~~ United States tt'&Al\\ Department of 

i..%,~~..t,} l Agriculture 

~ Forest Service 

e United States 

Department 

of the Interior 

Bureau of 

Land Management 

General Technical 

Report PNW-164 

March 1984 

/ ED1TOR'S' The Seen FILE coP~ 

and Unseen World 

of the Fallen Tree 
Cov

er: Ant's view of the seedling, moss, 

lichen, and mushroom cover on the 

surface of a fallen class Ill Douglas-fir 

tree. 

Authors 

CHRIS MASER U.S. Department of the lntenor 

Bureau of Land Management 

Forestry Sctences Laboratory 

Corvallis, Oregon 97331 

JAMES M 1RAPPE 

U.S. Department of Agncullure, Forest Servtce 

Pactfic Northwest Forest and Range 

Expenment Statton 

Forestry Sc1ences Laboratory 

Corvallis, Oregon 97331 

STEVEN P CLINE 

Oregon State Untverstty 

School of Forestry, Depanment of 

Forest Sc1ence 

Corvallis, Oregon 97331 

KERMIT CROMACK, JR. 

Oregon State Unlverstty 

School of Forestry, Department of 

Forest Sctence 

Corvallis, Oregon 97331 

HELMUT BLASCHKE 

Lehrstuhl fur Forstbotantk 

UntverSttat Munchen 

Amahenstrasse 52 

D·8000 Munchen 40 

West Germany 

JAMES A SEDELL U.S. Department of Agnculture, Forest Servtce 

Pacific Northwest Forest and Range 

Expenment Statton 

Forestry Sctences Laboratory 

Corvallis, Oregon 97331 

FREDERICK J SWANSON U.S Department of Agnculture, Forest Servtce 

Pac11tc Northwest Forest and Range 

Expenment Statton 

Forestry Sc1ences Laboratory 

Corvallis, Oregon 97331 
The Seen and Unseen World

of the Fallen Tree

Chris Maser

James M. Trappe

Technical Editors

Pacific Northwest Forest and Range Experiment Station

U.S. Department of Agriculture

Forest Service

General Technical Report PNW-164

March 1984

Published in cooperation with

the Bureau of Land Management

U.S. Department of the Interior

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

Washington, D.C., 20402
This page was intentionally left blank
Abstract Maser, Chris; Trappe, James M., tech. eds. The seen and unseen world of the fallen

tree. Gen. Tech. Rep. PNW-164. Portland, OR: U.S. Department of Agriculture, Forest

Service, Pacific Northwest Forest and Range Experiment Station; 1984. 56 p. In

cooperation with: U.S. Department of the Interior, Bureau of Land Management.

Large, fallen trees in various stages of decay contribute much-needed diversity to

terrestrial and aquatic habitats in western forests. When most biological activity in soil

is limited by low moisture availability in summer, the fallen tree-soil interface offers a

relatively cool, moist habitat for animals and a substrate for microbial and root activity.

Intensified utilization and management can deprive future forests of large, fallen trees.

The impact of this loss on habitat diversity and on long-term forest productivity must be

determined because managers need sound information on which to base resource

management decisions.

Keywords: Fallen trees, decay (wood), decomposition, old-growth stands, Douglas-fir,

Pseudotsuga menziesii, mycorrhizae, soil moisture.
Contents 1

1

2

2

3

4

5

7

7

8

10

12

13

13

15

15

16

17

19

19

36

38

39

41

42

44

46

47

47

48

49

49

50

50

51

Prolog

Introduction

The Seen World of the Fallen Tree

Recruitment of Fallen Trees

Mass of Fallen Trees

Placement of Fallen Trees

Decay Classes of Fallen Douglas-Fir Trees

A Fallen Tree’s Relationship With Time

Determining the Age of a Fallen Tree

Size and Shape of Fallen Douglas-Fir

Substrates as Niches

Internal Surface Area

Temperature

Moisture

Plant Rooting

Insect Galleries and Frass (Excrement)

The Unseen World of the Fallen Tree

Microhabitats and Inhabitants

Nutrient Cycling

Interactions-Nutrients, Plants, and Animals

Biotic Succession Within and Around Fallen Trees

Stream Characteristics and Fallen Trees

A Historical Sketch of Woody Debris in Northwest Waterways

Vegetation of Streamsides and Gravel Bars

Decomposition and Nutrients in Streams

Aquatic Invertebrates

Fish Habitat

Management Impacts on Fallen Trees

On Land

In Streams

Research Needs

Epilog

Common and Scientific Names

Acknowledgments

Literature Cited
Figure 4.-When they fall, trees 

and snags immediately enter one 

of the first four decay classes 

(modified from Maser and others 

1979). 

Mass of Fallen Trees 

---~r-----1 

-!!_ t"~' ~ I -1"~ 

t' 

,.$ ! ~( 

·. Ar,\. 

