heart rot

(Farr et al. 1976, Kimmey 1956). Wood decay was found to be associated with

hemlock dwarf mistletoe bole infections in thinned young-growth stands in Washington and Oregon (Goheen et al. 1980).

Tree Hazards

Witches’ brooms of hemlock dwarf mistletoe can eventually grow to a very large

size weighing several hundred kilograms (Buckland and Marbles 1952) (fig. 22).

Witches’ brooms are potentially hazardous in recreation areas and around homes

because they frequently have decay and often break out of trees during storms or

when laden with snow (Hadfield 1977, Shea and Stewart 1972). In many hemlock

dwarf mistletoe-infested stands, the forest floor is littered, sometimes densely, with

witches’ brooms that have broken out of trees. Wallis et al. (1980) described the

potential hazard by hemlock dwarf mistletoe-infected trees in British Columbia.

Russell (1966) suggested removing dead hemlock dwarf mistletoe witches’ brooms

from trees in recreation sites in state parks in Washington. Dead tops and snags are

other potentially hazardous conditions caused by hemlock dwarf mistletoe.

Bole infections

and large witches’

brooms on primary

branches near the

bole frequently

harbor wood decay

or heart rot fungi.
41

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Figure 21—(a) Wood decay and decay fungi sporophores

associated with hemlock dwarf mistletoe on western

hemlock; (b) hemlock dwarf mistletoe broom with

Phellinus hartigii sporophore.

a

b
42

GENERAL TECHNICAL REPORT PNW-GTR-718

Effects on Trees, Stands, and Forests

Infection Rating Systems

Studies on growth loss associated with hemlock dwarf mistletoe (described below)

use one of several systems for rating the severity of infection. A system for quantifying dwarf mistletoe infection is necessary for evaluating effects of the disease

because, hypothetically, tree responses, e.g., reduced growth, should be proportional to the amount of hemlock dwarf mistletoe infestation in a tree.

Hawksworth and Lusher (1956) and Hawksworth (1977) developed a six-class

dwarf mistletoe rating (DMR), which is the most widely used system for estimating

dwarf mistletoe severity. To estimate a DMR, the live crown of an infected tree is

visually divided into equal vertical thirds. Each third is assigned a value: 0 for no

Figure 22—Large hemlock dwarf mistletoe brooms on western hemlock.
43

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

branches infected; 1 for one-half or fewer of the branches infected; and 2 for more

than half of the branches infected. The sum of the three values is the DMR, which

ranges from 0 to 6. A DMR for a stand of trees is calculated by averaging all host

DMRs. A stand dwarf mistletoe index (DMI) is calculated from the average of all

mistletoe-infected trees for the susceptible host species (Geils and Mathiasen 1990).

The DMR was developed for open pine forests. Variations on this system

have been used by other workers and for other dwarf mistletoes (Hawksworth

and Wiens 1996). For example, Tinnin (1998) used an estimate of the proportion of each crown-third occupied by witches’ brooms (broom volume rating,

BVR), rather than the proportion of infected branches, to rate Douglas-fir infected

by Douglas-fir dwarf mistletoe. In northern Arizona, Parker and Mathiasen (2004)

evaluated DMR, BVR, and total broom volume, a system that they developed and

recommended to rate southwestern dwarf mistletoe witches’ brooms in ponderosa

pine. Dooling (1978) gave an excellent tabulation of dwarf mistletoe surveys and

rating systems.

Smith (1969) adapted the six-class system outlined by Baranyay et al. (1971)

for a nine-point rating of mistletoe-infected western hemlock. He included large

witches’ brooms and bole swellings with three points per one-third interval of the

whole tree height, not just the “live” foliated crown. Smith (1969) recognized that

infected hemlock trees were difficult to rate because of lower dead and missing

branches and upper crowns that were mostly unobservable from the ground. He

recommended rating only the middle third of the tree, which was readily examined

and contained the greatest number of live mistletoe infections. His rating system

included ratings for bole swellings and volunteer-leader witches’ brooms.

Laurent (1981) argued that the traditional six-point rating system was inappropriate in southeast Alaska. Tops of trees are often difficult to see, and the lower

crown often has dead infected branches. A modification of this system, where

only the middle third of the live crown is rated, has been used in other forest ecosystems with uneven-aged stands (Myers et al. 1976) and is suggested for use in

southeast Alaska (Laurent 1981). If data on each third of the live crown were recorded separately during surveys, then these or other options would be available for

analysis. Shaw et al. (2000) found that for hemlock dwarf mistletoe in old-growth

western hemlock trees at the Wind River Canopy Crane Research Facility, 10 to 40

percent of 140 trees were accurately rated by the six-class DMR. Four of five observers correctly rated mistletoe-infected old-growth hemlock trees with an accuracy of 70 to 95 percent. Other rating systems for western hemlock dwarf mistletoe

were not evaluated.
44

GENERAL TECHNICAL REPORT PNW-GTR-718

Our and previous workers’ observations of live mistletoe-infected branches

below what is usually considered to be the live or green crown suggest that a

reevaluation of the DMR for western hemlock should be undertaken. If all of the

lower infected branches were included, we suspect that a DMR rating for an

infected tree would generally be higher, but a reevaluation of ratings is required to

determine this. It would be useful to have one accepted rating system so that results

on spread rates and tree effects of hemlock dwarf mistletoe in different regions

could be compared.

Effects on Growth of Trees and Stands

Information on losses owing to reduced annual growth and mortality caused by

hemlock dwarf mistletoe is important for considering how to manage infested

forests. We suggest that most management plans and objectives should include a

target level or threshold of acceptable or desirable dwarf mistletoe impact. Some

studies indicated that annual growth loss in very severely infected old-growth

stands could reach 17 m3

/ha-1 (Smith 1969), a level that might be unacceptable for

any management that includes timber as an objective. However, other studies have

indicated fewer or no effects.

British Columbia—

Several studies in British Columbia provided detailed information on growth loss

and other effects to trees caused by hemlock dwarf mistletoe. Wellwood (1956)

examined the effects of hemlock dwarf mistletoe infection on 10 western hemlock

trees. Stem analyses were used to determine how infection affected tree growth.

Trees ranged in size from 71 to 97 cm diameter at 1.2 m height and ages from

146 to 256 years. Trees were grouped into three infection classes based on the

number of witches’ brooms and bole infections. This early study did not use the

Hawksworth and Lusher (1956) six-class DMR system. Wellwood found that growth

was not significantly different between lightly and moderately infected

trees. However, severely infected trees had significantly less growth. Several of

the dissected trees had severe heart rot that apparently was caused by decay fungi

that infected the trees through bole cankers caused by hemlock dwarf mistletoe.

Smith (1969) quantified growth loss caused by hemlock dwarf mistletoe in

trees approximately 110 years old on Vancouver Island, British Columbia. For

rating severity of infection, Smith used a nine-point rating system (described

above) similar to the six-class system. Uninfected and lightly infected western

We suggest that

most management

plans and objectives should include

a target level or

threshold of acceptable or desirable

dwarf mistletoe

impact.
45

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

hemlock trees had a 41 percent greater annual volume growth than severely infected dominant and codominant trees. Severely infected trees averaged 133 years

age; moderately infected, 104 years; and lightly infected, 94 years.

Severely infected trees were actually larger than lightly and moderately infected trees early in their lives, but by the time they were destructively sampled

for stem analysis, they had significantly less volume, diameter, and height. Height/

diameter ratios were less for severely infected trees. Increased taper in the lower

boles of severely infected trees was also noted. Ring analysis revealed that radial

growth fell off severely in 28 years prior to destructive sampling for severely infected trees, as compared to the 28-year period previous to that. A striking finding

was that uninfected and lightly infected trees had 81 percent more height growth

than severely infected trees toward the end of the study. For a 1.5-ha plot that encompassed the study area, annual volume loss (compared to healthy trees) was 15

to 27 percent of total tree growth, increasing to 40 percent in the last 2 years. This

amounted to a reduction in growth of 17 m3

·ha-1·yr-1 based on growth reductions of

moderately and severely infected dominant and codominant trees.

Smith (1969) found that branches in the lower and middle portions of the

crown were approximately 30 percent larger in severely infected trees than in

lightly infected trees. Smith suggested that these large knots will have adverse effects on the yield and quality of forest products. Based on a sample of branches, the

number of infections ranged from 23 to 1,351 per tree. The quantity and vigor of

dwarf mistletoe aerial shoots increased sharply with increasing height above the

ground. Smith speculated that low light intensity was responsible for reducing

shoot production on infections in the lower crown.

Smith (1969) found that the difference in number of infections between lightly

and severely infected trees was greatest in the middle one-third of the tree, a portion of the tree more easily observed than the top third. Volume produced by severely infected trees was found to be 37 and 43 percent less than lightly infected

trees by using two different methods of rating trees, i.e., rating all three thirds and

rating only the lower and middle-third, respectively. There was considerable variation in radial growth among the sample trees that could not be explained by dwarf

mistletoe infection levels.

Thomson and Smith (1983) examined the growth of 22-year-old western hemlock trees that had been planted around a residual infected tree previously studied

by Smith (1966, 1973, 1977). Hemlock dwarf mistletoe infection had a significant

effect on height increment but not on radial growth. Radial growth was affected

Uninfected and

lightly infected

western hemlock

trees had a 41 percent greater annual

volume growth than

severely infected

dominant and

codominant trees.
46

GENERAL TECHNICAL REPORT PNW-GTR-718

by microsite variables (especially growth of neighboring trees) and March to May

precipitation levels (Thomson and Smith 1983).

Thomson et al. (1984, 1985b) reviewed and analyzed data from previous

studies by Smith (1969) and Alfaro (1985) in British Columbia to determine

growth loss associated with hemlock dwarf mistletoe. Difficulties in sampling,

particularly the selection of trees to be used as controls for a comparison of growth,

are discussed in their papers. They indicated that control trees—healthy or lightly

infected—should ideally share all of the characteristics of age, microsite, and stand

history of infected trees, but these types of trees can rarely be found near moderately or severely infected trees. Their analyses and interpretations of these data

from earlier studies—described as very conservative—indicate volume reductions of 15 and 25 percent for moderately and severely infected trees compared to

healthy trees for stands of up to 80 years of age (Thomson et al. 1984). They suggested that these growth reductions would likely continue in older infected stands.

In another major study in British Columbia, Thomson et al. (1985a) analyzed

growth of mistletoe-infected western hemlock trees in five stands on northern

Vancouver Island. Infected western hemlock trees were rated for dwarf mistletoe

infection severity by using the six-class system and then dissected systematically

to determine growth patterns by ring analysis. Approximately 6 to 16 trees were

sampled in each infection class on each of the five sites. Dissected trees ranged in

age from 45 to 126 years, mostly owing to variable periods of suppressed growth

before release. The authors found that growth patterns were complicated by variations in the growth history of trees before stand-opening events, presumably gaplevel blowdown and mortality. They corrected for differences in suppression by

comparing growth patterns after release at a selected comparison age of 45 to 80

years for each stand. Severely infected (DMR 5 or 6) and moderately infected

(DMR 3 or 4) trees had an average of 39 percent and 23 percent less volume

growth, respectively, than uninfected or lightly infected trees (DMR 0, 1, or 2).

Lightly infected trees were pooled with uninfected trees because light infection

is known to have no significant impact on growth (Hawksworth 1961, Parmeter

1978).

Thomson et al. (1985a) stated that their results reflected growth loss on an

individual tree basis; their study did not examine loss on a stand basis. Because

severely infected trees occupy a significant portion of the canopy and site for long

periods, the authors reasoned that neighboring healthy and/or less severely infected

Severely infected

and moderately infected trees had an

average of 39 percent and 23 percent

less volume growth,

respectively, than

uninfected or lightly

infected trees.
47

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

trees probably did not compensate for very much of the growth loss. They suggested that their results can be used to estimate impacts per stand based on the

proportion of the trees within each infection class.

Washington and Oregon—

Data from a limited study of 50- to 60-year-old stands in coastal Washington suggested that western hemlock trees severely infected with hemlock dwarf mistletoe

had about 40 percent less volume than uninfected but otherwise comparable stands

(Sterling 1962). This difference appeared unusually large compared to results from

other studies.

In two locations in Washington, Hadfield (1980) measured trees to determine

impacts of hemlock dwarf mistletoe in second-growth trees. In one unthinned area,

five plots with a total of 36 infected trees averaging 43 years old were established

around single, obviously infected residual trees. There were no significant differences for total tree volume or the most recent 5- or 10-year radial growth measured at a 10-cm diameter top for trees with different DMRs and distances from the

residual infected trees. Possibly the variations in growth among the relatively few

trees, 2 to 14 per DMR, obscured differences or differences were not pronounced

at the bole height where disks were taken. Hadfield (1980) remarked that hemlock

dwarf mistletoe spread appeared to be much less than expected from these residual

trees in the densely stocked (1,200 trees per hectare) stand and that trees were not

infected severely enough to cause significant growth reductions.

In a 75-year-old stand thinned 26 years previously, Hadfield (1980) compared

growth of 40 trees in a thinned area and 40 trees in an unthinned area containing

hemlock dwarf mistletoe. Trees in the thinned stand with DMR 3-6 were significantly larger and had significantly greater radial growth at stump height, but not

at the height of a 10-cm diameter top, than trees with DMR 0 to 2. There were no

significant differences between trees in the same DMR classes in the thinned versus

unthinned areas. Hadfield reported that some of the more severely infected trees

had indications of recent reduced growth but concluded that hemlock dwarf mistletoe effects on growth would not be significant because most second-growth stands

managed for timber would be harvested by age 40 to 100. Hadfield also selected

42 trees in the same area to determine effects of hemlock dwarf mistletoe on log

grades and found that there were no effects. He reported that infected hemlock

trees had live branches below the main portion of the live crown, but did not mention any hemlock dwarf mistletoe infection on these branches. Despite finding no
48

GENERAL TECHNICAL REPORT PNW-GTR-718

detrimental effects on growth of young hemlock trees, Hadfield (1980) recommended that hemlock dwarf mistletoe should not be ignored during clearcut

harvesting and that control treatments were justified if large numbers of infected

trees remained after harvesting.

In an extensive study of wood decay in paired, thinned and unthinned stands of

40- to 100-year-old western hemlock trees in Washington and Oregon, Goheen et

al. (1980) also recorded occurrences of hemlock dwarf mistletoe-infected trees by

DMR. Stands were thinned in 1959 to 1972, approximately 10 to 20 years before

the study. Hemlock dwarf mistletoe occurred in six of the eight areas studied.

The percentage of hemlock dwarf mistletoe-infected trees in each DMR category

increased after thinning in 12 stands, decreased in 10 stands, and remained unchanged in three. In mistletoe-infested stands, trees in thinned areas were significantly larger than in unthinned areas.

Alaska—

New unpublished information on the association of hemlock dwarf mistletoe

infection and western hemlock radial growth is available from southeast Alaska.

These results were an extension of a study that evaluated the long-term spread and

intensification of hemlock dwarf mistletoe following wind disturbance (Trummer

et al. 1998). Wind disturbance was intended to function as a surrogate for selection

harvest. Permanent plots were established in 1994 in forests that recovered from

stand-replacing storm events in the 1880s. Some large and small residual trees remained after the storms and provided the inoculum for spread of hemlock dwarf

mistletoe to the newly regenerated forest. Most plots were on Kuiu Island, but some

additional plots on Chichagof Island were in the same forest condition and disturbance history.

Repeated diameter at breast height measurements were made over a 10-year

period on western hemlock trees to determine tree growth losses associated with

varying levels of hemlock dwarf mistletoe severity. Measurements of tree height

were found to have large measurement error and thus this analysis was limited to

diameter growth. The original plan was to contrast the diameter growth of infected

trees with uninfected trees. The sample size of uninfected residual trees was too

small, however. Also, for the new cohort tree class where the number of uninfected

trees sampled was adequate, their growth rate was unexpectedly slower than for

trees in any other infection class. This result was difficult to explain and probably

indicated some difference in tree density, site quality, dominance position, or confusion in distinguishing the new cohort trees from small residuals. Because of these
49

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

issues, the growth of moderately and heavily infected trees was compared with the

growth of lightly infected trees (table 2). Previous research from British Columbia

(Thomson et al. 1985a) demonstrated no difference in the growth between

uninfected and lightly infected trees. Also, lightly infected and uninfected trees are

often pooled together in evaluations of growth loss for other species of dwarf

mistletoe (Hawksworth 1961, Parmeter 1978).

Radial growth rates are summarized by tree size class and hemlock dwarf

mistletoe infection level in table 2. Note that no trees were growing rapidly owing

to growth phase of trees in this stand age (i.e., 115 years). Analysis of variance

indicated a significant difference in tree radial growth rate among dwarf mistletoe

severity classes for new cohort trees and small residuals, but not for large residuals

where the sample size was too small (table 2). Also, the large standard errors indicated widely variable tree growth due to other factors in these forests. As noted

above, uninfected new cohort trees had the slowest growth, suggesting that some

of the differences were due to variations in site quality or competition relative to

infected trees. Growth rates were negatively associated with higher dwarf mistletoe

levels for each of the size classes of trees, with approximately 10 percent growth

loss for moderately infected trees and 34 percent growth loss for severely infected

trees.

Results from an additional 308 western hemlock trees growing in similar winddisturbed 115-year-old forests on Chichagof Island, southeast Alaska are not shown

in table 2. Too few residual trees were included for analysis, but, for the new

cohort size class, hemlock dwarf mistletoe severity was significantly associated

with tree growth, with a 16 and 35 percent reduction in growth for moderately

and heavily infected trees relative to lightly infected trees.

A second method using increment cores was employed to evaluate the effect of

hemlock dwarf mistletoe infection levels on tree radial growth. Increment cores

were extracted and more intensive canopy measurements were made on a subset (n

= 167) of the new cohort tree class on Kuiu Island described above. Two cores

were removed from each tree, mounted, and sanded. Decadal growth over the

previous 10, 20, and 30 years was measured to the nearest 0.5 mm. For each tree,

canopy diameter was measured in two perpendicular directions and averaged, and

the degree of overtopping of the tree crown by adjacent trees was estimated. These

canopy measurements were made to assess any confounding effects of intertree

competition on the impact of hemlock dwarf mistletoe.
50

GENERAL TECHNICAL REPORT PNW-GTR-718

Canopy diameters were similar among the dwarf mistletoe infection levels

(mean = 5.0 to 5.7 m), except they were smaller for uninfected trees (mean = 4.4

m). The smaller crowns of uninfected trees indicated greater intertree competition

and may explain the relatively poor radial growth of uninfected trees reported

above. Crown overtopping was variable, ranging from none to 80 percent for all

trees. Lightly infected trees had the smallest degree of overtopping (24 percent)

compared to the other classes (33 to 37 percent).

Analysis of covariance was used to produce adjusted means of decadal radial

tree growth (i.e., 1975-1984, 1985-1994, and 1995-2004) with canopy overtopping

as the covariate. Overtopping was significantly associated with reduced tree growth

Table 2—Annual diameter growth of western hemlock trees with different hemlock

dwarf mistletoe ratings over a 10-year measurement period in 115-year-old

forests of wind throw origin on Kuiu Island, southeast Alaska

Radial growth

Tree classa

-

severity ratingb Trees Mean Standard error Growth lossc

Number – – Millimeters per year – – Percent

New cohort***:

None 206 1.08 2.19 —

Light 315 1.73 2.16 —

Moderate 225 1.61 1.62 7.5

Severe 68 1.16 1.43 33.1

Small residuals*:

None 14 3.08 1.30 —

Light 30 3.00 1.42 —

Moderate 61 2.64 1.58 12.2

Severe 28 1.96 2.18 34.8

Large residuals:

None

Light 7 2.90 1.73 —

Moderate 18 2.62 2.01 9.6

Severe 41 2.36 1.80 18.4

a

 New cohort were trees that regenerated after the wind disturbance; residuals were classified as

small or large if they were <20 cm or ≥20 cm diameter at breast height at the time of the disturbance (Trummer et al. 1998).

b

 Dwarf mistletoe rating (DMR) severity were none, light = 1 or 2, moderate = 3 or 4, severe = 5

or 6. Symbols *, **, and *** indicate significant differences among severity ratings for mean

radial growth for each size class of trees at α = 0.10., 0.05, and 0.001, respectively.

c

 Growth of moderately and severely infected trees was compared to lightly infected trees because

uninfected trees were underrepresented in some classes and, where adequately sampled (i.e., in the

new cohort tree class), had an inexplicable lower growth rate compared to infected trees (see text).
51

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

for each decade. Infection levels of hemlock dwarf mistletoe were nonsignificantly

(α = 0.09), marginally (α = 0.05), and significantly (α = 0.006) related to radial

tree growth in the each of the three successive decades, respectively (fig. 23). Moderately and severely infected trees had an approximate 12 and 30 percent reduction in growth relative to lightly infected trees. These estimated effects of hemlock

dwarf mistletoe on tree growth were in general agreement with those from repeated

diameter at breast height measurements from the larger pool of trees in these plots,

as well as those reported from stem dissections by Thomson et al. (1985a).

Thus, we concluded that both crown competition and hemlock dwarf mistletoe

are important factors in predicting radial growth. These results demonstrated the

difficulty of quantifying the effect of hemlock dwarf mistletoe on western hemlock

tree growth without using experimental methods. With variable-canopy structure in

partially disturbed and retention-harvested forests, intertree competition is expected

Figure 23—Mean annual radial growth in diameter at breast height (DBH) as measured in 10-year

intervals from increment cores of 115-year-old new cohort western hemlock trees with varying

dwarf mistletoe infection levels in wind-disturbed forests on Kuiu Island, southeast Alaska. These

adjusted mean and standard error values were derived from analysis of covariance by using canopy

overtopping as the covariate. Values are shown in the midpoint year for the decade covered (although

they are offset slightly for clarification). Infection levels (based on dwarf mistletoe ratings [DMR])

were none, light (DMR 1 or 2), moderate (DMR 3 or 4), and severe (DMR 5 or 6).

We concluded that

both crown competition and hemlock

dwarf mistletoe are

important factors in

predicting radial

growth.
52

GENERAL TECHNICAL REPORT PNW-GTR-718

to be a major factor in tree growth that confounds the effects of hemlock dwarf

mistletoe.

The same trees on Kuiu Island, Alaska, in partial wind-disturbed stands described in the growth-loss section were monitored for mortality associated with

hemlock dwarf mistletoe. Different rates and processes of mortality might be expected among the size classes of tree; thus, new cohort, small residuals, and large

residuals were displayed separately in table 3. Hemlock dwarf mistletoe may slow

the growth of new cohort trees and contribute to their death through suppression

where intertree competition is important. For large residual trees that have already

reached a codominant or dominant crown position, hemlock dwarf mistletoe may

contribute more directly to their death through physiological processes associated

with allocation of resources to excessive broom formation. For all of the tree size

classes, greater mortality was associated with more severe infection by hemlock

dwarf mistletoe with the highest mortality rates in the severe infection class. For

the new cohort, which was the most extensively sampled tree size class, moderately

and severely infected trees experienced approximately a 100 and 150 percent increased mortality rate, respectively, relative to lightly infected trees. As with the

poor radial growth rate in uninfected new cohort trees (see section above), a

relatively high mortality was found in uninfected western hemlocks. This was difficult to explain but might be related to differences in site quality or stand stagnation where residual trees did not occur. Conceivably, either the presence of residual

trees or hemlock dwarf mistletoe contributes to the differentiation of trees and

thereby helps avoid stagnation in this age class of forest.

Summary of tree and stand-level effects—

Hemlock dwarf mistletoe effects on tree growth have been linked to the severity of

infection on a tree and tree age. Usually at least half or more of the branches on a

tree must have visible hemlock dwarf mistletoe infection before significant growth

reduction occurs. The six-class DMR continues to be used for rating hemlock dwarf

mistletoe severity in western hemlock forests, but should be used and interpreted

cautiously. The major problem is that the dense canopy in many forests obscures

hemlock dwarf mistletoe infections, and the top crown intervals of tall western

hemlock trees are particularly difficult to observe from the ground.

Generally, no significant growth effects occur in young trees less than approximately 20 years old, but large variations in growth of young trees due to genetic

differences per tree or to microsite differences probably obscure effects of hemlock

dwarf mistletoe (Thomson and Smith 1983). Small seedlings and young trees with

Moderately and

severely infected

trees experienced

approximately a 100

and 150 percent

increased mortality

rate.
53

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

hemlock dwarf mistletoe infection occasionally die, but it is difficult to determine

if the cause of mortality was hemlock dwarf mistletoe, wood-boring moths, hyperparasitic canker fungi, other agents, or shading.

By tree age 80 to 100, growth rates usually are significantly different between

trees with DMRs 3 to 6 (moderately to severely infected) and trees with DMRs 0

to 2 (none to lightly infected). The Hadfield (1980) results described above where

more severely infected trees grew faster than lightly infected trees are an anomally.

However, there is a large amount of variation in tree growth rates within a local

area, and trees must be selected carefully to avoid confounding effects. Often, trees

with different DMRs have large differences in growth history, with severely infected trees often having an extended period of suppressed growth due to their

understory position or to dwarf mistletoe infection before a stand disturbance

(Thomson et al. 1984, 1985a, 1985b).

Table 3—Tree mortality of western hemlock associated with hemlock

dwarf mistletoe in 115-year-old wind-disturbed forests on Kuiu Island,

southeast Alaska, in a 10-year measurement period

Tree mortality

Tree classa

- Trees Annual

DMR ratingb sampled Trees dead mortality ratec

Number – – – – – – – Percent – – – – – – –

New cohort:

None 208 13.9 1.4

Light 308 6.2 0.6

Moderate 225 12.9 1.3

Severe 69 15.9 1.6

Small residuals:

None 14 0 0.0

Light 30 0 0.0

Moderate 61 1.6 0.2

Severe 28 7.1 0.7

Large residuals:

None 0 — —

Light 7 0 0.0

Moderate 18 5.6 0.6

Severe 40 12.5 1.3

a

 New cohort are trees that regenerated after the wind disturbance; residuals were

classified as small or large if they were <20 cm or ≥20 cm diameter at breast height at

the time of the disturbance by Trummer et al. (1998).

b

 Dwarf mistletoe ratings (DMR) were none, light = 1 or 2, moderate = 3 or 4, severe =

5 or 6.

c

 Annual mortality estimated over the 10-year measurement period from 1994 to 2004.
54

GENERAL TECHNICAL REPORT PNW-GTR-718

DeMars (1987) pointed out that a large number of factors, many interrelated,

can influence tree growth, including hemlock dwarf mistletoe severity and infection history. He recommended determining hemlock dwarf mistletoe impacts by

remeasuring infected and healthy trees in sets of paired, long-term plots, ideally

over many decades. Uncertainty about selecting appropriate control trees or stands

will continue to be a problem, however. Unfortunately, to our knowledge there

have never been sufficient resources available or dedicated to establishing and remeasuring a set of plots that represent a sufficiently wide range of ecosystems, forest stands and incidence conditions for hemlock dwarf mistletoe in western forests.

Several extensive long-term installations that have been remeasured over several

years have produced valuable tree-growth data, including hemlock dwarf mistletoe

incidence and severity as described above, but in most of these studies, the early

status of hemlock dwarf mistletoe is unknown. In some instances, plots were

selected to represent healthy and well-stocked stands, and we suspect that stands

with severely infected trees were avoided.

A large proportion of the information on growth loss for western hemlock due

to hemlock dwarf mistletoe comes from studies in British Columbia. The values

reported from these studies are not identical, but they generally agree that lightly

infected trees have no measurable growth loss, moderately infected trees lose about

20 percent of their current annual growth, and severely infected trees, about 40 percent. The recent information from Alaska presented above and similar results from

southern Washington (Shaw et al. 2007) generally agrees with these growth loss

values, suggesting that growth loss associated with various infection levels is similar throughout the natural range of hemlock dwarf mistletoe. This is in contrast to

the rate of life cycle development and spread rates, which vary substantially by

latitude.

With the exception of the few trees sampled by Wellwood (1956) and the information on infected residual trees in Alaska presented above, trees older than 150

years have not been sampled, and there is little information on growth loss caused

by hemlock dwarf mistletoe in old-growth trees. Based on the 60 percent reduction in carbon assimilation of old-growth-infected trees reported by Meinzer et al.

(2004), growth loss in old, slow-growing trees would likely exceed that of fastgrowing younger trees.

Since the 1990s, forestry practices have changed dramatically with more

residual and older trees now being deliberately reserved in harvested areas. Data

on hemlock dwarf mistletoe growth effects in trees older that 100 years are now
55

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

pertinent and highly relevant. In many regions, clearcut harvesting that removes all

or most residual trees has become an exceptional rather than a common practice.

Forest-Level Effects on Growth

Several approaches have been taken to estimate regional or forest-level losses from

dwarf mistletoes, including hemlock dwarf mistletoe (Drummond 1978, Muir and

Moody 2002).

Based on results of growth studies or best estimates, projections have been

made for hemlock dwarf mistletoe impacts in several regions. For Oregon and

Washington, both Childs and Shea (1967) and Shea and Stewart (1972) estimated

that the annual growth loss was approximately 1.2 million m3

. In British Columbia,

Van Sickle and Smith (1978) estimated that annual growth loss was 1.7 million m3

per year. Their estimate was based on 4 million hectares of western hemlock forest

with approximately 15 percent infested by dwarf mistletoe and a growth reduction

of 4 to 5 m3·ha-1·yr-1, based on results of Smith (1969) for a 110-year-old stand.

Bolsinger (1978) summarized information in California, Oregon, and

Washington for dwarf mistletoes from 17,000 inventory plots that sampled an area

of approximately 20 million ha; 84 percent of the total commercial forest area of

these Western States. Of 1.5 million ha of western hemlock and mountain hemlock

forests in western Oregon and Washington, approximately 21 percent were infested

with dwarf mistletoes, presumably either western hemlock or mountain hemlock

dwarf mistletoe. On the plots that had more than 40 percent of the trees infected

by dwarf mistletoe, approximately 15 percent of the infected trees had an average

DMR of more than 2 and would be expected to have some reduction in tree growth.

These results have to be viewed with caution because Drummond (1978) and others

have noted that inventory crews usually underestimate occurrences and severity of

dwarf mistletoe.

Drummond (1982), Childs and Shea (1967), and Shea and Howard (1968) estimated total growth reductions for hemlock dwarf mistletoe for Alaska, Washington,

and Oregon. However, these estimates apparently were based on extrapolations of

limited data.

In one of the most extensive studies and review of A. tsugense occurrence

and effects, Hildebrand (1995) reported that hemlock dwarf mistletoe was widespread in old-growth and young western hemlock forests in Washington and

Oregon. Mountain hemlock dwarf mistletoe was common in the Cascade Mountains throughout Oregon and reported in 10 to 14 percent of the inventory plots in

Washington. However, the distribution in Washington was considered uncertain due
56

GENERAL TECHNICAL REPORT PNW-GTR-718

to unverified identification of the mountain hemlock dwarf mistletoe in many of

the reported areas. In 1994, A. tsugense was included in a “record of decision”

because of the association of hemlock dwarf mistletoe with both the northern

spotted owl and the old-growth forest stage associated with the owl. The decision

stipulated that, on land managed by the U.S. Forest Service and the Bureau of

Land Management in Oregon and Washington, A. tsugense had to be surveyed and,

if present, managed to maintain the viability of the species. Hildebrand’s (1995)

report and supporting statements by recognized experts that viable populations of

hemlock dwarf mistletoe were not threatened in this area reversed this decision.

In addition to an extensive review of the literature, Hildebrand (1995) reviewed

survey data on hemlock dwarf mistletoe occurrence and presented extensive inventory plot data on western hemlock and mountain hemlock dwarf mistletoe incidence and severity. On national forests within the range of the northern spotted

owl, 26 percent of 2,260 plots with western hemlock and 21 percent of 940 plots

with mountain hemlock had dwarf mistletoe. Some of the forests had no A.

tsugense on any plot. The highest occurrence was in the Olympic National Forest

where 50 percent of the plots had A. tsugense.

It is well recognized, e.g., Muir and Geils (2002), that data on dwarf mistletoe

collected by inventory crews with relatively limited experience are of variable

quality and usually underestimate incidence and severity of dwarf mistletoe infection. However, detailed plot data summarized by Hildebrand (1995, see App. B,

Part III, p. 44) could be used for each forest in conjunction with data on area and

growth rates to estimate hemlock dwarf mistletoe growth reductions. These data

indicate that on average, for all of the Washington and Oregon coastal forests,

approximately 10 percent of the western hemlock trees were infected by hemlock

dwarf mistletoe with 5 percent having a DMR of 3 to 6. These percentages were

similar to survey data previously reported by Bolsinger (1978). These moderately

to severely dwarf mistletoe-infected trees probably experience a current annual

growth reduction of 15 to 40 percent. These data support estimates of substantial

regional growth impacts caused by hemlock dwarf mistletoe.

Muir et al. (2004b) extrapolated Hildebrand’s (1995) data to an area of 2.8

million ha of western hemlock forests in British Columbia with an estimated average productivity of 6 m3

·ha-1·yr-1 to estimate a growth reduction of 0.8 million m3

per year due to hemlock dwarf mistletoe.

Approximately 12 percent of forest land in southeast Alaska is infested

with hemlock dwarf mistletoe (table 4). Ignoring the inaccessible wilderness not

sampled, hemlock dwarf mistletoe occurs on an estimated 335 000 ha. Most of the
57

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

occurrence is in the old sawtimber classes, and both the young and old sawtimber

classes have a higher proportional occurrence (19.5 and 13.5 percent, respectively)

than the smaller size classes. These values are likely conservative estimates because

hemlock dwarf mistletoe might not have been recorded when other damage agents

were present. Also, scattered larger trees might have been present in the plots

designated as smaller and younger classes. This could explain, in part, the higher

level of hemlock dwarf mistletoe in the young sawtimber class.

Ecology

Ecological Factors and Geographic Distribution

Reasons for hemlock dwarf mistletoe absence in some western hemlock forests,

such as in western hemlock north of Haines, Alaska, or in the extensive 300-km

area farther west, around the Gulf of Alaska, are unknown. Parmeter (1978)

pointed out that these types of absences are known for many dwarf mistletoes and

likely are caused by a variety of factors. Early autumn frosts and/or short growing

seasons or colder winters probably inhibit or prevent hemlock dwarf mistletoe

occurrence. Gill (1957) suggested that the upper elevational limit of lodgepole

pine dwarf mistletoe in the Rocky Mountains coincided with a 0 °C mean annual

isotherm, suggesting that a similar temperature threshold might be determined for

hemlock dwarf mistletoe. However, with recent climatic warming, it is also possible that hemlock dwarf mistletoe is migrating into these areas. Presumably during

Table 4—Occurrence of hemlock dwarf mistletoe on Forest Inventory

and Analysis (FIA) plots in southeast Alaska

Accessible forest Mistletoe

Stand size classa Sampledb occurrence

 – – Hectares (thousand) – – Percent

Seedling/sapling 270 11 3.9

Poletimber 171 4 2.4

Young sawtimber 283 56 19.5

Old sawtimber 1,968 265 13.5

Nonstocked 88 0 0.0

All size classes 2,780 335 12.0

a

 Size classes defined by plurality of stocking by live, growing stock trees. Poletimber

sized trees: diameter at breast height ≥12.7 cm and < 22.9 cm; sawtimber sized trees:

≥22.9 cm for softwoods and ≥27.9 cm for hardwoods. Young sawtimber and old

sawtimber are distinguished by age of sample trees.

b

 Includes all forest lands in southeast Alaska extending to the Malaspina Glacier

northwest of Yakutat; does not include wilderness areas not sampled by FIA.

With recent climatic

warming, it is also

possible that hemlock dwarf mistletoe

is migrating into

these areas.
58

GENERAL TECHNICAL REPORT PNW-GTR-718

the Pleistocene Epoch, hemlock dwarf mistletoe in southeast Alaska and/or British

Columbia survived the northern continental glaciation in refugia that were not

covered by continental ice. These refugia functioned as epicenters for migrating

vegetation upon climatic warming and deglaciation. Dwarf mistletoes including

A. tsugense recolonize areas slower than their hosts do (Scharpf 1984). Birds are

believed to be largely responsible for stand-to-stand spread of hemlock dwarf

mistletoe, but in many areas such as southeast Alaska, their migratory patterns in

autumn when dwarf mistletoe seeds are produced are not in the appropriate direction to facilitate spread into noncolonized areas (Hawksworth and Wiens 1996).

Regardless of the reasons for the absence of hemlock dwarf mistletoe in areas such

as the entire Yakutat Ranger District and Chugach National Forest in Alaska, its

absence precludes the need for hemlock dwarf mistletoe management there.

With the exception of a few limited occurrences in major river valleys, hemlock dwarf mistletoe is absent in western hemlock forests in the interior region of

British Columbia east of the Cascade Mountains. However, based on results of experimental inoculations, most of these populations of western hemlock are susceptible to hemlock dwarf mistletoe (Smith 1965, 1974) and the parasite is capable

of surviving in these areas (Smith and Wass 1979). Geographic barriers, autumn

frosts, short growing seasons, and slow reinvasion are likely reasons for the absence

of hemlock dwarf mistletoe in these interior hemlock forests.

Upper elevation limits are another interesting ecological association. Dwarf

mistletoes, including A. tsugense, generally occur only up to a few hundred meters

or more below the upper elevation occurrence of their hosts (Hawksworth and

Wiens 1996). The upper elevation limits for dwarf mistletoes generally decrease

from southern to northern latitudes. Mountain hemlock dwarf mistletoe occurs

up to 2500 m elevation in northern California and up to 1200 m in northern

Washington (Geils et al. 2002). Pacific silver fir dwarf mistletoe occurs from

1100 m to 1700 m in central Oregon (Mathiasen and Daugherty 2007). The shore

pine dwarf mistletoe occurs from near sea level up to 800 m in southern British

Columbia (Wass and Mathiasen 2003). The western hemlock dwarf mistletoe

occurs from sea level up to approximately 1250 m elevation in southern Oregon.

The upper elevation limit for the western hemlock dwarf mistletoe in southern

British Columbia was stated to be 1500 m (Alfaro 1985), but we believe that this is

an overestimate. The upper elevation limit in southern British Columbia appears to

be near 800 m, but further surveys are needed (Muir 2004).
59

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Drummond and Hawksworth (1979) stated that hemlock dwarf mistletoe occurred up to 150 m and rarely up to 300 m in southeast Alaska. These observations

are confirmed by Forest Inventory and Analysis plot data (fig. 24). There is an

apparent threshold at approximately 150 m, above which the parasite is uncommon.

This occurs both in productive forests, generally closed-canopy conditions with

good soil drainage, and in unproductive forests, which are more open-canopied on

poorly drained soils. The principal host, western hemlock is common well above

150 m elevation, suggesting that some climatic factor limits the distribution of

hemlock dwarf mistletoe at higher elevations.

Another interesting ecological feature that invites further study is a frequent

association of hemlock dwarf mistletoe with riparian zones (Muir 2004, Swanson

et al. 2006). Like many other dwarf mistletoe species, A. tsugense often is very

common on poor or low-quality sites. Dwarf mistletoes usually occur more commonly on ridges or upper slope positions. However, these associations have not

been quantitatively documented or tested for hemlock dwarf mistletoe. Swanson et

Figure 24—Occurrence of hemlock dwarf mistletoe in southeast Alaska by 30.5-m elevation classes.

Data represent lands supporting either “unproductive” or “productive” (stocked and either not capable or capable of producing 1.4 m3

·ha-1·yr-1 at culmination of mean annual increment, respectively).

Data from Pacific Northwest Research Station, Forest Inventory and Analysis plots distributed

throughout southeast Alaska except inaccessible wilderness areas and collected from 1995 to 2000.
60

GENERAL TECHNICAL REPORT PNW-GTR-718

al. (2006) found hemlock dwarf mistletoe-infected stands associated with riparian

areas in 150- but not in 500-year-old Douglas-fir/western hemlock forests in southern Washington. An analysis of transects indicated statistically significant associations of mistletoe-infected hemlock trees with riparian zones. Swanson et al. (2006)

attributed the occurrence of hemlock dwarf mistletoe in these riparian areas to

survival of infected hemlock trees after wildfires.