~ ' lif 

l ! j I ,,') 

_ _l A STAGE 1 I STAGE 2 

Ll~ _ ~ECLINING __ 

DECAY CLASS V 

I 

~~~_,~ ·~: 1___.......~.--~ ~- i L_______J 

Trees recruited to the forest floor accumulate through time (table 2, fig. 5); accumulation 

and rate of decomposition vary with the type of forest, slope, and aspect (Franklin and 

others 1981, Grier 1978, Graham and Cromack 1982, Maser and others 1979). In one 

midelevation stand of unmanaged 470-year-old Douglas-fir in western Oregon, the 

recruitment rate was estimated to be 0.49 fallen tree per acre (1.2 trees/ha) per year 

(Grier and Logan 1978). The decaying Douglas-fir trees represented from 53.4 to 

265.4 short tons per acre (120 to 595 metric tons(t)/ha) (MacMillan and others 1977). 

Franklin and Waring (1980) showed values ranging from 31.2 to 69.4 short tons per 

acre (70 to 156 t/ha) in stands of widely different ages; their work included both young 

stands with large, carryover pieces of rotting wood and old-growth forests. Grier and 

Logan ( 1978) found that as much as 60 percent of the annual litter fall in a 450-year-old 

Douglas-fir stand may be woody debris. Sollins (1982) found that coarse woody debris 

contributed about 50 percent of the litter on a long-term basis. Further, decomposing 

trees in western Douglas-fir forests represent more volume above ground than is 

represented by the aboveground woody debris of typical deciduous forests in the Eastern 

United States (Day and Monk 197 4, McFee and Stone 1966, Sollins and others 1980) 

(fig. 6). 

Table 2-Fallen trees in an old-growth Douglas-fir stand, by decay class 1 

Decay 

class 

I 

ll 

Ill 

IV 

v 

Total 

Fallen trees 

per hectare 

27 

15 

31 

39 

185 

297 

1 Adapted from MacMillan and others (1977). 

Number 

Fallen trees 

per acre 

11 

6 

13 

16 

75 

121 

3 
Figure 5.-0ouglas-firtree, decay 

class I, fell before the Douglas-fir 

snag, which entered decay class 

Ill. 

Figure 6.-Substantial volumes of 

decomposing fallen trees are 

typical of old-growth forests in the 

Douglas-fir region. 

Figure 7.-A fallen tree oriented 

along the contour of a slope. The 

upslope side is filled with humus 

and inorganic material that allows 

invertebrates and small vertebrates to tunnel alongside. The 

downslope side provides protective cover for larger vertebrates. 

Placement of Fallen Tre(ts Fallen trees that are oriented along the contours of a slope seem to be used more by 

vertebrates than are trees oriented across contours, especially on steep slopes. Large, 

stable trees lying along contours help reduce erosion by forming ·a barrier to creeping 

and raveling soils (fig. 7). Soil and nutrients deposited along the upslope side of fallen 

trees reduce loss of nutrients from the site. Such spots are excellent for the establishment 

and growth of vegetation, including tree seedlings. 

4 

Vegetation becomes established on and helps stabilize this "new soil" (Maser and others 

1979), and as invertebrates and small vertebrates begin to burrow into the new soil, 

they not only nutritionally enrich it with their feces and urine but also constantly mix it 

by their burrowing activities. 

The interactions of fallen trees with soil are directly affected by steepness of slope and 

ruggedness of terrain; a fallen tree on flat ground, for example, is much more likely to 

contact the soil over its entire length than is one oriented either across or along contours 

on steep or rough terrain. The proportion of a tree in contact with the soil affects the 

water-holding capacity of the wood (Graham 1925). In our studies of fallen trees in 

old-growth Douglas-fir forests, the moisture retention through the summer drought was 

best in the side of trees in contact with the soil. The moisture-holding capacity of the 

wood affects in turn its internal processes and therefore the succession of plants and 

animals. In addition, the orientation of a fallen tree to aspect and compass direction and 

the amount and duration of sunlight it receives, drastically affect its internal processes 

and biotic community (Graham 1924, 1925; Graham and Knight 1965). 
Decay Classes of Fallen Dead, fallen trees decay continuously, passing through recognizable stages or classes

Douglas-Fir Trees of decomposition. Fogel and others (1973) described broad classes of decay, based

on the physical condition of the bark, wood, and twigs; the presence and pattern of

vegetation on a fallen tree and its degree of root development; and the genera of fungi

(identified from fruiting bodies or sporocarps) associated with the fallen tree. The five

decay classes (Maser and others 1979; table 3), refined by subsequent study (Sollins

1982, Triska and Cromack 1980) are indispensable to research on wood decomposition

for three reasons:

1. These stages of decay are inevitable; despite variation in the original material and

surrounding environment, a classification of decay based on general processes can be

used. (A publication by Minore (1966) can be used to key fallen trees to species.)

2. Decay class can be used without having to determine when the tree fell, a difficult

problem that requires destructive techniques.