Only a few dwarf mistletoes have been reported to occur in riparian areas. A.

pusillum on eastern black spruce is common near bogs and lakes (Hawksworth and

Wiens 1996). Arceuthobium americanum occurs on lodgepole pine in riparian

zones along the Columbia and Kootenay Rivers in southeastern British Columbia.

In a study of shore pine dwarf mistletoe, Wass (1976) noted that the few western

hemlock dwarf mistletoe infestations near Sooke, British Columbia, occurred in

valley bottoms. In contrast, the shore pine dwarf mistletoe occurred on rocky

knolls, ridgetops, and mountain tops. Western spruce dwarf mistletoe also commonly occurs along stream bottoms on lower slopes of drainages on blue spruce in

the Southwestern United States (see footnote 2). However, these drainages are not

major river bottoms, but rather are small stream courses with or without perennial

streams. These areas are subject to cold air drainage, and here blue spruce grows

quite well. Western spruce dwarf mistletoe severely parasitizes blue spruce along

these drainages, but it also occurs up slope on both blue and Engelmann spruce.

Muir (2004) reported several infestations of hemlock dwarf mistletoe in riparian zones in southern and eastern Vancouver Island, British Columbia. In this

“rainshadow” region that corresponds to the coastal Douglas-fir biogeoclimatic

zone (Meidinger and Pojar 1991), precipitation is 1250 mm or less per year. Here,

western hemlock survives wildfire and competition from other tree species predominantly in riparian zones. However, Muir also observed a similar close association of hemlock dwarf mistletoe with riparian zones in northern Vancouver Island

(fig. 25) in much wetter (annual precipitation greater than 1250 mm) areas of the

coastal western hemlock (CWH) zone where western hemlock commonly grows

across the landscape in most topographic positions.

Hemlock dwarf mistletoe shoots and seed production appear more common and

prolific on trees in riparian areas probably because in these areas, stands are more

open and thus trees are exposed to more sunlight. In upslope sites above riparian

zones in the CWH, stands are usually very dense and light is a limiting factor for

hemlock dwarf mistletoe reproduction. However, riparian zones are also subject to

cold-air drainage and possible frost damage to hemlock dwarf mistletoe witches’
61

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

brooms (Kuijt 1960) and berries (Baranyay and Smith 1974). The local microclimate and light regimes need to be determined for these areas.

Hemlock dwarf mistletoe in riparian zones appears to be an important issue

because strips or blocks of old-growth trees are usually retained or reserved from

harvesting in these areas. Occurrences of hemlock dwarf mistletoe in riparian zones

pose a dilemma for management: Should hemlock dwarf mistletoe-infected trees

be retained to protect riparian zones if trees are a risk to increase hemlock dwarf

mistletoe spread and future effects on tree growth and stability? Increased retention

of hemlock dwarf mistletoe-infected trees might be desirable in some northerly

localities where incidence of hemlock dwarf mistletoe has been drastically reduced

by clearcut harvesting and/or extensive, severe windstorms.

The association of hemlock dwarf mistletoe with riparian zones appears less

pronounced in the hypermaritime CWH subzone near Ucluelet and Tofino, British

Columbia. In this subzone of extreme precipitation that extends along the coast and

inland 10 to 15 km, western hemlock usually grows only on small hummocks or

ridges, and western redcedar predominates on low-lying, often flooded terrain.

Here, most of the older western hemlock trees appear severely infected by hemlock

dwarf mistletoe. This situation is similar to southeast Alaska, where hemlock dwarf

mistletoe is no more abundant in riparian areas than in upland forests. The lack of

Figure 25—Hemlock dwarf mistletoe in a riparian zone near Port McNeill, British Columbia.
62

GENERAL TECHNICAL REPORT PNW-GTR-718

fire as a disturbance process probably explains the widespread occurrence of

hemlock dwarf mistletoe in these areas.

Disturbance Regimes

Dwarf mistletoes influence fire behavior in several ways, and wildfires were

probably the primary natural control for most dwarf mistletoe species throughout

most of the Western United States (Hawksworth 1978, Parker et al. 2006). Wildfire is the primary natural factor influencing the distribution and abundance of

many dwarf mistletoes in the Western United States and Canada (Hawksworth

and Wiens 1996, Wright and Baily 1982) and of A. tsugense in the southern Cascades of Washington and Oregon (Swanson et al. 2006). However, in northern

hypermaritme coastal regions, wildfire is an uncommon event and windstorms

typically are the primary natural factors that influence new infestations of hemlock dwarf mistletoe (Trummer et al. 1998).

Swanson et al. (2006) indicated that hemlock dwarf mistletoe survival, spread,

and subsequent impacts are largely dictated by wildfires in the southern portion of

its geographic range and by windstorms in northern regions. Areas where either

wildfire or windstorms are the primary disturbance regime probably overlap and

depend on local climate. In areas of Washington, Oregon, and on the southeastern

coast of Vancouver Island where wildfires are predominant, hemlock dwarf mistletoe-infected western hemlock trees apparently survive in more open or less vulnerable stands on poor or low production sites and in riparian zones. Generally, windstorms appear to be a more important factor than wildfires in high-rainfall areas of

the west coast of Vancouver Island north to southeastern Alaska.

In more northerly, wind-affected areas, hemlock dwarf mistletoe-infected

trees hypothetically occur more often in landscapes not subjected to stand-replacing

windstorms, which in southeast Alaska and along the north coast British Columbia

are on protected north- and northwest-facing slopes. However, as reported here, in

some areas on northern Vancouver Island and possibly farther north, hemlock

dwarf mistletoe appears closely associated with riparian zones. Perhaps western

hemlock survives windstorms more frequently in riparian areas, or riparian areas

might have ecological conditions that favor hemlock dwarf mistletoe survival and

spread. These ecological associations and conditions need to be determined.

Although catastrophic windstorms are often regarded as determining factors,

Hennon and McClellan (2003) found that local, more frequent and lower intensity

storms, as well as other tree mortality agents, had a major influence on creating

Wildfire is the primary natural factor

influencing the

distribution and

abundance of many

dwarf mistletoes in

the Western United

States and Canada.
63

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

gaps and openings that encourage hemlock mistletoe spread and impacts. Even in

catastrophically wind-disturbed forests, some western hemlock trees survive,

enabling hemlock dwarf mistletoe spread to regenerating stands.

Stand Structure and Species Composition

By causing changes to individual infected trees, including formation of witches’

brooms, reduced height growth, top-kill, and tree mortality, hemlock dwarf mistletoe significantly alters the structure and composition of forest stands. Witches’

brooms are important and unique elements that contribute to structural diversity of

forest stands (Mathiasen 1996, Tinnin et al. 1982).

In mixed-species stands infested with hemlock dwarf mistletoe, trees that are

less susceptible and growing near severely infected western hemlock trees can have

a competitive advantage. Annual height growth of severely infected western hemlock can be reduced by up to 80 percent (Smith 1969). In Alaska, the growth and

dominance of individual Sitka spruce trees appear enhanced in stands with hemlock

dwarf mistletoe-infected western hemlock. Relatively slow-growing yellow-cedar

trees appear able to maintain their position in the upper canopy in mixed stands on

productive soils only when the growth of western hemlock is slowed by hemlock

dwarf mistletoe infection. In several stands near Sitka, Alaska, yellow-cedar trees

in mixed stands grow in the upper dominant and codominant crown positions with

dwarf mistletoe-infected western hemlock in lower crown positions. Hemlock

dwarf mistletoe infestations can enhance a greater diversity of tree species by

suppressing the growth of the prolifically reproducing western hemlock.

Dwarf mistletoe probably alters other ecological processes in old-growth forests. Severe mistletoe infections kill trees and create canopy gaps by which oldgrowth forests are maintained (fig. 26). Another process contributing indirectly

to canopy gap formation is that old bole swellings of hemlock mistletoe provide

infection courts for heart rot fungi that result in heart rot and bole breakage (fig.

27). Both of these processes of tree mortality—snag formation and bole breakage—

do not cause substantial soil disturbance or encourage reestablishment of western

hemlock. Uprooting, a third major cause of canopy gap formation, causes considerable mixing of organic and inorganic layers, forms pit/mound topography, alters

soil development processes, and probably leads to a different composition of

understory flora and tree species. Thus, the manner in which trees die, a process in

which hemlock dwarf mistletoe often participates, has a number of pronounced and

complicated effects on forest structure, composition, and developmental processes.

Hemlock dwarf

mistletoe infestations can enhance

a greater diversity

of tree species by

suppressing the

growth of the prolifically reproducing

western hemlock.
64

GENERAL TECHNICAL REPORT PNW-GTR-718

Figure 26—Canopy gap associated with dead hemlock dwarf mistletoe infected tree.

Figure 27—Bole breakage of western

hemlock and canopy gap associated

with hemlock dwarf mistletoe infection.
65

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Larger scale effects can develop in very severely infested western hemlock

stands. Top-kill, tree mortality, reduced height growth, and the reduction in wood

volume production can develop to such an extent that stands have a broken canopy

and stunted trees. In some forest ecosystems, this results in a succession of mistletoe-resistant tree species. However, in nearly pure stands of severely infected western hemlock, western hemlock regenerates prolifically and predominates in all

canopy levels. A high incidence of hemlock dwarf mistletoe develops as understory hemlock trees grow in the canopy gaps that resulted from the death of dwarf

mistletoe-infected trees in the overstory. Hemlock dwarf mistletoe is able to maintain itself in the upper canopy level, and its negative effects on height growth often

stunts trees. In severe cases, the entire stand eventually becomes stunted and lacks

the typical vertical stand structure that develops in the absence of hemlock dwarf

mistletoe. This topic is covered in more detail in the section on dwarf mistletoe

“Effects of Stand Development.”

The role of hemlock dwarf mistletoe in creation of canopy openings or gaps

needs further study. Several dwarf mistletoe species have been reported to cause

canopy gaps in forest canopies including A. pusillum on black spruce (Baker and

French 1991) in eastern forests, A. americanum on jack pine (Muir and Robins

1973) in Alberta, hemlock dwarf mistletoe on western hemlock in Alaska (Hennon

1995, Hennon and McClellan 2003), and other dwarf mistletoes on other tree

species (Mathiasen 1996). Gaps caused by Douglas-fir dwarf mistletoe can have

positive, neutral, or negative ecological effects (Lundquist et al. 2002), but these

effects have yet to be determined for hemlock dwarf mistletoe. In British Columbia, canopy gaps associated with or caused by hemlock dwarf mistletoe occur in

western hemlock forests as young as 80 to 90 years (Muir 2004).

Hennon and McClellan (2003) investigated patterns of canopy gaps and tree

death in 27 old-growth forests in southeast Alaska, 18 of which contained hemlock

dwarf mistletoe. The annual mortality rate of codominant and dominant trees, i.e.,

the “gapmakers,” in these 27 forests was approximately 0.4 percent through the

1900s. Most hemlock trees died standing, but approximately 20 percent each died

through bole breakage and uprooting. Small canopy gaps that were formed by the

death of individual dead trees and without soil disturbance (by dead standing trees

or bole breakage) favored the regeneration of shade-tolerant western hemlock.

Evaluating the mode of tree death in older dead trees was difficult because broken

boles can result from sudden death through stem snap or from dead standing trees

deteriorating to the stage where boles eventually break. By contrast, uprooting as a

cause of tree death was evident long after death. Examining the more recently
66

GENERAL TECHNICAL REPORT PNW-GTR-718

killed trees indicated that in these forests, most trees died standing. The incidence

of these types of tree death did not differ substantially in forests where hemlock

dwarf mistletoe was present or absent. Hemlock dwarf mistletoe was just one of

many mortality agents of large western hemlock trees that led to canopy gaps. In

forests with hemlock dwarf mistletoe, approximately 20 to 25 percent of dead

western hemlock trees had evidence of infection (witches’ brooms). The ecological

maintenance of old-growth forests through tree death and canopy gap formation

does not appear to require the occurrence of hemlock dwarf mistletoe; however, it

could affect rates and types of tree death in heavily infested forests.

A key factor in gap creation appears to be death and/or bole breakage of large

trees that were the source trees for the hemlock dwarf mistletoe infestation. Large

old hemlock dwarf mistletoe-infected trees often have pronounced swellings on

stems that are frequently extensively decayed by wood-decay fungi (e.g., Hennon

1995). However, their occurrence and deterioration are variable. In Washington,

Shaw et al. (2005) found that large, severely infected trees had few, if any, bole

infections, and only a few smaller understory trees had bole infections.

Canopy gaps caused by hemlock dwarf mistletoe are believed to play an important role in the ecology and development of western hemlock and western

redcedar forests on northern Vancouver Island and the adjacent coastal mainland

areas.6

 In windstorms or in topographic positions exposed to wind, hemlock dwarf

mistletoe-infected trees often break off at large stem swellings rather than being

uprooted. Many broken-off trees survive and continue as source trees for further

hemlock dwarf mistletoe infection. Breakage of trees and creation of small gaps

or openings encourages regeneration of western hemlock and discourages western

redcedar, which requires full exposure to light and mineral soil for successful

establishment (see footnote 6). Further research is needed to test these hypotheses,

but it appears likely that gap creation by hemlock dwarf mistletoe encourages

regeneration of its host species.

Shaw et al. (2005) studied the distribution and severity of hemlock dwarf

mistletoe in an old-growth Douglas-fir and western hemlock forest at the Wind

River Canopy Crane Research Facility, Washington. A 75-m-tall construction crane

at the research site was used to examine hemlock dwarf mistletoe closely in tree

crowns. The forest was somewhat unique in that western hemlock trees gradually

replaced many of the older Douglas-fir trees over the past century (Franklin and

6

 Kimmins, J.P. Professor, 2006. Personal communication. Department of Forest Sciences,

University of British Columbia, Vancouver, BC.
67

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

DeBell 1988). The forest canopy was complex with western hemlock foliage predominating from 15 to 35 m height, western hemlock and Douglas-fir codominant

from 35 to 45 m, and Douglas-fir predominant from 45 to 55 m (Parker 1997).

Hemlock dwarf mistletoe infestations at the site developed in relatively discrete

patches (Shaw et al. 2005). Infected western hemlock trees were larger than uninfected trees, and severely infected trees were larger than moderately or lightly

infected trees. Shaw et al. (2005) suggested that the more severe infection of taller

trees resulted from birds that dispersed hemlock dwarf mistletoe seeds preferentially to the taller and larger trees. Several bird species that were observed feeding

in mistletoe-infected trees during dwarf mistletoe seed dispersal at Wind River Canopy Crane Research Facility are known from studies in other regions to be potential

vectors for long-distance dispersal. They suggested that birds might be responsible

in whole or part for establishment of mistletoe infestation centers in old-growth

forests and for the common occurrence of hemlock dwarf mistletoe infections in

the upper crown of severely infected trees. However, they recognized other possible

scenarios for explaining the patchy occurrence of hemlock dwarf mistletoe including survival of the mistletoe on scattered trees after wildfire that grew to become

some of the older and taller trees.

Effects of Stand Development

The abundance, reproductive success, and effects of hemlock dwarf mistletoe on

trees and ecosystems are closely linked to and depend on stand development. Oliver

and Larsen (1990) developed terminology and described several developmental

stages that represent the regrowth of even-aged forests after a disturbance such as

wildfire, catastrophic windthrow, or clearcut harvesting. These and several other

models of stand development are tabulated and compared by Franklin et al. (2002).

Note that most of the models of stand development depend on a stand-replacing

disturbance, either through clearcut harvesting or some natural event, and most do

not apply to partial disturbance or retention harvesting. Partial disturbances or

selection harvesting lead to multiaged forests with decidedly more complicated

and more serious, scenarios for hemlock dwarf mistletoe spread. Details of hemlock dwarf mistletoe biology and spread that are described in previous sections are

highlighted in the following stand-development scenarios presented by Franklin et

al. (2002) and summarized in table 5. Depending on the climate and disturbance

regimes, several differences in stand development and effects on development of

hemlock dwarf mistletoe are apparent.

The abundance,

reproductive success, and effects

of hemlock dwarf

mistletoe on trees

and ecosystems are

closely linked to and

depend on stand

development.
68

GENERAL TECHNICAL REPORT PNW-GTR-718

Table 5—Summary of events and processes in development of coastal old-growth forestsa in wind- or wildfiredisturbed regions and effects on hemlock dwarf mistletoe (HDM)

Stand development effects on HDM

Stand-development Stand age

process (years) Wind disturbance regime Wildfire disturbance regime

Disturbance <1 Common in high rainfall climates Common in low rainfall climates

where western hemlock is the where Douglas-fir is the

predominant pioneer tree species. predominant pioneer tree species.

Residual forest structure 0 HDM survives on residual infected HDM survives on residual infected

trees depending on severity of trees depending on severity of

disturbance, topographical features disturbance, topographical features

of the landscape. HDM infection of the landscape, particularly in

affects tree vulnerability to bole moist riparian zones. HDM-infection

breakage and uprooting in the affects tree vulnerability. Broken

actual disturbance event. witches’ brooms and large branches

at the base of the bole increase

damage. Severe HDM infection in

lower crown increases vulnerability

to ground fires.

Establishment of new 1-20 (+) Many western hemlock seedlings Mostly seedlings established from

cohort of trees that were previously suppressed seed dispersed from residual trees

and survive “release” after after disturbance. Douglas-fir, only

disturbance and often achieve rarely infected by HDM, often

dominant position and numbers. predominates. Western hemlock

HDM that survives on residual survives and grows well in shady

trees is stimulated to produce or moist microsites. HDM that

seeds and infect young trees. survives on residual trees is

Extent of HDM infection of stimulated to produce seeds and

young trees and subsequent infect young trees.

effects depends on the number

of infected residual trees.

Canopy closure 25-35 (+) HDM on lower crown branches HDM on lower crown branches

is suppressed and loses aerial is suppressed and loses aerial

shoots but survives on bole and on shoots but survives on bole and

nonfoliated branches of surviving on nonfoliated branches of

trees. HDM-infected small or unthrifty surviving trees. HDM-infected

trees die through competition. small or unthrifty trees die through

competition.

Biomass 35-100 HDM upward spread in tree crowns HDM upward spread in tree crowns

accumulation, continues in open stands or, in continues in open stands or, in

tree mortality, dense stands, slows, is arrested, or dense stands, slows, is arrested, or

canopy gap is eradicated. Growth of severely is eradicated. Growth of severely

development HDM-infected trees is reduced, HDM-infected trees is reduced, and

and infected trees can be overtaken infected trees can be overtaken and

and replaced by nonhost and/or less replaced by nonhost trees. In dense
69

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Table 5—Summary of events and processes in development of coastal old-growth forestsa in wind- or wildfiredisturbed regions and effects on hemlock dwarf mistletoe (HDM) (continued)

Stand development effects on HDM

Stand-development Stand age

process (years) Wind disturbance regime Wildfire disturbance regime

severely infected trees. Dead HDM- stands with large numbers of

infected trees and other disturbances Douglas-fir, HDM could be

initiate canopy gaps. Depending on eradicated. Disturbances initiate

gap size, western hemlock trees canopy gaps. Depending on gap

regenerate and are infected by size, western hemlock trees

HDM from nearby live trees. regenerate and are infected by HDM

from nearby live trees in moist areas

or riparian zones.

Maturation of pioneer 150+ Western hemlock trees mature and Douglas-fir trees mature at 200 to

trees, canopy reach maximum height at 150 years 300 years and survive to 1,200+

development, and and survive to 350 to 500 years. years. Canopy gaps develop slowly

loss of pioneer trees In canopy gaps, infected trees over several hundred years with

refoliate, and suppressed infected progressive invasion and

trees grow to a codominant or replacement by shade-tolerant tree

dominant position. Depending species such as western hemlock

on frequency and severity of with extensive, severe HDM

disturbances, western hemlock can infection. Depending on frequency

become extensively and severely and severity of disturbances, western

infected and replaced by other hemlock can become extensively

nonsusceptible species such as and severely infected and replaced

western redcedar, yellow-cedar, by other nonsusceptible species such

and/or Sitka spruce. Or, in pure as western redcedar, yellow-cedar,

western hemlock stands, HDM and/or Sitka spruce. Large witches’

can become so prevalent that brooms with large-diameter branches

trees become stunted with large develop on infected trees. Oldwitches’ brooms with large- growth infected trees commonly die,

diameter branches developing on creating gaps colonized by more

most trees. Old-growth infected western hemlock.

trees commonly die, creating gaps

colonized by more western hemlock.

a Based on Franklin et al. (2002), tables 2 and 3.
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GENERAL TECHNICAL REPORT PNW-GTR-718

Disturbances and establishment of seedlings and saplings—

In northern high-rainfall areas that have very few or no wildfires and are affected

mostly by windstorm disturbances, some newly developing forests predominately

consist of residual trees, i.e., those that were previously established as understory

trees in the preceding stand (Pearson et al. 2003). In others, there is a mixture of

residual and postdisturbance regenerated trees (Shaw 1982). In areas of drier

climates where wildfires are the major disturbance, newly regenerated trees that

were established after a wildfire predominate. In these drier zones, mistletoeinfected and noninfected western hemlock trees survive in moist riparian zones

(Muir 2004, Swanson et al. 2006).

Many understory seedlings and saplings survive local or catastrophic disturbance. In particular, western hemlock seedlings and saplings, which are abundant

on the forest floor of many old stands, survive local or large-scale windthrow

events and clearcut harvesting, but almost all are killed by wildfire and landslide

events. Western hemlock saplings that survive and are infected with hemlock dwarf

mistletoe from the old stand are sources for the parasite to spread into the regenerating forest (fig. 28). The type and intensity of the disturbance affects the number

and distribution of residual hemlock dwarf mistletoe-infected trees. These have a

major and long-term effect on the amount and extent of hemlock dwarf mistletoe

that develops in the resulting forest (Smith 1966, 1973, 1977; Trummer et al.

1998; see footnote 4).

On productive sites, the growth of residual saplings and newly regenerated trees

is rapid because of the availability of light and nutrients. Residual saplings have a

competitive advantage and usually maintain a dominant position in the new stand.

Regeneration typically becomes dense, and as the stand approaches the age of about

25 to 30 years, it forms a closed continuous canopy. As trees continue their rapid

height growth, many lower branches die from shading. Hemlock dwarf mistletoe

can survive for several decades or more on lower shaded branches, but it must

spread vertically fast enough to maintain its presence in the upper well-exposed

canopy to be able to continue to produce and disperse seeds. On several sites in

Alaska, the height growth of young western hemlock trees was twice the vertical

spread rate of the parasite (Shaw and Hennon 1991). These results suggested that

on productive sites, the rapid height growth of young western hemlock trees results

in dense canopies that shade out lower branches, prevent hemlock dwarf mistletoe

spread, and possibly even eliminate hemlock dwarf mistletoe from some stands.

On less productive sites with young-growth stands that have more open canopy and

The type and intensity of the disturbance affects the

number and distribution of residual

hemlock dwarf

mistletoe-infected

trees. These have a

major and long-term

effect on the amount

and extent of hemlock dwarf mistletoe.
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

slower growing trees, hemlock dwarf mistletoe could spread vertically fast enough

to keep pace with the height growth of western hemlock and thereby retain its

position in the upper canopy. Hence, a large proportion of trees could have numerous infections in the upper third of the crowns. However, hemlock dwarf mistletoe

spread and intensification are likely affected by its host tree physiology as recently

shown for southwestern dwarf mistletoe on ponderosa pine (Bickford et al. 2005).

If so, vertical spread of hemlock dwarf mistletoe might be less on less productive

sites. Some estimated rates are described below, but further information is needed

on vertical spread rates of hemlock dwarf mistletoe on various sites.

Competition and stem exclusion—

In this phase of stand development, growing space becomes fully occupied, tree

mortality increases due to intertree competition, and little to no new regeneration

occurs. Tree competition for light, soil moisture and nutrients intensifies as tree

crowns and roots become more crowded. Many smaller trees are disadvantaged,

become suppressed, and die from increasing competition. As the rapid height

growth of trees in the closed-canopy forest continues in the pole stage, hemlock

dwarf mistletoe is further suppressed or even eradicated from the stand. Severely

infected trees can become suppressed and die. Hemlock dwarf mistletoe can survive

on lower branches or on the bole of some infected trees, but under dense shade,

Figure 28—Hemlock dwarf mistletoe-infected residuals in recently harvested area at Denman

Island, British Columbia.
72

GENERAL TECHNICAL REPORT PNW-GTR-718

these infections generally cease producing reproductive shoots. This suppresses

further spread at least until the next disturbance. Even if reproductive shoots were

to persist, spread of mistletoe seeds from lower branch and bole infections up into

the live canopy generally does not occur because of the dense foliage.

A comprehensive examination of even-aged stands in the pole stage in southeast Alaska using an existing plot system installed to evaluate the effects of thinning

(DeMars 2000) indicates a nearly complete absence of hemlock dwarf mistletoe. In

the few instances of occurrence, hemlock dwarf mistletoe in pole-sized trees was

always associated with spread from nearby, large residual trees that had survived

the initial disturbance event.

Results from several studies in British Columbia that were undertaken in

mature stands to determine growth effects of hemlock dwarf mistletoe (Smith

1969; Thomson et al. 1984, 1985a, 1985b) supported this scenario but suggested

another interpretation. Stands with moderate to severe hemlock dwarf mistletoe

infection were typically uneven-aged, with most of the severely infected trees

20 to 80 years older than the lightly or uninfected trees. The older tree age was

evidently due to periods of suppressed growth as understory trees before the

disturbance. All trees grew rapidly after the disturbance, but several decades later,

growth of moderately to severely infected western hemlock trees had significantly

slowed compared to uninfected and lightly infected trees. Nevertheless, these older

trees had an initial height advantage that enabled them to maintain a dominant

position until recent years. These results indicated that substantial infestations of

hemlock dwarf mistletoe could develop progressively in some western hemlock

forests on good or highly productive sites and cause significant growth reductions,

as long as some residual trees remained after the harvest or disturbance.

Rate of increase in hemlock dwarf mistletoe infestations appears to be correlated with survival of hemlock dwarf mistletoe-infected understory trees that are

able to grow rapidly to maintain a dominant position in the new stand. Apparently

in these situations, hemlock dwarf mistletoe-infected residual trees that survive in

sufficient numbers enable the parasite to spread vertically, possibly by contributing

to the spread of hemlock dwarf mistletoe between infected trees (Bloomberg and

Smith 1982, Bloomberg et al. 1980, Knutson and Tinnin 1980). Further detailed

examinations of tree growth, establishment of hemlock dwarf mistletoe, and stand

development are needed.
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Transition to old growth—

Older immature or pole-sized stands eventually begin to “break up” in transition to

the next true, old-growth stage. Toppling, breakage and/or mortality of dominant

and codominant trees create canopy gaps due to a variety of causal or contributing

agents (Hennon 1995, Hennon and McClellan 2003). These include wind, heartwood decay, root disease, hemlock dwarf mistletoe, and insects. Trees particularly

susceptible to windthrow and breakage are tall, small-diameter trees with excessive

slenderness as expressed by an extreme ratio, i.e., 90 or more, of total height to

diameter at breast height. In this stage of stand development, trees occupy almost

all of the rooting space and carrying capacity of a site and their height growth

begins to slow. In some areas, hemlock dwarf mistletoe has already been largely or

completely eliminated from the stand during earlier stages. In other areas, hemlock

dwarf mistletoe survives in riparian areas or on suppressed understory trees and

reinvades as western hemlock regenerates. In this instance, hemlock dwarf mistletoe

incidence and impacts depend on the number, size, and infection levels of residual

hemlock trees that survived the disturbance that gave rise to the new stand.

Stands with hemlock dwarf mistletoe in the transition stage to old-growth are

not particularly common in southeast Alaska. Casual observation of five stands

approximately 150 years old suggested that hemlock dwarf mistletoe was absent at

this stage in all but one stand. Hemlock dwarf mistletoe was found near Sitka,

Alaska, in the Verstovia forest, which as a former Russian territory, was harvested

in the 1860s. Hemlock dwarf mistletoe in the stand probably survived on some of

the residual hemlock trees left during the harvest. In British Columbia, several

examples of western hemlock with hemlock dwarf mistletoe in this type of transitional stage were observed in 120-year-old trees south of Beaver Cove near Naka

Creek and in a 90- to100-year-old stand near Vancouver (Muir 2004).

Old growth—

Trees of multiple ages, various sizes, and numerous canopy levels characterize the

old-growth stage. Throughout its range, hemlock dwarf mistletoe is known to thrive

at this stage of stand development (Buckland and Marples 1952) (fig. 29) particularly on steep slopes (see footnote 5). Disturbance events are typically small and

scattered throughout the stand where one or a few trees die through uprooting, bole

breakage, or by any number of agents that leave dead trees standing. These disturbances are often described as canopy-gap or gap-phase disturbance processes. A

small-scale disturbance regime is highly favorable for the development of hemlock

dwarf mistletoe if trees with the parasite are present. Hemlock dwarf mistletoe

A small-scale disturbance regime is

highly favorable for

the development of

hemlock dwarf

mistletoe if trees

with the parasite are

present.
74

GENERAL TECHNICAL REPORT PNW-GTR-718

spreads efficiently over the short distance from infected large trees on the periphery

of the gap to smaller hemlocks regenerating or releasing in the gap. In some instances, large fallen but only partially uprooted hemlock dwarf mistletoe-infected

trees survive with hemlock dwarf mistletoe infections near ground level in proximity to regenerating trees (fig. 30). The increased availability of light in stand openings enhances aerial shoot growth and reproduction of the parasite (Shaw and Weiss

2000). Abundant hemlock dwarf mistletoe seed production in peripheral large

hemlock trees leads to a greater distance of seed dispersal and increased interception

of seeds by young trees.

The common occurrence of hemlock dwarf mistletoe in old-growth stands is

difficult to explain in view of the scarcity of the parasite in many young and

Figure 29—Hemlock dwarf mistletoe in an uneven-aged stand of

western hemlock at Tow Hill, Naikoon Provincial Park, Queen

Charlotte Islands (Haida Gwaii), British Columbia.
75

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

immature stands. One explanation, at least in northern forest ecosystems not

subjected to wildfire, could be that old-growth forests usually are only partially

disturbed and rarely experience a severe stand-replacing disturbance that removes

all overhead sources of hemlock mistletoe seeds. Thus, occurrence of hemlock

dwarf mistletoe in a forest could be used as an indicator that there has been no

relatively recent catastrophic disturbance.

In areas where hemlock dwarf mistletoe has been suppressed and/or eradicated,

long-range dispersal of hemlock dwarf mistletoe seeds, thought to occur by birds

or mammals inadvertently carrying seeds to the upper live canopy, might be the

means by which the parasite recolonizes the stand. In a 350-year-old Douglas-fir

and western hemlock forest at the Wind River Canopy Crane Research Facility

in southern Washington, Shaw et al. (2005) found relatively distinct and separate

hemlock dwarf mistletoe infection centers that they believed could have been

established by long-distance spread of hemlock dwarf mistletoe seeds. Shaw et al.

(2005) observed several animals including the Douglas squirrel and several bird

species including red crossbills that were present during the hemlock dwarf mistletoe seed-dispersal period.

However, long-distance hemlock dwarf mistletoe seed dispersal appears to be

an extremely rare event. Another possibility is that hemlock dwarf mistletoe infections could survive for many decades on lower branches of trees (Trummer et al.

Figure 30—Hemlock dwarf mistletoe infection on fallen, partially uprooted tree near

Port McNeill, British Columbia.
76

GENERAL TECHNICAL REPORT PNW-GTR-718

1998). As old-growth stands break up, these infections could begin to produce

aerial shoots and hemlock dwarf mistletoe seeds, which infect nearby regenerating

trees.

If reintroduction of hemlock dwarf mistletoe into old-growth stands is a rare

event, then it might take many centuries before hemlock dwarf mistletoe is reestablished and builds to its commonly observed, abundant levels. If so, then in some

areas the practice of clearcut harvesting or possibly even severe selection cutting

(Deal et al. 2002) could remove hemlock dwarf mistletoe from a site for a long

time. The presence or abundance of hemlock dwarf mistletoe might indicate how

long a forest has persisted in the old-growth condition.

Analyses of pollen deposits have provided several insights on development

of forest vegetation and dwarf mistletoe in coastal regions, including analyses of

surface pollen in different ecological zones in British Columbia (Allen et al. 1999).

Dwarf mistletoe pollen records indicate local occurrences of the mistletoe because

dwarf mistletoe pollen is dispersed to distances of only a few hundred meters

(Hawksworth and Wiens 1996), whereas tree pollen is distributed more extensively

and pollen records indicate regional occurrences (Brown and Hebda 2002). Results

of several studies (Allen et al. 1999; Brown and Hebda 2002; Greenwald and

Brubaker 2001; Hansen and Easterbrook 1974; Hebda 1983; Heusser 1973a, 1973b,

1974, 1977, 1978, 1983; Mathewes 1973; Mathewes and Rouse 1975; Peterson et

al. 1983) suggested that although western hemlock was abundant there were

lengthy periods of occurrences and absences of hemlock dwarf mistletoe in the

Holocene Epoch. Although limited, these data indicated that in some areas, hemlock dwarf mistletoe incidence varied considerably over periods of several hundred

years. Periods of high incidence of hemlock dwarf mistletoe often were interspersed with periods of low incidence possibly because of local extinction by wildfire and/or climatic changes and reintroduction or due to long-term persistence of

infections without shoots and flowers at very low levels of incidence until conditions favor renewed flowering.

The role that dwarf mistletoe plays in either maintaining old-growth forests

in a fairly sustainable equilibrium by causing scattered mortality or producing a

stunted, stagnant and unproductive forest is not known. Examples of both scenarios

can be found in old-growth forests. Also, well-functioning western hemlockdominated old-growth forests can be found that lack hemlock dwarf mistletoe.

Wood decay fungi, other pathogens, and other disturbance agents also lead to

scattered tree mortality and diverse tree structures such as large standing snags,
77

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

uprooted trees, trees with broken boles, and large cavities (Hennon 1995, Hennon

and McClellan 2003). These raise questions about whether or not hemlock dwarf

mistletoe represents a keystone species in these forests. Hemlock dwarf mistletoeinfested gaps likely have a variety of positive, negative, and no ecological effects as

reported by Lundquist et al. (2002) for gaps associated with Douglas-fir dwarf

mistletoe.

Ecological conditions and processes leading to a mistletoe-stunted western

hemlock forest climax are largely unknown or speculative. We propose the following scenario: As hemlock dwarf mistletoe becomes common in an old-growth forest, canopy gap disturbance favors its spread and buildup. As trees become severely

infected and die, hemlock dwarf mistletoe becomes one of the common mortality

factors responsible for gap formation. Hemlock dwarf mistletoe infection is often

associated with wood decay, which predisposes old trees to bole breakage, another

cause of gap enlargement. An old-growth forest is in a dynamic equilibrium with

gaps opening and closing across the landscape. Small gaps with little or no soil

disturbance (i.e., trees do not die by uprooting with its associated soil mixing) lead

to forest stands composed almost entirely of western hemlock, because (1) small

openings inhibit establishment of nonhost trees such as Sitka spruce that reduce

hemlock dwarf mistletoe spread and (2) small openings and the lack of soil disturbance enhance regeneration of western hemlock, further favoring hemlock dwarf

mistletoe infection. The parasite continues to spread, eventually reaching intense

levels where every hemlock is severely infected by hemlock dwarf mistletoe. As

hemlock dwarf mistletoe becomes abundant in the upper canopy, infected trees experience greatly reduced height growth and top-kill, resulting in a stunting of the

height development of the entire stand. In this scenario, an old-growth forest

culminates in a stunted, mistletoe-infested, forest complex until the occurrence

of a stand-replacing disturbance. Because of the reduced height development,

these forests would be less susceptible to catastrophic windthrow events.

Wildlife Habitat and Biodiversity

Dwarf mistletoe infestations create or are associated with gaps or openings in forest canopies that increase biodiversity of tree species, tree ages, and other vegetation (Mathiasen 1996). Trees with large witches’ brooms create habitat particularly

favorable for wildlife. Results of several previous studies indicate that animals and

birds roost and nest in pine and Douglas-fir stands infested by dwarf mistletoe.

Hawksworth and Wiens (1996) and Geils et al. (2002) summarized information

These raise questions about whether

or not hemlock

dwarf mistletoe

represents a keystone species in

these forests.
78

GENERAL TECHNICAL REPORT PNW-GTR-718

on the interaction of wildlife and other dwarf mistletoe species. Currently, there is

little knowledge about wildlife use of hemlock dwarf mistletoe.

Wildlife—

Large hemlock dwarf mistletoe witches’ brooms are often flat, densely branched

structures that could be suitable as nesting platforms for marbled murrelets (Lank et

al. 2003). Marbled murrelet nests were found on hemlock dwarf mistletoe-infected

branches of western hemlock in Washington state (Nelson and Wilson 2002), but in

British Columbia, most of the few murrelet nests located were not associated with

hemlock dwarf mistletoe-infected trees (Burger 2002). Further work is needed on

this association.

Older hemlock dwarf mistletoe-infected trees frequently have dead tops and

decayed branches that appear suitable for cavity-nesting birds and animals. In

hemlock dwarf mistletoe-infested stands, large amounts of fallen, hemlock dwarf

mistletoe-infected branches accumulate, and these could provide substantial cover

for small animals. In Alaska, Bakker and Hastings (2002) found that northern

flying squirrels made dens in hemlock dwarf mistletoe-infected western hemlock

trees.

Several animals have been reported to use dwarf mistletoes for food by consuming dwarf mistletoe shoots and fruits or by chewing on the swollen bark tissue,

which contains nourishing carbohydrates (Geils et al. 2002, Hawksworth and Wiens

1996, Knutson 1979). Red squirrels on Vancouver Island, British Columbia, fed on

shore pine dwarf mistletoe infections on shore pine but not on western hemlock

dwarf mistletoe on western hemlock (Wass 1976). Shaw and Hennon (1991) noted

that a mammal, perhaps the northern flying squirrel, frequently and selectively

consumed the bark of hemlock dwarf mistletoe infections in young-growth western

hemlock forests on Prince of Wales Island, Alaska.

Given the abundance of hemlock dwarf mistletoe witches’ brooms in western

hemlock forests and the dense cover that they provide, it is possible that they are

important for hiding, nesting, and feeding habitat. Animal use may be overlooked

because from the ground it is difficult to inspect hemlock dwarf mistletoe witches’

brooms. In some instances, it might be desirable to actively manage hemlock dwarf

mistletoe-infested trees to maintain or enhance some of these features for wildlife.

However, further work is needed to determine what wildlife species are using

witches’ brooms associated with hemlock dwarf mistletoe infection and how

important this habitat is for these species.
79

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

General biodiversity—

Effects of hemlock dwarf mistletoe on general biodiversity are little known, but it

is suspected that hemlock dwarf mistletoe infestations encourage biodiversity as was

found for other dwarf mistletoes (Geils et al. 2002, Hawksworth and Wiens 1996,

Mathiasen 1996). In pine forests of Colorado, the number of individual birds and

bird species detected was positively correlated with the level of dwarf mistletoe,

i.e., more birds and more bird species with more dwarf mistletoe (Bennetts et al.

1996). Mammals (mule deer and elk) were also more numerous, as estimated by

pellet counts, in dwarf mistletoe-infested pine forests than in uninfested forests in

Colorado (Bennetts et al. 1996).

Stevens and Hawksworth (1970, 1984) summarized the ecological relationships

between insects and dwarf mistletoe species. There is little information on these

interactions for hemlock dwarf mistletoe, but it likely plays an important part in

the complex food web as do other dwarf mistletoes and leafy mistletoes. Conceivably, hemlock dwarf mistletoe witches’ brooms provide a combination of hiding

cover and tissue of high nutritional value that encourages a flourishing community

of invertebrates. Insectivorous birds have been observed spending considerable

amounts of time feeding in hemlock dwarf mistletoe witches’ brooms in southeast

Alaska7

 and southern Washington (Shaw et al. 2002). Perhaps a greater diversity or

biomass of insects occurs in these witches’ brooms.

The Johnson’s hairstreak butterfly (Loranthomitoura johnsonii) is common in

Oregon but is considered to be a rare and possibly an endangered species in British

Columbia and Washington (Guppy and Shepard 2001, McCorkle 1962). In Washington, a permit is required to collect the butterfly (see footnote 3). The larvae feed

exclusively on hemlock dwarf mistletoe shoots. With most of the shoots on mistletoe infections occurring high in tree crowns, the insect is rarely collected. Larvae

are present on shoots in late autumn to early spring, and Muir observed a specimen

on aerial shoots of hemlock mistletoe on a fallen witches’ broom at Vancouver,

British Columbia. Larvae are well camouflaged by their remarkable mimicry of

mistletoe shoots (See Hawksworth and Wiens 1996, p. 80, for an illustration of a

related species L. spinetorum).

In addition to the reports described above, more information on wildlife

interactions with dwarf mistletoe is discussed in the section “Long-Distance Seed

Dispersal.”

7

Willson, M. 2000. Ecologist. Personal communication, 5230 Terrace Pl. Juneau, AK

99801.
80

GENERAL TECHNICAL REPORT PNW-GTR-718

Management of Hemlock Dwarf Mistletoe-Infested

Western Hemlock Forests

There is a critical need to have detailed plans for the management of coastal forests

that have hemlock dwarf mistletoe infestations. Coastal forests are reserved and/or

managed for a variety of purposes and resources. These include:

• Retention of natural forest ecosystem features and functions

• Maintenance of viewscapes and landscapes for visual quality objectives

• Wildlife habitat and reserves

• Watersheds and riparian zones

• Control erosion and soil stability on steep slopes

• Recreation and tourist facilities (campsites)

• Townsites and urban development

• Commercial production of timber and nontimber resources

As outlined by previous authors and/or as described in this review, hemlock

dwarf mistletoe infestations can affect and/or confound management for all of the

above objectives. Amaranthus et al. (1998) and Weigand (1998) reported an example of management of hemlock dwarf mistletoe for its possible effects on

nontimber resources. They included treatments to reduce infestations of hemlock

and lodgepole pine dwarf mistletoe in their adaptive trials in southern Oregon to

enhance tree growth and promote production of valuable American matsutake

(Tricholoma magnivelare) mushrooms.

One of the complicating factors in planning for management of mistletoeinfested stands is that hemlock dwarf mistletoe infestations are dynamic, with substantial changes to infestations, trees, and stand structures occurring within periods

of one to several decades (Muir and Geils 2002). Hemlock dwarf mistletoe-infested

forests therefore require detailed management plans, including what kinds of harvesting practices and silvicultural treatments will be applied to certain blocks or

units at specified times. There are instances where hemlock dwarf mistletoe infestations should be prevented or reduced and possibly other instances where hemlock

dwarf mistletoe should be reintroduced, encouraged to reestablish, or maintained to

enhance hemlock dwarf mistletoe effects on forest ecosystems. These actions need

to be specified for local forest ecosystems, depending on climate, topography, and

other biophysical factors. The particular management objective for each piece of

ground may help guide the establishment of desirable levels of hemlock dwarf

mistletoe.
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Strategies and treatments to manage hemlock dwarf mistletoe need to be based

on an understanding of hemlock dwarf mistletoe spread and effects in various

stages of stand development as outlined in the preceding section. In the next two

sections, major harvesting/silviculture regimes are outlined for management of

hemlock dwarf mistletoe-infested coastal forests: (1) clearcut harvesting of small

blocks and establishment of even-aged stands and (2) selection or retention harvesting to establish and maintain uneven-aged stands. Strategies and techniques for

these approaches were outlined in general for dwarf mistletoes by Muir and Geils

(2002).

Clearcut Harvesting and Even-Aged Stands

Clearcut harvesting of western hemlock forests continues to be a common harvesting practice in some areas. Currently, cutblocks or openings are usually limited

to a maximum size of 15 ha on public lands in Alaska and 40 ha in coastal British

Columbia. Most cutblock openings are designed to be no wider than twice the

height of the residual trees at the margins of the block in British Columbia. Blocks

or reserves of residual trees often are left in cutblocks to retain attributes of oldgrowth forests for wildlife, biodiversity, and/or protection of riparian zones. These

reserves will influence the maintenance and spread of hemlock dwarf mistletoe.

Ruth and Harris (1979) described steps for silvicultural control of hemlock

dwarf mistletoe in clearcut harvesting and even-aged management including detection surveys, careful layout boundaries of cutting units, sanitation cuttings,

and the management of nonsusceptible species such as Sitka spruce. Shaw (1981)

outlined control options, e.g., doing nothing, conversion to desirable nonhost species, clearcut harvesting, and seed tree cuts, based on stand conditions and management objectives in Alaska. These earlier recommendations were generally aimed at

reducing infection levels to limit detrimental effects to the timber resource. Recommendations were outlined more recently by Hennon et al. (2001) and Muir et al.

(2004b) with broader resource management objectives.

Spread into clearcut areas—

One frequently asked question is how rapidly and how far will hemlock dwarf

mistletoe spread from residual hemlock dwarf mistletoe-infected trees at the margins of a cutblock into the young regenerating trees. Results of studies of hemlock

dwarf mistletoe seed dispersal that were derived from one tree in British Columbia

by Smith (1966, 1973, 1977) indicated that most initial hemlock dwarf mistletoe
82

GENERAL TECHNICAL REPORT PNW-GTR-718

infection occurs within 10 m of residual mistletoe-infected trees. These results have

been assumed to apply over a wide geographic area. In Washington and Oregon,

Stewart (1976) determined that hemlock dwarf mistletoe had spread into young

stands within 10 m of single residual infected trees to infect 5 percent of trees at

age 15 and up to 70 percent at age 25 to 30. Shaw (1982) determined that dwarf

mistletoe spread from single residual trees in Alaska was less frequent, amounting

to 5 to 17 percent of young trees infected at ages 17 to 43 years.

Clearcut harvesting of small openings and retention of large reserves of trees in

clearcut harvested areas will likely continue as preferred management practices in

some areas. Frequently, in these situations, there appears to be very slow spread of

hemlock dwarf mistletoe to young fast-growing trees. In several areas in Alaska

(Shaw 1982) and on northern Vancouver Island, British Columbia (Bloomberg

1987), hemlock dwarf mistletoe incidence adjacent to stand margins appears to be

much less than reported by Smith (1973, 1977) and Stewart (1976). Data on

hemlock dwarf mistletoe spread from stand margins into adjacent young stands

have not been reported for these areas, and the causes for these low incidences are

unknown. Suppression of young tree growth adjacent to tall residual trees could be

important for western hemlock as previously reported for lodgepole pine dwarf

mistletoe (Hawksworth and Graham 1963). Possibly, as reported previously by

Muir (1970, 2002) for lodgepole pine dwarf mistletoe in Alberta, hemlock dwarf

mistletoe spreads more frequently and farther from single residual trees such as

studied by Smith (1966, 1973, 1977) and Stewart (1976) than from infected trees

at the margins of residual stands.

In some instances, severely infected hemlock trees along margins of cutblocks

have been cut to reduce infection of young trees. Usually the most severely infected

trees are cut, and lightly infected trees are retained. Often DMR is used to rate infection severity, but it is unknown if DMR is correlated with the amount of hemlock dwarf mistletoe seed that will be dispersed from an infected residual tree.

Smith (1969) reported that hemlock dwarf mistletoe shoot growth was greater on

infections in the upper crowns of more severely infected, 110-year-old trees. These

trees had approximately 1,300 infections with aerial shoots, and half of these were

probably pistillate infections with berries. In contrast, a nearby 15-m tall residual

tree used in several studies of seed dispersal (Smith 1966, 1973, 1977) had approximately 2,200 pistillate infections with 73,000 berries (Smith 1973). Evidently, small residual infected trees can be more substantial sources of hemlock

dwarf mistletoe seeds than taller and older infected trees, but further studies would

be desirable to support these data.
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Generally, we expect that hemlock dwarf mistletoe seed production is stimulated in infected trees recently exposed to increased light. However, in 2005 in a

limited study near Ucluelet, British Columbia, at 10 years after harvesting there

was little or no hemlock dwarf mistletoe seed dispersed from two hemlock dwarf

mistletoe-infected residual trees (fig. 31). Apparently, host factors in addition to

severity of infection influence hemlock dwarf mistletoe seed production on and

dispersal from residual trees. Despite considerable searching in British Columbia,

relatively few areas have been found that have residual and young regenerating,

hemlock dwarf mistletoe-infected trees with prolific hemlock dwarf mistletoe

shoots and berries (Muir 2004). The factors that are responsible for certain infected

trees such as the tree studied by Smith (1966, 1973, 1977) becoming prolific

sources of hemlock dwarf mistletoe seed have yet to be determined.

Previously in British Columbia, cutting permits for clearcut harvesting operations usually included a “3-m knockdown” rule where all trees 3 m or taller were

required to be felled. This rule initially was applied for safety considerations, but

recently it has been used or required to ensure that most hemlock dwarf mistletoeinfected trees are felled to reduce the potential spread of the parasite. However,

the risks of spread from mistletoe-infected residual trees of various heights into

young trees have not been determined. In several instances, hemlock dwarf mistletoe-infected advanced regeneration, i.e., small trees that were established in the

understory of the preceding stand became virulent sources of hemlock dwarf

mistletoe seeds (Smith 1966, 1973, 1977) (fig. 32). These trees appear to be more

important sources for hemlock dwarf mistletoe infection of young trees than taller

infected trees that were components of the upper canopy layer.

Response by hemlock dwarf mistletoe to clearcut harvesting—

The traditional practice of clearcut harvesting was an effective method of controlling hemlock dwarf mistletoe (Harris 1974, Hennon and Shaw 1988, Ruth and

Harris 1979, Shea 1966, Taylor 1973). Hemlock dwarf mistletoe control was accomplished through removal of infected residual trees, particularly any overhead

sources of hemlock dwarf mistletoe seeds. As described above, the process of stand

development that follows clearcut harvesting also could reduce or eliminate hemlock dwarf mistletoe. Shea and Stewart (1972) stated that throughout the range of

hemlock dwarf mistletoe, regeneration that develops after clearcut harvesting is

seldom seriously infected. However, Stewart (1976) determined that the incidence

of hemlock mistletoe in young trees near residual infected trees ranged from 5 percent at age 15 to 70 percent at age 25 to 30. The key factor in determining the

Process of stand

development that

follows clearcut

harvesting also

could reduce or

eliminate hemlock

dwarf mistletoe.
84

GENERAL TECHNICAL REPORT PNW-GTR-718

Figure 31—Hemlock dwarf mistletoe-infected residual tree with few hemlock dwarf

mistletoe seeds dispersed as measured by seed traps in 2005 near Ucluelet, British

Columbia.
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Figure 32—Residual trees with large amounts of hemlock dwarf mistletoe shoots and

seeds near Courtenay, British Columbia.
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GENERAL TECHNICAL REPORT PNW-GTR-718

incidence or occurrence of hemlock dwarf mistletoe in young stands is the number

and distribution of infected residual trees (Smith 1973, 1977; Trummer et al. 1998).

The primary silvicultural method of reducing hemlock dwarf mistletoe in early

stages of regenerating even-aged stands was to cut most or all of the hemlock

dwarf mistletoe-infected residual trees that remained after harvest. These residual

trees are sometimes as large as 20 to 30 cm in diameter and are frequently found

throughout harvested units. Residual trees are the primary source for introducing

hemlock dwarf mistletoe into the next stand. Some trees were left in harvested

units because they were considered unmerchantable, although more recently, trees

have been deliberately retained for other purposes. The degree to which these trees

occur in clearcut units varies considerably. Within units, residual trees may be common near unit boundaries and in reserve strips in riparian zones, but few residual

trees survive around landings because nearly all of the trees left are knocked over

by yarding.

Cutting residual infected trees is usually performed at the same time as precommercial thinning in southeast Alaska, at a stand age of approximately 15 years.

Costs for residual control were approximately $30 per hectare when done directly

following clearcut harvesting (Curtis and Swanson 1972) and are even less if conducted during a spacing or precommercial thinning, thereby avoiding a separate

entry. Note that this cost value is decades old. Typically, chain saws are used to

make a double cut into the lower bole of infected trees through sapwood that generally results in tree death 3 to 4 years later (Hennon and Shaw 1988). As an

obligate parasite, the dwarf mistletoe dies when its host dies.

In British Columbia, a common practice is to remove residual trees including

small hemlock trees (“whips”) 3 m or taller soon after harvest. Previously, when

complete mistletoe control was an objective, this had the advantage of removing

almost all infected trees before they could spread hemlock dwarf mistletoe seeds

to young-growth western hemlock trees during the years before precommercial

thinning. In many regions, spacing and/or precommercial thinning are no longer

practiced due to low value of western hemlock trees, and knock-down of residuals

is usually performed soon after harvesting. Laurent (1981) urged managers to

perform surveys to confirm that residual trees are actually infected with hemlock

dwarf mistletoe before conducting control measures. However, our impression is

that mistletoe infection of residuals is difficult to determine and extremely variable.

Small hemlock that were understory trees in old forests, but were left after the

harvest, often have irregular branches that can be confused with hemlock dwarf

mistletoe infections. We suspect that it is probably cheaper to fell all residual trees
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

in designated areas rather than conducting a separate mistletoe survey, at least in

areas where dwarf mistletoe could be present. Precommercial thinning is still a

common silvicultural activity in southeast Alaska, but killing residual trees for

dwarf mistletoe control usually is not included.

Alaska—

Perhaps the most substantial evidence of a minimal occurrence of hemlock dwarf

mistletoe in even-aged stands in Alaska came from an examination of trees in a

large stand density study (DeMars 2000). This plot system was designed to quantify

the growth of even-aged trees in different thinning treatments. A total of 62 installations representing stand ages from approximately 15 to 115 years was selected

throughout southeast Alaska. Given the abundance of hemlock dwarf mistletoe

in this region, the parasite was probably present in many of the old-growth forests before the disturbance (windthrow or harvesting) that gave rise to these young,

even-aged stands. However, the specific location of some plots was likely selected

to avoid unusual stand conditions and damage from agents such as hemlock dwarf

mistletoe. Therefore, these plots might not be a completely unbiased sample or

representation of young managed forests in southeastern Alaska.

In 1990 and 1991, 59 of the 62 installations having a total of 9,279 western

hemlock and 6,651 Sitka spruce trees were examined for hemlock dwarf mistletoe. Overall, only 1 percent of the western hemlock trees and no Sitka spruce trees

were infected. Most of the infected western hemlock trees were lightly infected.

Only 6 of the 59 stands, or 10 percent, had hemlock dwarf mistletoe. In the infested stands, an average of 15 percent of the western hemlock trees had some level

of hemlock dwarf mistletoe infection.

Only one plot at Touchit Cove, Long Island, had hemlock dwarf mistletoe

severe enough to reduce stand growth. An inspection revealed large residual hemlock dwarf mistletoe-infected trees in and just outside of the plot, thereby explaining the abundance of hemlock dwarf mistletoe in the young-growth stand. These

trees were left after logging disturbance, which occurred about 60 years before

sampling. This plot did not resemble an even-aged stand that usually results from

a complete clearcut harvest or similar disturbance. Therefore, the young stand was

the result of conducting what we would now consider as a selection or retention

harvest of a hemlock dwarf mistletoe-infested, old-growth stand.

The low incidence of hemlock dwarf mistletoe in young western hemlock

forests in southeast Alaska was further indicated by the results of Tait et al. (1985).

They sampled 16, 12- to 27-year-old, even-aged, thinned or unthinned stands in the
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GENERAL TECHNICAL REPORT PNW-GTR-718

vicinity of Prince of Wales Island for the occurrence of general insect and disease

activity. Although infected residuals were encountered, hemlock dwarf mistletoe

was not found on any of the 160 western hemlock crop trees on the plots.

Shaw (1982) examined the distribution of hemlock dwarf mistletoe in younggrowth forests age 17 to 43 years on Prince of Wales Island and neighboring islands

to determine how the parasite intensified around small unmerchantable residual

trees. The areas were clearcut harvested leaving unmerchantable residual trees.

Shaw (1982) determined the percentage of young-growth western hemlock trees

located within 9 m of single infected residual western hemlock trees (diameter at

breast height 30 to 70 cm) by detailed examination and destructive sampling of

3,429 young-growth trees on 19 plots at three sites. On average, only 10 percent

of trees were infected with only 1 to 2 percent of trees having two or more live

infections. Western hemlock advanced regeneration was more often infected (16

percent) than western hemlock trees that regenerated after the harvest (6.5 percent).

Results from this study indicated fewer infections in Alaska adjacent to source trees

than reported in southern British Columbia by Smith (1977) for western hemlock

trees planted around one source tree.

Shaw (1982) reported that 92 percent of the infections were alive in stands

aged 17, 19, and 35 years, but only 51 percent of the infections were still alive in

the stand aged 43 years. This initiated the hypothesis that hemlock dwarf mistletoe

begins to die out during crown closure in even-aged stands. However, in some areas

in British Columbia, many of these “dead” infections are actually alive despite their

occurrence on lower branches that lack foliage, leading to an alternate hypothesis

of long-term persistence.

Shaw and Hennon (1991) investigated hemlock dwarf mistletoe development in

two young (15 years old at the study onset) thinned stands of western hemlock near

Thorne Bay, Prince of Wales Island, Alaska. Plots were established around unmerchantable, severely infected trees, 25 to 30 cm diameter at breast height, to determine the pattern of dwarf mistletoe colonization of the surrounding young hemlock

trees. In total, 206 young trees on 11 plots were examined systematically every 2

years from 1981 to 1987. These trees were not destructively sampled so that hemlock dwarf mistletoe development could be monitored through time. Each branch

of every tree was searched carefully for any evidence of infection by hemlock

dwarf mistletoe. Tree heights and crown dimensions were measured every 2 years

to compare the height growth of hemlock trees to the vertical spread of the parasite

in infected trees. The plots were thinned before the first measurement and, to conform to current practices, residual trees were girdled to kill them. One site was
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

developed into a demonstration area to inform foresters, students, and others of the

biology and management options for hemlock dwarf mistletoe (Hennon and Shaw

1988).

Shaw and Hennon (1991) found that the percentage of infected trees increased

during their study, reaching 79 percent during the last sampling year when trees

were 21 years old. All plots except one had at least one infected young-growth

western hemlock tree. Although most of the trees were infected, few trees had 3

or more infections and only eight trees had 10 or more infections. Most infections

were small swellings, but a few had developed small witches’ brooms. The numbers of infections were insufficient to measurably reduce tree growth or to cause

mortality. There was no significant orientation by distance or direction of infected

trees from the infected residual trees. Infections were significantly more common

in the lower and inner portions of the crowns than elsewhere in the infected trees.

From 1981 to 1987, the height growth of hemlock trees on these plots was approximately double the vertical advancement of hemlock dwarf mistletoe. Infected trees

grew about 21 cm per year more in height than the parasite spread vertically.

As suggested by Drummond and Hawksworth (1979), hemlock dwarf mistletoe develops more slowly in Alaska than farther south on Vancouver Island,

Washington, and Oregon. The potentially detrimental effects of hemlock dwarf

mistletoe with even-aged management in Alaska were overemphasized in the past

due to reliance on research results from British Columbia (Drummond and

Hawksworth 1979).

There are several possible explanations for the lower infection rates of hemlock

dwarf mistletoe in Alaska. Disease of hemlock dwarf mistletoe caused by hyperparasites, early fall frost damage of berries, ineffective pollination due to weather

or reduced insect pollinator activity, abortion of fertilized embryos, and seeds being

dislodged by snow or rain during winter before they can germinate are some of the

possible factors. Bloomberg (1987) suggested that differences in infection levels

of hemlock dwarf mistletoe in various portions of its range might be due to effects

of weather on seed production and spread, stand composition and growth rate, and

hemlock dwarf mistletoe biotypes, i.e., genetically different populations of hemlock dwarf mistletoe. Regardless of causal mechanisms, these differences emphasize

the need to gather more information on intensification and spread by regions and to

exercise caution in extrapolating research results to large geographic areas.

In Alaska, Drummond and Hawksworth (1979) and Shaw and Hennon (1991)

concluded that intensive control efforts are not necessary for hemlock dwarf

mistletoe in clearcut harvested, “residual-free,” even-aged managed stands. In these

Infected trees grew

about 21 cm per year

more in height than

the parasite spread

vertically.
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GENERAL TECHNICAL REPORT PNW-GTR-718

even-aged forests in Alaska and possibly other areas, hemlock dwarf mistletoe is

reduced in incidence and might even die out because of stand developmental

processes, i.e., shading and the inability of the parasite to spread vertically, on sites

with average and above-average productivity. Hemlock dwarf mistletoe effects on

tree mortality and growth loss will probably be negligible in these situations. On

these sites under clearcut harvesting and even-age silvicultural regimes, hemlock

dwarf mistletoe effects will likely be minimal as long as short rotations are used.

In Alaska in recent years, clearcut harvesting has rarely removed all of the

residual structure from the previous forest. Often, infected seedlings, saplings,

small trees, and even large trees remain. Utilization standards have changed frequently, with the result that codominant and dominant old-growth trees 20 to 30

cm diameter have been left because of excessive internal defect, i.e., wood decay.

These harvest practices no longer resemble clearcut harvesting and, with regard to

spread and intensification of hemlock dwarf mistletoe, should be considered to be

more similar to retention harvests discussed below.

British Columbia, Washington, and Oregon—

As in Alaska, forest managers in British Columbia, Washington, and Oregon have

been concerned about the spread of hemlock dwarf mistletoe from residual trees

that remain in clearcut areas. Much of the applied research on hemlock dwarf

mistletoe has focused on intensification of the parasite in the presence of these small

residual trees. In one survey in British Columbia, Morris and Wood (1978) found

that approximately half of the residual hemlock trees left after harvesting were

infected with hemlock dwarf mistletoe. In Washington and Oregon, leaving residual

trees resulted in 70 percent infection of nearby young trees at 25 to 30 years after

harvesting (Stewart 1976). In British Columbia, Tripp et al. (1979) suggested that

treatment of residuals at the time of precommercial thinning is too late; 98 percent

of selected crop trees were already infected with an average of 20 infections (range

1 to 170) per tree. Morris and Wood (1978) and Muir (1985) recommended using

the 3-m knockdown clause to reduce the spread of hemlock dwarf mistletoe in

British Columbia. To further reduce hemlock dwarf mistletoe incidence, Muir

(1985) recommended laying out harvest boundaries for clearcutting to minimize

infected trees along the perimeter of units, thinning out infected young-growth trees

during spacing activities, and leaving infected young hemlock stands on very poor

sites unspaced. More recent recommendations were outlined by Hennon et al.

(2001) and Muir et al. (2004a). In recent years, few young hemlock stands have

been spaced or precommercially thinned in British Columbia, but this practice
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

continues in Alaska. In many areas, removal of larger infected trees as important

sources of hemlock dwarf mistletoe infection is still recommended, but the biological basis for the practice has yet to be established.

Bloomberg and Smith (1982) investigated infection levels of hemlock dwarf

mistletoe in seven western hemlock stands that had a mix of residual and younggrowth trees on southern Vancouver Island, British Columbia. The number of

infections per tree varied considerably, from 373 to 4,058 on residual trees and

from 3 to 455 on young-growth trees. Taller and larger diameter young-growth

trees had more infections, presumably because they were older, were exposed to

infection for longer periods, and were larger targets for intercepting mistletoe seeds

from the residual trees. The authors recorded dead infections and found that these

were significantly related to crown position; more dead infections were found in

the lower crowns. Severity of infection of the young-growth trees was directly

proportional to the number of residual hemlock trees and inversely proportional

to the number of nonhost trees, e.g., Douglas-fir, Pacific silver fir, and Sitka

spruce. The authors concluded that the primary factors affecting hemlock dwarf

mistletoe infection levels in young trees were number of residuals, stand composition (proportion of different species), growth rates, and stand density. The data

verified results from using a previously developed hemlock dwarf mistletoe model

(Bloomberg et al. 1980) to predict spread and infection of hemlock dwarf mistletoe

in the young trees.

Alfaro et al. (1985) reported data on occurrence and infection by hemlock

dwarf mistletoe from seven western hemlock stands in British Columbia. The

stands were selected from inventory maps to represent even-aged stands. Average

tree age ranged from 40 to 100 years per stand with total volumes of 120 to 750

m3

·ha-1. At three of the locations, approximately 45 trees were felled and growth

increments measured. Growth reductions caused by hemlock dwarf mistletoe were

variable depending on tree size, age, and position within each stand. Reductions in

annual growth rates in moderately and severely infected trees at age 80 were

estimated at 15 and 25 percent, respectively.

In western Washington and Oregon, Stewart (1976) developed an equation to

estimate the percentage of infected western hemlock in young trees within a 10-m

distance of infected residual trees. The percentage of infected trees increased exponentially up to the oldest stand sampled (25 to 30 years old) at which time 70

percent of young hemlock trees were infected.

In British Columbia, for clearcut-harvested units, Smith (1977) estimated that

86 evenly spaced residual infected trees per hectare similar to the one he studied
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GENERAL TECHNICAL REPORT PNW-GTR-718

would be sufficient to severely infect every young-growth hemlock. In an earlier

report, Smith (1966) stated that only 25 evenly spaced residual trees per hectare

would be needed, but the discrepancy between these two predictions was due to a

difference in the numbers of hemlock dwarf mistletoe seeds dispersed from the

same tree reported in 1966 and 1977. Based on these studies, intensification of

hemlock dwarf mistletoe in southern British Columbia is faster than reported by

Shaw (1982) in Alaska. However, the studies involved different conditions that are

not necessarily equivalent. In British Columbia, trees were planted around one

infected residual tree at 30+ years after harvest, and in Alaska, naturally regenerated trees around infected residual trees were examined 17 to 43 years after harvest.

Van der Kamp and Wilford (1981) examined the intensification of hemlock

dwarf mistletoe in two western hemlock stands in British Columbia. They found

that the number of infections doubled every 2 years during the early stage of stand

development. They predicted that intensification would slow considerably as the

stand developed, primarily through mortality of the infections by branch senescence at the base of tree crowns. They suggested that high levels of hemlock dwarf

mistletoe that are visible at an early stand stage do not necessarily translate into

high, or damaging hemlock dwarf mistletoe levels later in stand development.

In two young stands in British Columbia, Richardson and van der Kamp (1972)

found that vertical spread of hemlock dwarf mistletoe was faster (65 ± 4 cm per

year) on trees that were widely spaced than on trees that were growing in a dense

stand (30 ± 4 cm per year). The height growth of trees in the widely spaced and

dense stands was 58 and 33 cm per year, respectively. However, in southeast

Alaska, Shaw and Hennon (1991) reported a 20-cm-per-year vertical spread

rate for dwarf mistletoe and a 40-cm-per-year hemlock height growth. In British

Columbia, Wass (see footnote 4) remeasured height growth and vertical spread of

hemlock dwarf mistletoe in 28, 45-year-old trees that were planted and survived in

the experimental trial established by Smith (1966, 1973, 1977). On average, lightly

infected (DMR 0, 1, 2) trees grew 64 cm per year in height with a vertical spread

of 20 cm per year for the parasite; moderately infected (DMR 3, 4) trees grew 55

cm per year with a vertical spread rate of hemlock dwarf mistletoe of 25 cm per

year.

Van der Kamp (1987) suggested that site quality is important in determining

the effects of hemlock dwarf mistletoe in even-aged managed stands. In dense

stands on productive sites with rapid height growth, hemlock dwarf mistletoe is

relegated to the lower crown. On poor sites, the mistletoe is able to spread vertically at the same rate as the height growth of western hemlock trees and therefore
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

is able to spread into the upper canopy as the stand develops. Thus, hemlock dwarf

mistletoe is expected to continue to be a factor in hemlocks growing on poor sites.

However, the hypothesis of differences of hemlock dwarf mistletoe spread rates

between good and poor sites has yet to be tested in a randomized, replicated study

across a representative range of sites with different densities, numbers, and distributions of residual, infected trees.

Hadfield (1980) concluded that crown closure and rapid height growth of

western hemlock stands in Washington were factors that minimized hemlock

dwarf mistletoe damage in 40- to 80-year-old even-aged stands. Hadfield urged

managers to consider no residual removal after clearcut harvesting unless the

number of residual trees was “excessive,” but he did not specify a number. Hadfield

also recommended that managers consider stand thinning, a technique that they

were hesitant to use because of the anticipated increase in dwarf mistletoe seed

dispersal. Thinning, he argued, increased the height growth of the remaining trees,

which restricted dwarf mistletoe occurrence to the lower crown. Concern was so

high about light and stand opening increasing the spread of hemlock dwarf mistletoe (Morris and Wood 1978) that Russell (1978) suggested that severely infected

western hemlock stands should not be thinned in Washington. It is generally expected that thinning will increase the abundance of hemlock dwarf mistletoe,

probably for a short time, by increasing seed production and reducing barriers to

dispersal. However, in some stands, there was a decrease or no effect of thinning

on mistletoe abundance (Goheen et al. 1980). On productive sites, fast-growing,

released trees could outgrow the vertical spread of the parasite, which would

probably reduce its impact on growth rates of infected trees as they matured.

This scenario is expected particularly where there is a large difference between

the height growth of western hemlock trees and the upward spread of hemlock

dwarf mistletoe (Shaw and Hennon 1991).

Selection or Retention Harvesting and Uneven-Aged Stands

As discussed by Muir and Geils (2002), previous experience and reports indicated

that selection harvesting or partial cutting initially decreased, but eventually increased hemlock dwarf mistletoe incidence or severity. Initially, removal of infected trees reduces dwarf mistletoe incidence and severity, but the more open stand

conditions are believed to enhance dwarf mistletoe seed production and dispersal,

so that in a few years, dwarf mistletoe incidence and severity could be higher in the

treated stand.
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GENERAL TECHNICAL REPORT PNW-GTR-718

The often-dramatic increase in hemlock dwarf mistletoe in some thinned stands

has persuaded some workers to recommend that stands infected by dwarf mistletoe

should not be thinned. However, 10- to 20-year data for thinned, infested-western

hemlock stands demonstrated that hemlock dwarf mistletoe severity increased in

some stands, decreased in some stands, and did not change in others when compared to nearby unthinned stands (Goheen et al. 1980). Regardless of mistletoe

incidence, tree volumes were greater in thinned than unthinned stands. These data

supported the contention that the best strategy to manage dwarf mistletoe-infested

stands is to apply an appropriate silviculture treatment that will enhance tree

growth (Muir and Geils 2002). When thinning infected stands, incidence and

severity of hemlock dwarf mistletoe will undoubtedly increase, and tree growth

will be affected. However, the overall increase in tree growth in thinned hemlock

dwarf mistletoe-infested stands could justify the costs of thinning even though the

growth rate in infested stands was less than in uninfested stands.

With selection harvesting, there are opportunities to maintain overall tree health

and stand productivity by periodically cutting severely infected trees. For a discussion of various strategies for treatments and related references, see Muir and Geils

(2002). Tree growth rates are usually not significantly reduced until 50 percent or

more of the live branches are visibly infected. This is equivalent to a dwarf mistletoe rating (DMR) of 3 or more. Usually DMR increases by a maximum of one class

per decade so that entries for sanitation cutting and thinning should be made at approximately 20-year intervals. In recreation and high-use sites, tree health can be

maintained by pruning infected branches and large witches’ brooms. Not more than

50 percent of the live crown should be removed. These approaches should be effective for maintaining hemlock dwarf mistletoe-infected trees, but experimental data

are needed for confirmation.

Factors that influence intensification in uneven-aged stands—

Several factors described below will most likely have a major influence on the

amounts and impacts of hemlock dwarf mistletoe that develop in uneven-aged

stands that grow following selection or retention harvesting.

Presence and severity of dwarf mistletoe in residual trees—

If hemlock dwarf mistletoe is absent in a stand before harvest, then introduction is

unlikely in the short term and management need not be concerned about it. For

example, stands at higher elevations, e.g., above 150 to 300 m in Alaska, are often

completely free of hemlock dwarf mistletoe and will probably remain so. If hemlock dwarf mistletoe is present but only a few trees are infected, then infected or

The best strategy

to manage dwarf

mistletoe-infested

stands is to apply an

appropriate silviculture treatment that

will enhance tree

growth.
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

uninfected trees could be selected and harvested to achieve the desired proportion of

infected to uninfected trees after logging. This option is not possible in some

western hemlock stands where every tree is infected. Hemlock dwarf mistletoe

incidence and severity data are needed to plan appropriate management activities.

Species composition—

In mixed-species stands, selection and/or retention of western hemlock trees could

modify the incidence of hemlock dwarf mistletoe. Less susceptible or immune tree

species such as Sitka spruce, Pacific silver fir, mountain hemlock, western redcedar,

and yellow-cedar, could provide partial barriers for mistletoe spread if they are tall

enough to block dispersing mistletoe seeds from infected western hemlock trees.

However, because many of these species are more valuable than western hemlock,

selective harvesting might favor their removal. Many old-growth stands are dominated by western hemlock trees to an extent that there are few other species available for retention. Several methods of selection or retention harvesting, especially

those that produce soil disturbances and larger stand openings, could favor the

regeneration of Sitka spruce and other species instead of western hemlock. This

could result in a more diverse understory with a higher proportion of trees less

susceptible to hemlock dwarf mistletoe. Planting species other than western hemlock after harvesting is another common mistletoe-reduction strategy in many areas.

However, in some forests where maintenance of biodiversity is a major objective,

extensive planting of less susceptible tree species might significantly reduce the

occurrence of western hemlock below acceptable levels (Edwards 2002).

Heights of infected trees and vertical position of infections—

Hypothetically, hemlock dwarf mistletoe infections located in small trees or confined to the lower crowns of large trees should not have the same potential for

spread and impact in a stand as higher positioned infections. Infections high in the

crowns of large trees are overhead sources of hemlock dwarf mistletoe seeds that

could infect adjacent, younger trees for decades to possibly centuries. Nearby

western hemlock trees could be exposed to hemlock dwarf mistletoe seeds lodging

in their upper crowns for a typical harvest rotation of 80 to 100 years and perhaps

for their entire lifespan of 300 to 400 years. Hemlock dwarf mistletoe seed sources

in lower crown positions, i.e., on infected small residuals and lower branch infections on large residuals, will likely be overtopped and suppressed by regenerating

and releasing western hemlock trees, particularly if harvest patterns allow for

adequate light to encourage fast height growth. The leaders of some of the regenerating western hemlock trees will become infected, resulting in witches’ broom
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GENERAL TECHNICAL REPORT PNW-GTR-718

formation and permanent stunting. Without an overhead source of hemlock dwarf

mistletoe seeds, many infected small residuals will respond to release following

selection harvest by fast height growth; the result is mistletoe-free upper crown,

because the height growth of the residuals should exceed the vertical spread of the

parasite. However, small, infected trees in a cutblock often appear important. These

residual infected trees can become prolific sources of hemlock mistletoe seeds

(Smith 1966, 1973, 1977), producing more seeds than nearby overstory old-growth

trees (Smith 1969). Due to their initial taller height than regenerating younger trees,

their rapid height growth after a disturbance, and their frequent large numbers, they

are potential sources for rapid horizontal and vertical spread, presumably because

of intertree, reciprocal seed dispersal and infection (Bloomberg and Smith 1982).

More research is needed on this topic of the amount of inoculum produced and then

disseminated from trees of different heights.

Size and horizontal distribution of harvested canopy openings—

The size and pattern of openings and the number and distribution of retained infected trees determine the amount and distribution of hemlock dwarf mistletoe in

the developing western hemlock forest. However, predicting hemlock dwarf mistletoe abundance is complicated by the manner in which important ecological parameters are affected by the size of openings. There is little direct information from

which to predict responses by hemlock dwarf mistletoe other than the knowledge

that minimal hemlock dwarf mistletoe levels develop in very large clearcut openings. With our current understanding of the biology of hemlock dwarf mistletoe,

some qualitative speculation can be made on how dwarf mistletoe will respond to

different harvesting patterns.

Harvesting that creates numerous, small openings such as individual tree selection or small group selection will remove fewer hemlock dwarf mistletoe-infected

trees, result in slower hemlock tree growth, and favor the short-range dispersal of

hemlock dwarf mistletoe seeds. These factors will probably maintain or increase

hemlock dwarf mistletoe incidence and severity. Conceivably, small openings might

not provide adequate light to increase hemlock dwarf mistletoe seed production

(Baranyay 1962) enough to lead to a potentially large population increase.

The actual size and spatial distribution of openings will likely have a pronounced effect on the amount of infection by hemlock dwarf mistletoe. The number of hemlock dwarf mistletoe seeds produced and distance that seeds disperse

dictate how far hemlock dwarf mistletoe spreads into openings as they regenerate

to western hemlock. Secondary spread of mistletoe among young-growth trees is
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

likely to be quite slow (Alfaro et al. 1985) and of less consequence. Because most

mistletoe seeds fall within about 5 to 10 m from their source, openings with a

diameter larger than 10 m (0.03 ha) are likely to have an interior zone with trees

relatively free of hemlock dwarf mistletoe. Conversely, in openings with a diameter

smaller than 10 m, all trees probably are exposed to hemlock dwarf mistletoe seeds

from infected trees on the periphery. Openings with a diameter of 100 m (0.8 ha)

would have a central portion of about 78 percent of the area not exposed to infection from the margins of the opening.

Numbers and sizes of residual trees—

As described above, Smith (1966, 1973, 1977) studied hemlock dwarf mistletoe

seed dispersal from one small residual tree on southern Vancouver Island, British

Columbia, and predicted that 86 hemlock dwarf mistletoe-infected residual trees per

hectare would result in almost 100 percent infection of regenerating young trees.

In southeast Alaska, Trummer et al. (1998) determined mistletoe severity for

10 western hemlock stands of 1.5 to 20 ha that were affected by near-catastrophic

windstorms in the late 1880s. They expected that their results would be similar to

those expected from selection harvesting that leaves large, old-growth trees. Wind

damage varied considerably within each site. Some portions of the stands had no

surviving residual trees and other portions had few to many surviving trees. Plots

were selected and results grouped for areas having 0, 1-32, and 33 or more residual

trees in each stand. Trummer et al. (1998) determined that the number and basal

area of large and small residual trees could be used to predict the severity of infection in a stand, in this case the mean DMR for a stand. The size, number, and

infection levels of residual trees were highly correlated with infection severity

in western hemlock that regenerated after each disturbance. Mean DMRs in the

wind-damaged stands were less than expected based on the results of Smith (1966,

1973, 1977) in British Columbia. Trummer et al. (1998) also determined that large

mistletoe infections on the lower bole and branches were up to 200 years old. A

linear relationship was found between the estimated age of infection for a bole

infection and the diameter of the bole swelling: infection age was equal to approximately 35 plus five times the swelling diameter in centimeters.

Other factors—

Other factors that affect the amount of hemlock dwarf mistletoe after selection

harvesting include slope and aspect of the site. Hemlock dwarf mistletoe is expected

to spread faster and farther from uphill sources to downhill targets. Uphill spread is

reduced with increasing slope gradient (Bloomberg et al. 1980). Greater shoot and
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GENERAL TECHNICAL REPORT PNW-GTR-718

seed production occurred on the southwest portion of an infected western hemlock

tree in British Columbia (Smith 1977). Smith (1977) found differences by quadrant

in the number of hemlock dwarf mistletoe infections on one residual tree and on the

young-growth trees surrounding the hemlock dwarf mistletoe seed source. Approximate percentages of infections per southwest, northwest, northeast, and southeast

quadrants were 48, 22, 16, and 14 percent, respectively. These results supported the

supposition that spread of hemlock dwarf mistletoe would be greatest downhill on

sites with a southwest aspect, but seed dispersal has to be determined on different

slopes and exposures before any predictions can be made confidently.

Response by hemlock dwarf mistletoe to selection or retention harvesting—

Although a substantial reduction of hemlock dwarf mistletoe can be expected by

clearcut harvesting followed by the development of even-aged stands, selection

harvesting in old-growth forests is expected to dramatically increase the incidence

of hemlock dwarf mistletoe. Any harvesting system that leaves dwarf mistletoeinfected western hemlock trees and encourages regeneration of western hemlock is

expected to result in intensification of the hemlock dwarf mistletoe. By contrast,

Deal et al. (2002) found that any level of selection harvesting reduced dwarf mistletoe levels over those found in adjacent unharvested old-growth stands. Apparently,

removing any infected trees during harvest reduced dwarf mistletoe to a greater

extent than it could spread and intensify. Reconstructive methods were used; thus,

a long-term experimental approach is needed to resolve this question in Alaska.

Generally, leaving tall infected trees guarantees parasite seed sources high in the

canopy from which spread to regenerating or understory trees can occur for long

periods. In these situations, hemlock dwarf mistletoe will probably spread extensively to young trees and intensify rapidly within tree crowns.

There are numerous possible harvesting patterns that could be considered as

selection or retention harvesting. To the degree that each prescribed system leaves

large, infected residuals and creates canopy openings, hemlock dwarf mistletoe

will likely be affected in some way. Malot (1991) argued that the previous view

of forest management conducting a “battle” with hemlock dwarf mistletoe must be

abandoned now that conservation of biodiversity is a central goal of forest management planning and that selection harvesting of old-growth forests will become the

tool to achieve that end. However, in our view, a selection harvesting and silvicultural approach that ignores hemlock dwarf mistletoe is too simplistic because a

specific prescription could result in large variations in the severity of infection over

a rotation and even over a single cutting cycle. Potentially, selection harvesting
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

gives flexibility in achieving various amounts of mistletoe in managed forests, but

the end results will often differ widely depending on the site, number, and distribution of infected residuals, and their severity of infection.

Buckland and Marples (1952) observed the spread of hemlock dwarf mistletoe

on Turnour Island in British Columbia (latitude 50 degrees) following three harvesting techniques. These included (1) clearcut harvesting; (2) harvesting with

retention of small blocks of residual trees; and (3) selective thinning or highgrading of large overstory trees, which left a large number of scattered nonmerchantable trees. Their conclusions were based on general observations rather than

rigorous data collection and analysis. Sites were logged operationally, and it was

not known if site or stand factors differed among the three harvesting techniques

before logging. Hemlock dwarf mistletoe was present in only widely spaced

locations in the even-aged stand that regenerated in the clearcut units. Buckland

and Marples (1952) assumed that birds or other animals had introduced hemlock

dwarf mistletoe into the clearcut units through seed vectoring. Hemlock dwarf

mistletoe had not spread far from the residual blocks, but was abundant in the

areas colonized. Hemlock dwarf mistletoe was both abundant and well distributed

throughout units after selective cutting (high-grading) of overstory trees. They

concluded that the practice of leaving some western hemlock trees as seed sources

for regeneration exacerbated hemlock dwarf mistletoe infestation. Buckland and

Marples (1952) argued that the practice of leaving smaller and poorer quality

trees encouraged slower growing trees with greater susceptibility to hemlock

dwarf mistletoe. They suggested that any form of selective (high-grading) or

selection harvesting will maintain or increase hemlock dwarf mistletoe incidence

and severity.

In southeast Alaska, Hennon established four large stem-mapped plots to determine the distribution and severity of infection of hemlock dwarf mistletoe under

selection or retention management. Two plots were located in stands that were

selectively harvested about 60 years before sampling and two were in stands that

had extensive windthrow that mimicked selective harvesting about 110 years before

sampling. Figure 33 shows the distribution of residual and young-growth trees in

the two plots disturbed by wind. Infection severity was classified as absent, DMR 0;

light, DMR 1 or 2; moderate, DMR 3 or 4; and severe, DMR 5 or 6.

The infected young-growth trees were closely associated with infected residual

trees, as was particularly evident in the Halleck Harbor No.1 plot, which had only

two infected residuals. In the other plots, a larger number of infected residual trees

allowed hemlock dwarf mistletoe to occupy most of the plot. About 80 percent of

Potentially, selection

harvesting gives

flexibility in achieving various amounts

of mistletoe in managed forests, but

the end results will

often differ widely

depending on the

site, number, and

distribution of infected residuals,

and their severity

of infection.
100

GENERAL TECHNICAL REPORT PNW-GTR-718

Figure 33—Stand Visualization Software (SVS) images of Halleck Harbor plots 1 (above) and 2 (below), Kuiu, Alaska,

depicting the spread of hemlock dwarf mistletoe from two and five infected residual trees, respectively, to surrounding younggrowth trees. Stem maps show the number, infection level, and location of residual trees that represented the inoculum for

eventual spread. DMR = dwarf mistletoe rating. See Trummer et al. (2007) for developing SVS images that depict dwarf

mistletoe.
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

the western hemlock trees were infected on these plots. Although the incidence of

infection was high, the severities of infection (DMRs) were low in these three plots.

Growth loss and mortality are substantial only for severely infected trees, which

represented only about 10 percent of the infected trees.

A 10-percent growth loss of moderately infected trees and 40 percent growth

loss of severely infected trees were applied to the proportion of young-growth

hemlock trees in each DMR to calculate an average growth reduction by hemlock

dwarf mistletoe per plot for young-growth hemlock trees. The residual trees were

of various sizes and infection levels, but only infected residuals were included in

table 6. Information from these plots suggested only a modest amount of mortality

and growth loss resulting from hemlock dwarf mistletoe 60 years after selective

harvesting and 110 years after a windstorm. Four plots did not adequately sample

the range of conditions needed to provide information on different silvicultural

approaches to selection harvesting. However, they indicated that leaving some

residual trees does not necessarily result in substantial levels of hemlock dwarf

mistletoe in southeast Alaska.

Short-term effects of hemlock dwarf mistletoe under selection harvesting is

being investigated in a large multidisciplinary ecosystem management study jointly

conducted by the Pacific Northwest Research Station and Alaska Region of the

U.S. Forest Service. The study, called “Alternatives to Clearcutting,” involves different patterns of selection harvesting in old-growth forests within an experimental

design and randomized assignment of treatments (McClellan et al. 2000). Hemlock

dwarf mistletoe intensification after harvest treatment is one part of this study. In

addition, the role of hemlock dwarf mistletoe as a mortality agent and the cause of

gaps in old-growth western hemlock unmanaged forests was measured before treatments at all sites (Hennon and McClellan 2003). The third and last block was harvested in 2005; early results on short-term intensification after harvest will be

available following the first 5-year measurement cycle.

Another approach in the “Alernatives to Clearcutting” study is to examine

forests that had been partially harvested in the early 1900s, attempt to reconstruct

how much of the stand had been removed, and describe subsequent development

(Deal et al. 2002). Eighteen stands were studied, mostly near shorelines, with plots

installed in different parts of the stand that had received different harvest intensities. A control plot was placed in an adjacent unharvested area at each site.

Preliminary results from the study indicated that dwarf mistletoe severity was

generally light in the selected stands. There was a tendency for lighter severity with

more intense harvests, suggesting that hemlock dwarf mistletoe levels were reduced
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GENERAL TECHNICAL REPORT PNW-GTR-718

by removing infected trees during the harvest. Selective harvesting maintained

hemlock dwarf mistletoe but did not lead to greater severity of infection in residual

western hemlock trees. Caution should be used in interpreting results from this reconstructive study, however, because hemlock dwarf mistletoe severity at the time

of harvest was unknown and because of the variable spatial distribution of hemlock

dwarf mistletoe in stands. In addition, plots placed in adjacent unharvested stands

might not serve as appropriate controls.

Additional research is needed to provide information on how much hemlock

dwarf mistletoe will result from different selection harvesting schemes and what

impact different severities of hemlock dwarf mistletoe infection will have on the

residual forest development. The most reliable information will come from repeatedly measured permanent plots. Along with blowdown and tree wounding, intensification of hemlock dwarf mistletoe is a concern with partial harvesting in western

hemlock forests (Beese et al. 2003, Jaeck et al. 1984, McClellan et al. 2000). Considerable divergence of opinion among forest managers and scientists will continue

until new information demonstrates the amount or extent of hemlock dwarf mistletoe growth losses and other effects under the various types of selection or retention

harvesting.

Results of previous studies and available information on hemlock dwarf mistletoe spread and effects in selection- or retention-harvested areas are summarized

below.

Individual tree selection—

Theoretically, individual tree selection is similar to natural small-scale disturbance

in old-growth stands where individual trees die leaving small gaps in the canopy. In

some instances where overstory trees are cut, crowns of neighboring canopy-level

Table 6—Incidence of hemlock dwarf mistletoe and

estimated growth loss based on the severity of infection

of western hemlock trees in four plots in southeast Alaska

Estimated

Site Age Residuals growth loss

Years No./ha Percent

Halleck No. 1 110 10 1.0

Halleck No. 2 110 25 6.0

Todd 60 65 4.0

Touchit 60 40 3.5
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

trees grow into the canopy space and a new codominant or dominant tree does not

develop (Deal et al. 2002). If a subcanopy tree grows into the small vacant space,

typically, the tree is fully exposed to hemlock dwarf mistletoe seeds from neighboring trees. Hemlock dwarf mistletoe could intensify in the old-growth trees due to

the effects of stand opening. A small-size disturbance generally favors the spread

of hemlock dwarf mistletoe and leads to severe infestations in old-growth stands.

However, a small opening might not provide enough additional light to increase

spread and intensification of hemlock dwarf mistletoe.

With this exception, individual tree selection is believed to enhance nearly all

factors that lead to higher severity of hemlock dwarf mistletoe. However, there are

exceptions to this view. In Alaska, a reconstructive study of western hemlock stands

selectively harvested in the early 1900s indicated that hemlock dwarf mistletoe was

maintained but severity did not increase significantly in these stands (Deal et al.

2002). In this case, trees infected by hemlock dwarf mistletoe were probably

removed during the harvest, and spread and intensification presumably did not

return dwarf mistletoe levels to the preharvest condition.

Dispersed residual trees—

The density, distribution, size, and infection severity of residual trees will influence

the pattern of hemlock dwarf mistletoe that results from this silvicultural treatment.

If individual infected residual trees were widely spaced, then dwarf mistletoe will

be aggregated in discrete infection centers of young-growth trees around these residuals with uninfected or lightly infected young-growth trees between. If a supply

of infested trees and witches’ brooms were important for wildlife, such a pattern

of dwarf mistletoe might enhance some aspects of wildlife habitat while still maintaining timber productivity. However, there are concerns about the wind firmness

of these exposed residual trees. Most windthrow occurs within the first few years

following harvest, but it is difficult to predict which trees will be wind-thrown.

Thus, spread of hemlock dwarf mistletoe may be of short duration from some of

these retained trees.

Small group selections—

The size of the opening created and occurrence of infected trees on the perimeters

will influence the amount of resulting hemlock dwarf mistletoe in small group

selections. Openings smaller than a 10-m radius (0.03 ha) could be fully colonized

by hemlock dwarf mistletoe if the parasite were present in the perimeter trees.

Hemlock dwarf mistletoe can intensify in the perimeter old-growth trees due to

The size of the

opening created

and occurrence

of infected trees

on the perimeters

will influence the

amount of resulting hemlock dwarf

mistletoe in small

group selections.
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GENERAL TECHNICAL REPORT PNW-GTR-718

the effects of stand opening. Small openings will likely regenerate primarily with

western hemlock. Openings larger than 0.03 ha will likely develop with a central

portion free of hemlock dwarf mistletoe and could regenerate a higher proportion

of less susceptible tree species.

Thinning from above (diameter-limit harvest)—

Some silvicultural prescriptions remove large trees and retain smaller ones. Such

systems remove the tallest trees, and therefore, the highest sources of hemlock

dwarf mistletoe. Considerable infection may often occur in the smaller trees that

remain. Hemlock dwarf mistletoe may intensify immediately after the stand treatment as conditions favor reproduction and seed dispersal. On productive sites,

however, these same conditions favor fast height growth of western hemlock trees.

If all or most high sources of hemlock dwarf mistletoe seeds are removed, then

most infected trees are anticipated to outgrow the vertical spread of the parasite.

Under this scenario, they would develop upper crowns that are free of dwarf mistletoe and probably not experience any large reductions in annual growth due to infection. Trees could have brooms or bole infections in the lower crown, however,

which may reduce wood quality.

Monitoring programs for certification and forest-level effects—

Recently, many private, corporate, and government forestry organizations that harvest and manage private or publicly owned forest land have registered with one or

more institutions that certify the long-term sustainability of forest management

plans and practices. In many areas, including British Columbia, certification is a

statutory requirement for forest harvesting licenses and agreements. Generally, certification requires a forestry organization to provide periodic reports with data that

demonstrate acceptable and effective results of their practices. In Canada these data

are often based on a set of criteria and indicators developed by the Canadian Council of Resource and Environmental Ministers. These indicators include a measure

of the area of forest damaged by forest insects and diseases. However, these data are

too general to adequately characterize many of the effects of most forest insects and

diseases, particularly the effects of hemlock dwarf mistletoe (Muir et al. 2004a).

Monitoring is needed to determine incidence and severity of hemlock dwarf

mistletoe in harvested blocks over an extensive forest land base to adequately characterize hemlock dwarf mistletoe infestations and measure (or estimate) current or

future impacts. Monitoring is required in most coastal forest areas because of variable ecological conditions, incidence, severity, and development of hemlock dwarf

mistletoe infestations.
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Recently we began developing more detailed measurements for monitoring

hemlock dwarf mistletoe effects in young trees and retention-harvested stands

(Muir 2005), but further work is required. Criteria to be measured include tree

numbers, sizes, dwarf mistletoe ratings, and spatial distributions of infected trees.

Trees on the same plots should be remeasured over a period of several decades

after harvest or disturbance. Ideally, monitoring plots for hemlock dwarf mistletoe

should be integrated with the overall monitoring procedure used for certification.

However, with the generally low incidence of mistletoe-infected trees, a two-stage

sampling design will be needed to efficiently select an unbiased set of plots with

infected trees.

Modelling spread and impacts of hemlock dwarf mistletoe—

One approach to integrating information and available data on biology, ecology and

management of hemlock dwarf mistletoe (and other insects and pathogens) has been

to construct models. Various modelling approaches have been developed for dwarf

mistletoes including hemlock dwarf mistletoe (Bloomberg et al. 1980, Geils et al.

2002, Hawksworth and Wiens 1996, Robinson and Geils 2006, Robinson et al.

2002). A general overview of modeling and development of forest models is

presented by Amaro et al. (2005).

Bloomberg et al. (1980) produced a model to predict the development of hemlock dwarf mistletoe in British Columbia from infected residuals left after clearcut

harvesting. The experimental basis for the model involved only a few residual trees

up to 15 m in height. Where stand conditions include small residuals, the model is

very useful because it integrates results from several studies of spread and intensification of hemlock dwarf mistletoe.

The Bloomberg et al. (1980) hemlock mistletoe model includes a number of

factors. These include distribution of dwarf mistletoe infections in residual source

trees, seed production, escape from crown and dispersal, interception of seeds by

neighboring trees, distribution of seeds within crowns, development of infections,

mortality of dwarf mistletoe plants, and tree crown growth. The model accurately

predicted hemlock dwarf mistletoe intensification within trees and spread over a

10-year period at several locations (Bloomberg and Smith 1982). Most of the

assumptions used by the model were from data collected on southern Vancouver

Island, British Columbia. The model has not been verified for use in other regions

or for conditions where large residual trees occur. Recently, Robinson et al. (2002)

and Robinson and Geils (2006) incorporated many of the relationships and data of

the Bloomberg and Smith (1982) model into a more detailed model that was

modified for hemlock dwarf mistletoe and other dwarf mistletoes.
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GENERAL TECHNICAL REPORT PNW-GTR-718

In response to the well-recognized lack of data on “healthy” tree growth in

most areas, detailed tree growth models such as the Tree and Stand Simulator

(TASS) have been developed in British Columbia and used to project western

hemlock tree growth under various conditions, including retention-harvested

stands (Goudie and DiLucca 2004). Although based on detailed measurements of

tree growth, TASS and most other tree growth models have deliberately excluded

effects of most insects and diseases, including hemlock dwarf mistletoe (Muir et al.

2004a). To rectify this limitation, a modeling project was initiated (Muir 2003) to

develop a hemlock dwarf mistletoe component for TASS. The TASS/ hemlock

dwarf mistletoe model incorporates the detailed dwarf mistletoe model initially

developed for hemlock dwarf mistletoe by Bloomberg et al. (1980) and further

developed by Robinson et al. (2002). However, data collection and analyses for the

new model have yet to be completed, pending further funding. More model development will be required before a prototype TASS/hemlock dwarf mistletoe model

is available for testing.

In southeastern Alaska, Trummer et al. (1998) developed a set of regression

equations to predict the spread and intensification of hemlock dwarf mistletoe in

wind-disturbed hemlock stands that mimic selection harvests. They intend to link

the model to the SEAPROG variant of the forest vegetation simulator (FVS) used

for southeast Alaska (McClellan and Biles 2003) to project effects of hemlock

dwarf mistletoe in infested stands. However, the base model requires further

development before it is acceptable for use in uneven-aged stands in Alaska

(McClellan and Biles 2003).

J.P. Kimmins and associates at the University of British Columbia are developing a hemlock dwarf mistletoe component for their forest level and ecosystem

models (see footnote 6). Hemlock dwarf mistletoe plays a significant role in the

development and succession of western hemlock and western redcedar forests on

northern Vancouver Island and the adjacent mainland, and therefore, must be

included in landscape-level models.

Needs for Further Research

Although there is a large body of research on the biology and impacts of dwarf

mistletoes (Geils et al. 2002, Hawksworth and Wiens 1996), several controversies

and major gaps in our knowledge need to be addressed to improve the understanding and management of hemlock dwarf mistletoe. Over the extensive geographic

range of hemlock dwarf mistletoe, incidence and severity of the parasite vary
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

greatly, resulting in a wide diversity of effects on forest trees and ecosystems.

These in turn have resulted in a diversity of scientific data and controversial

opinions on hemlock dwarf mistletoe impacts and significance.

As noted by Trummer at al. (1998) and others, most of the information on

hemlock dwarf mistletoe reported from the 1960s to 1990s was based on or directed towards effects of the parasite on tree growth and silvicultural treatments,

particularly in conjunction with clearcut harvesting and even-age management.

More recent changes in forestry practices have emphasized selection harvesting,

reduced sizes of openings, and retention of scattered or blocks of old-growth

trees. These new practices create conditions that potentially favor rapid spread

and more severe impacts of hemlock dwarf mistletoe, but in many areas, actual

data on hemlock dwarf mistletoe spread and effects under these situations are

limited or lacking.

In British Columbia, hemlock dwarf mistletoe is severe in some second-growth

hemlock forests (Muir 2004) and partially cut areas (Buckland and Marples 1952)

that resemble retention-harvested areas. Apparently, conditions that enhance or

suppress hemlock dwarf mistletoe seed production, spread, and eventual impacts

on tree growth are highly variable. Hemlock dwarf mistletoe impacts could become

severe in many retention- or selection-harvested stands, but perhaps remain inconsequential in others. We urgently need more detailed studies of hemlock dwarf

mistletoe spread and impacts in retention- or selection-harvested forests to clarify

and quantify regional and local variations in hemlock dwarf mistletoe spread and

impacts. The limited and often controversial data and opinions on management of

hemlock dwarf mistletoe-infested forests under these new regimes prompted our

review of available literature and information on hemlock dwarf mistletoe.

If the intensification by hemlock dwarf mistletoe and impacts that it causes

under selection harvesting in old-growth forests can be quantified, the results

should be combined with growth loss estimates from studies in various regions and

developed into subroutines for stand and tree growth models. These models include

FVS for Washington and Oregon, PROGNOSIS BC stand model and TASS for

British Columbia, and for southeast Alaska, FVS (McClellan and Biles 2003).

Several basic features of reproductive biology are still not well understood.

For example, the methods of pollination for hemlock dwarf mistletoe, whether

by insects, wind, or both is unclear. The factor or stage in hemlock dwarf mistletoe reproduction that limits spread and intensification needs to be determined in

Over the extensive

geographic range

of hemlock dwarf

mistletoe, incidence

and severity of the

parasite vary greatly,

resulting in a wide

diversity of effects

on forest trees and

ecosystems.
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GENERAL TECHNICAL REPORT PNW-GTR-718

a wide range of environmental conditions. Currently, we do not know how stand

disturbances, e.g., selection harvesting, will influence hemlock dwarf mistletoe

shoot, fruit, and seed production; although we do know that they are greatly enhanced for other dwarf mistletoes. Responses to selection harvesting by hemlock

dwarf mistletoe appear extremely variable but are probably to be expected under

the wide range of environmental conditions and management regimes in coastal

forests. These topics require further research for a better understanding and prediction of how the parasite responds to different forest management practices.

Very little is known about wildlife associations with and use of hemlock dwarf

mistletoe. If wildlife-hemlock dwarf mistletoe interactions are as important as for

other dwarf mistletoes in other regions, then this type of information is needed to

determine and ensure levels of hemlock dwarf mistletoe that are beneficial for

wildlife management objectives. Maintaining particular levels of hemlock dwarf

mistletoe for wildlife habitat could be accomplished by using harvesting systems.

Clearcut harvesting appears to largely eliminate hemlock dwarf mistletoe from

many sites for a long time. Some form of selection harvesting could be used to

extract timber but maintain hemlock dwarf mistletoe at a favorable level for wildlife or other objectives. Thus, research on wildlife use of hemlock dwarf mistletoe

as food and as sites for resting, perching, and nesting should be con-ducted so that

managers can judge what tree, stand, and ecosystem effects are gained or lost by

retaining or encouraging different levels of hemlock dwarf mistletoe.

Several other more specific topics of needed research are listed in appendix 2.

Diverse Opinions on Hemlock Dwarf Mistletoe Impacts

Several studies of spread and effects of hemlock dwarf mistletoe have produced

contradictory results, controversial opinions, and considerable uncertainty about

future impacts of the parasite. Opinions regarding the potential impacts of hemlock

dwarf mistletoe and management practices often are divergent. We have encountered three prominent opinions:

• Hemlock dwarf mistletoe has severe impacts on tree growth and should be

eradicated or reduced as much as possible to protect timber values

• Hemlock dwarf mistletoe is an important ecological agent that contributes

to stands structure, biological diversity, and wildlife habitat and should be

maintained

• Hemlock dwarf mistletoe has insignificant impacts and can be ignored

Some form of selection harvesting

could be used to

extract timber but

maintain hemlock

dwarf mistletoe at a

favorable level for

wildlife or other

objectives.
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A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

The first view that hemlock dwarf mistletoe has severe effects on tree growth

and should be reduced in occurrence as much as possible is derived from several

early reports and studies beginning with Drake (1915), Weir (1915, 1916, 1918),

and others who reported severe infestations of hemlock dwarf mistletoe. Drake

(1915), who worked as a forest examiner, first reported hemlock dwarf mistletoe

from southeast Alaska near Tenakee and later at a number of other locations. Drake

stated that in many areas of the Tongass National Forest, typically 30 to 80 percent

of western hemlock trees were infected. These reports and views that hemlock

dwarf mistletoe infestations are damaging were reinforced by more recent authors

who examined hemlock dwarf mistletoe impacts in recently harvested areas and

recommended harvesting and silvicultural treatments for management of hemlock

dwarf mistletoe-infested stands (Buckland and Marples 1952, Shea 1966). Many of

the earliest reports were made before a DMR system was developed and before any

quantified estimates of impact were available. Also, these views were developed

during an era of resource management that was predominantly focused on harvesting and timber productivity.

The second view is that hemlock dwarf mistletoe has ecological value, or is

even a keystone species, and should be maintained in managed forests. Desirable

levels of hemlock dwarf mistletoe to help meet specific wildlife or other objective are rarely established, however. Thus, there are no target values to apply in

attempts to manipulate hemlock dwarf mistletoe. Another related perspective on

hemlock dwarf mistletoe is the degree to which selection harvests can be conducted

in a manner in which hemlock dwarf mistletoe is maintained, but not at levels that

are devastating to timber values or other resources. In this case, hemlock dwarf

mistletoe is not the primary focus of forest management, but there is concern that

certain types of timber harvesting may exacerbate the disease. This situation applies

to forests managed for multiple resource objectives and scenarios where there are

reasons to consider harvesting methods other than clearcutting.

The third view apparently derives from either a lack of understanding about the

importance of the parasite or a belief that the parasite spreads too slowly to cause

significant impacts. As we have outlined here, there are many studies that indicate

that hemlock dwarf mistletoe has distinct effects on both the timber resource and a

range of ecological factors, including stand structure and disturbance. Perhaps hemlock dwarf mistletoe can be ignored in unmanaged forests where natural processes

are allowed to run their course, but in forests with any level of management,

actions that may increase or decrease the parasite should be evaluated.
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GENERAL TECHNICAL REPORT PNW-GTR-718

The viewpoint that hemlock dwarf mistletoe can be ignored because it spreads

slowly is true for some areas. Examples of low rates of spread include several

studies from Alaska (Deal et al. 2002, Hennon et al. 2001, Shaw 1982, Shaw and

Hennon 1991, Trummer et al. 1998) and in British Columbia, the high-elevation

Montane Alternative Silviculture Systems (MASS) project site (Nevill and Wood

1995). However, the authors of these studies do not recommend that hemlock

dwarf mistletoe be ignored, but rather that certain selection harvest scenarios result

in only limited negative impacts of the disease. We suspect that much of the perception of low impacts of hemlock dwarf mistletoe is based on the fact that the parasite develops relatively slowly in young trees. Often, 10 to 20 years after harvesting

are required before infection of young trees becomes obvious to casual observers.

It is important to realize that hemlock dwarf mistletoe in southeast Alaska extends

from sea level to an elevation of approximately 150 to 300 m. In Alaska, hemlock

dwarf mistletoe probably spreads relatively slowly because of some environmental

factor associated with these higher elevations at this latitude. Similarly, the MASS

site in British Columbia at 750 m is near the upper elevation limit of hemlock

dwarf mistletoe in southern British Columbia. Low incidence and low rates of

spread of hemlock dwarf mistletoe could be expected there. Several authors including Baranyay and Smith (1974) and Hennon et al. (2001) suggested that cold temperatures limit hemlock dwarf mistletoe seed production and dispersal. Snow could

be a factor in the overwintering survival of seeds in higher elevation stands and in

more northerly populations of western hemlock.

We believe that the wide range in hemlock dwarf mistletoe behavior, including differing spread rates in the southern and northern portions of its range, has

contributed to the divergent views about increasing or decreasing its occurrence.

Differing views about the goals of resource management, which often have an

underlying philosophical basis, also have a strong influence in shaping these opposing opinions. The challenge is to determine what management approaches are

most appropriate for a particular ecological condition or resource management

objective, with different severities and incidences of hemlock dwarf mistletoe

infestation. Widespread implementation of selection- or retention-harvesting

practices in coastal forests could exacerbate effects of hemlock dwarf mistletoe

and thereby negatively affect the long-term productivity and sustainability of

managed forests. Alternatively, hemlock dwarf mistletoe could be substantially

eradicated by some practices, particularly clearcut harvesting, with potentially

substantial effects on stand structure, wildlife habitat, and biodiversity of forest
111

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

ecosystems. Fortunately, the relatively predictable nature of the spread and impact

of hemlock dwarf mistletoe facilitates the management of this important forest

parasite.

Conclusions

In some regions, maintenance of current levels of hemlock dwarf mistletoe might

not be a desirable strategy. In southeastern Alaska, several surveys indicate an

almost complete eradication of hemlock dwarf mistletoe due to clearcut harvesting

and the resulting development of even-aged stands. In the past, when timber was

the predominant management objective, reduction of hemlock dwarf mistletoe was

viewed as highly desirable. Management objectives now include a broader objective

of maintaining forest ecosystems. Attributes of a healthy forest include biological

diversity, structural diversity, essential functional components, and processes. In

this context, particular levels of hemlock dwarf mistletoe may enhance forest

health.

The challenge for forestry professionals is to adapt, but not rigidly apply, general guidelines for managing hemlock dwarf mistletoe to local or specific stands.

Specific management actions will probably not be directly aimed at hemlock dwarf

mistletoe, but the consequences of management on the parasite should be considered. Every location in the extensive western hemlock coastal forests probably has

a unique complex of environmental, biotic, climatic, and historical factors that

must be considered in determining suitable or appropriate management regimes and

silvicultural treatments. Hemlock dwarf mistletoe is inconspicuous, but its presence

and impacts are far easier to evaluate and predict than infestations by forest insects

and pathogens such as root diseases or wood decay fungi. Hemlock dwarf mistletoe

seed production, dispersal, infection, and effects on tree growth are relatively easier

to measure and predict. Tree- and forest-level models need to be developed to estimate hemlock dwarf mistletoe spread and impacts for particular situations. However, spread rates and impacts of hemlock dwarf mistletoe also need to be measured

and monitored over a wide range of ecological conditions as a basis for sustainable

forest management.

The differences in results of studies on the biology and impacts of hemlock

dwarf mistletoe are striking particularly between those in Alaska and British

Columbia. We suspect that a large part of the difference in results and opinions

derives from the types of stands and trees selected for study. A second-growth,

immature stand of western hemlock with no or few residual trees from the previous forest will likely have a low incidence of hemlock dwarf mistletoe. An older,

Fortunately, the

relatively predictable nature of the

spread and impact

of hemlock dwarf

mistletoe facilitates

the management of

this important forest

parasite.
112

GENERAL TECHNICAL REPORT PNW-GTR-718

second-growth stand with hemlock dwarf mistletoe infection ranging from light to

severe will likely include several residual trees from the previous stand that became

sources of mistletoe spread into the young stand. The incidence and impacts of

hemlock dwarf mistletoe are closely related to the number of residual infected

trees. Our review suggests that the two types of stands can be found in most

regions, but it is not known how frequently each scenario occurs.

We suggest that to resolve these differences and controversies, an extensive,

nonbiased sampling procedure is needed in each region to determine hemlock

dwarf mistletoe incidence and effects, and to form a basis for designed experimental studies. An inventory-based sample of hemlock dwarf mistletoe is available for

Washington and Oregon (Hildebrand 1995) and for nonwilderness areas of Alaska

(described in this report). Inventory plots or subsamples will likely have to be remeasured to ensure accuracy of records on mistletoe incidence and severity (see

references in Muir and Moody 2002).

Many, if not most, of the studies on biology and effects have been based on

selected, infected trees or stands of trees with varying attributes. Because of the

methods of selection in many previous studies, personal bias in the selection

process cannot be excluded. An extensive, stratified, random sampling method

with a two- (or more) stage procedure will be needed to select infested stands and

infected trees, but costs for implementing such a sampling design could be prohibitive. Until this type of a sampling design can be established, the extent and significance of the effects of hemlock dwarf mistletoe in many areas will likely remain

uncertain or controversial.

Management of coastal western hemlock forests will not focus solely on hemlock dwarf mistletoe, but will include many other factors. However, our review

indicates that if hemlock dwarf mistletoe influences on stand structure and productivity are ignored, all harvesting and management regimes could produce unintended and undesirable results.

Acknowledgments

Much of the information on hemlock dwarf mistletoe is unpublished, and we have

drawn on several of these sources for information and ideas. We are indebted to

several researchers and workers who have shared their opinions and information

with us.

In compiling this report, we made extensive use of the dwarf mistletoe literature database, which is maintained at the USDA Forest Service, Rocky Mountain
113

A Synthesis of the Literature on the Biology, Ecology, and Management of Western Hemlock Dwarf Mistletoe

Research Station in Flagstaff, Arizona. We deeply appreciate the efforts of the late

Frank Hawksworth and Brian Geils who initiated and maintained this invaluable

literature database over five decades. In addition, we thank Brian Geils and Terry

Shaw for searches of the database and citations of the literature.

We appreciate the help of Bill van Hees of the Forest Inventory and Analysis

staff in Anchorage who provided summaries of hemlock dwarf mistletoe occurrence in Alaska.

We are very grateful for the substantial efforts of Diane Hildebrand, Robert

Mathiasen, David Shaw, Dick Smith, and Lori Trummer who reviewed our extensive draft manuscripts and provided many helpful suggestions for improvement.

We are also grateful to U.S. Forest Service, Forest Health Protection, Pest

Trend Impact Plot System program, the Forest Investment Account, and International Forest Products Ltd., British Columbia, for funds that supported the preparation of this report. We thank Warren Warttig, International Forest Products Ltd.,

and Roderick Negrave, British Columbia Ministry of Forests and Range, Nanaimo,

for their support in preparing the shortened version of this review for British

Columbia.

English Equivalents

When you know: Multiply by: To find:

Millimeters 0.039 Inches

Centimeters (cm) .4 Inches

Meters (m) 3.3 Feet

Kilometers (km) .6 Miles

Hectares (ha) 2.5 Acres

Kilograms (kg) 2.2 Pounds

Cubic meters (m3) 35.3 Cubic feet

Cubic meters .0283 Cubic feet (ft3)

Cubic meters per hectare (m3/ha) 14.3 Cubic feet per acre

Trees per hectare .4 Trees per acre

Degrees Celsius (°C) 1.8 (and then add 32) Degrees Fahrenheit

Cubic meters per hectare .07 Cubic feet per acre

(ft3 acre)

Kilometers per hour .621 Miles per hour
114

GENERAL TECHNICAL REPORT PNW-GTR-718

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Appendix 1: Common and Scientific Names

Common name Scientific name

Trees:

Black spruce Picea mariana (Mill.) B.S.P.

Blue spruce Picea pungens Engelm.

Douglas-fir Pseudostuga menziesii (Mirb.) Franco

Engelmann spruce Picea engelmannii Parry ex Engelm.

Grand fir Abies grandis (Douglas ex D. Don) Lindl.

Jack pine Pinus banksiana Lamb.

Lodgepole pine Pinus contorta Dougl. ex Loud var. latifolia

Engelm. ex Wats.

Pinus contorta Dougl. ex Loud var. murrayana

 Engelm. ex Wats.

Mountain hemlock Tsuga mertensiana (Bong.) Carr.

Noble fir Abies procera Rehd.

Pacific silver fir Abies amabilis (Dougl.) Forbes

Ponderosa pine Pinus ponderosa Dougl. ex Laws.

Shore pine Pinus contorta Dougl. ex Loud. var. contorta

Sierra white fir Abies lowiana (Gordon) A. Murray

Sitka spruce Picea sitchensis (Bong.) Carr.

Subalpine fir Abies lasiocarpa (Hook.) Nutt.

Western hemlock Tsuga heterophylla (Raf.) Bong.

Western larch Larix occidentalis Nutt.

Western redcedar Thuja plicata Donn ex D. Don

Western white pine Pinus monticola Dougl. ex D. Don

Whitebark pine Pinus albicaulis Engelm.

Yellow-cedar Chamaecyparis nootkatensis (D. Don) Spach

Mistletoes:

Douglas-fir dwarf mistletoe Arceuthobium douglasii Engelm.

Eastern dwarf mistletoe Arceuthobium pusillum Pk.

Fir dwarf mistletoe Arceuthobium abietinum Engelm. ex Munz

Hemlock dwarf mistletoe Arceuthobium tsugense (Rosendahl) G.N. Jones

Subspecies:

Mountain hemlock dwarf mistletoe A. tsugense (Rosendahl) G.N. Jones subsp.

mertensianae Hawksworth & Nickrent

Shore pine dwarf mistletoe A. tsugense (Rosendahl) G.N. Jones subsp.

contortae Wass & Mathiasen

Pacific silver fir dwarf mistletoe A. tsugense (Rosendahl) G.N. Jones subsp.

amabilae Mathiasen & C. Daugherty
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GENERAL TECHNICAL REPORT PNW-GTR-718

Common name Scientific name

Western hemlock dwarf mistletoe A. tsugense (Rosendahl) G.N. Jones subsp.

tsugense

Lodgepole pine dwarf mistletoe Arceuthobium americanum Nutt.:Engelm.

Southwestern dwarf mistletoe Arceuthobium vaginatum (Willd.) Prsl. subsp.

cryptopodum (Engelm.) Hawksw. & Wiens

Western dwarf mistletoe Arceuthobium campylopodum Engelm.

Western larch dwarf mistletoe Arceuthobium laricis (Piper) St. John

Western spruce dwarf mistletoe Arceuthobium microcarpum (Engelm.)

Hawksw. & Wiens

Leafy (true) mistletoes Phoradendron, Viscum spp.

Fungi:

American matsutake Tricholoma magniverlare (S. Ito & S. Imai)

Singer

Colletotrichum canker Colletotrichum gloeosporioides (Penz.) Penz &

Sacc.

Neonectria canker Neonectria neomacrospora (Booth & Samuels)

Mantiri & Samuels

Annosus root disease Heterobasidium annosum (Fr.) Bref.

Artist’s conk Ganoderma applanatum (Pers.: Wallr.) Pat.

Hartig’s conk Phellinus hartigii (Allsch. & Schnabl.) Bond.

Paint fungus Echinodontium tinctorium (Ell. & Ev.) Ell. & Ev.

Pinicola conk Fomitopsis pinicola (Swartz ex Fr.) Karst.

Yellow cap fungus Pholiota adiposa (Batsch) P. Kumm.

Insects:

Hemlock looper Lambdina fiscellaria

Johnson’s hairstreak butterfly Loranthomitoura johnsonii

Blue mistletoe hairstreak Loranthomitoura spinetorum

Birds:

blue grouse Dendragapus obscurus

brown creeper Certhia americana

chippping sparrows Spizella passerina

dark-eyed junco Junco hyemalis

goshawk Accipiter gentilis

gray jay Perisoreus canadensis

great gray owl Strix nebulosa

hermit thrush Catharus guttatus

house wren Troglodytes aedon

long-eared owl Asio otus

marbled murrelet Brachyramphus marmoratus
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Common name Scientific name

mountain chickadee Parus gambeli

northern spotted owl Strix occidentalis

pine siskin Carduelis pinus

red crossbill Loxia curvirostra

red-breasted nuthatch Sitta canadensis

sharp-shinned hawk Accipiter striatus

Steller’s jay Cyanocitta stelleri

western tanager Piranga ludoviciana

Mammals:

Douglas squirrel Tamiasciurus douglasii

Elk Cervus elaphus

Marten Martes americana

Mule deer Odocoileus hemionus

Northern flying squirrel Glaucomys sabrinus

Red squirrel Tamiasciurus hudsonicus
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Appendix 2: Hemlock Dwarf Mistletoe Research

Topics or Needs for Further Study

Subject and/or research References

Medicinal uses Smith 1928

Host specificity of subspecies Mathiasen and Daugherty 2005

Temperature thresholds for geographic Gill 1957

and upper elevation occurrences

Association with riparian zones Muir 2004, Swanson et al. 2006

Disturbance regimes Hennon and McClellan 2003

Aerial shoot production in young stands Shaw and Weiss 2000

Seed production and dispersal from Smith 1966, 1973, 1977

old-growth trees

Replicated studies of life cycle in different Carpenter et al. 1979, Smith 1974,

regions Smith and Wass 1979, Shaw and

Loopstra 1991

Lack or variability of bole swelling Shaw et al. 2005

Animal and bird vectors of mistletoe seeds Shaw et al. 2002, 2005

Tree-to-tree variation in hemlock dwarf Smith 1966, 1973, 1977

mistletoe seed production and dispersal

Biocontrol agents Shamoun and DeWald 2002

Survival and effects of infection on

lower foliated and unfoliated branches

Genetically resistant western hemlock Shamoun and DeWald 2002

Physiological effects on trees Bickford et al. 2005, Meinzer et al.

2004

Reevaluation of dwarf mistletoe rating Shaw et al. 2000

for hemlock

Regional forest-level growth impacts Hildebrand 1995

Top-kill and whole tree mortality Hennon and McClellan 2003

Role in canopy gap formation Hennon 1995, Hennon and

McClellan 2003

Wildlife habitat and biodiversity Geils et al. 2002, Mathiasen 1996

Stand development and residual infected trees Trummer et al. 1998

How much hemlock dwarf mistletoe will

result from different harvesting schemes

Criteria, indicators, and monitoring Muir et al. 2004a

Models for spread and impact Robinson and Geils 2006
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