3. The original decay classes form ecological units that function as distinctive habitats

for plants and animals.

The major limiting factor of this decay classification is that it is based on the external

characteristics of a fallen tree and does not adequately convey the internal diversity of

niches. We have found, however, that internal development of niches relates reasonably

well to decay class.

A 450-year-old Douglas-fir stand on the H. J. Andrews Experimental Forest in the

Willamette National Forest in western Oregon had a distribution of fallen trees in decay

classes II to V that covered an average of 24 percent of the ground surface, ranging

from 11 to 35 percent on different subplots. Although class V trees produced the most

coverage, many were evident only as mossy or humus-covered mounds on the forest

floor. Class I fallen trees, on the other hand, accounted for the least coverage for two

reasons: (1) mortality rates are low in old-growth stands (Cline and others 1980) and

(2) a rapid rate of decomposition in class I trees quickly converts them to class II.

Subsequent change to higher decay classes progresses more and more slowly as the

most readily decomposed fractions of the wood are utilized by decomposers (Hulme

and Shields 1970).

Variation within and among fallen trees.--One must be aware of two sources of

variation when using the decay classification: (1) within a particular fallen tree and (2)

among fallen trees of the same decay class.

Variation within a fallen tree.-An old-growth Douglas-fir tapers along the length of its

trunk, from its root collar to the tip of its crown (fig. 8). Much of the variation within a

fallen tree relates to this taper. The smaller diameter parts of a fallen tree decay faster

than the larger ones because the volume of the wood decreases toward the crown, the

ratio of sapwood to heartwood increases (Dadswell and Hillis 1962), and fragmentation

and contact with the forest floor increase. As decay proceeds, variation within the tree

gradually changes; by the time the tree enters decay class V, the entire tree becomes

a relatively amorphous, homogeneous mass.
Table 3-A 5-class system of decay based on fallen Douglas-fir trees 1 

Characteristics 

of fallen trees 

Bark Intact 

Twigs, 1.18 inches 

(3cm) Present 

Texture Intact 

Shape Round 

Color of wood Original color 

Portion of tree Tree elevated 

on ground on support 

points 

Invading roots None 

1 Adapted from Maser and others (1979). 

70 

60 

50 

u; 

a: 

w 

..... 

w 

!. 40 ..... 

J: 

0 

w 

J: 

30 

20 

10 

II 

Intact 

Absent 

Intact to 

partly soft 

Round 

Original color 

Tree elevated on 

support points 

but sagging 

slightly 

None 

Decay class 

Ill IV 

Trace Absent 

Absent Absent 

Hard, large Small, soft 

pieces blocky pieces 

Round Round to oval 

Original color Light brown to 

to faded reddish brown 

Tree is sagging Alloftreeon 

near ground ground 

In sapwood In heartwood 

Figure 8.-Diameter-height relationships in 

seven individual old-growth Douglas-fir trees. 

v 

Absent 

Absent 

Soft and 

powdery 

Oval 

Red brown to 

dark brown 

All of tree on 

ground 

In heartwood 

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 

DIAMETER INSIDE BARK (CENTIMETERS) 

6 
A Fallen Tree's 

Relationship With Time 

Variation among fallen trees.- The second source of variation-among fallen trees of 

the same decay class-is partly a consequence of classification itself. A single decay 

class necessarily contains a variety of fallen trees because they are assigned to a 

discrete class from a decay continuum. Additional variation, however, is caused by 

differences among trees at the time they fall. For example, trees vary in size, original 

diameter, age, old growth versus young growth, decay condition, live tree versus standing 

dead tree, and presence of heart rot. 

The age of a fallen tre&-the number of years it lies on the forest floor-is positively 

correlated with decay class, but the relationship is logarithmic rather than linear. The 

time lapse between classes IV and V is exponentially greater than between decay 

classes Ill and IV, and so on. Decay classes I and II evolve rapidly, whereas the later 

classes develop slowly; variation in residence time within a decay class increases in 

the later classes. Estimating the residence time or age of a fallen tree becomes 

increasingly difficult as decay proceeds. Further, decay classes can evolve at different 

rates, depending on the physical setting of a forest stand, such as a north-facing slope 

versus a south-facing slope, or low elevation versus high elevation. 

Determining the Age of a The time a fallen tree has rested on the forest floor can be determined by two standard 

Fallen Tree methods: ( 1 ) aging the scars left on adjacent, living trees that were hit by the tree when 

it fell, and (2) aging the oldest tree growing on the fallen tree (fig. 9). 

Scars.- Aithough aging scars left on the living, woody vegetation by the tree as it fell 

is the most reliable way to age what is now the fallen tree, such scars are often difficult 

to locate because they heal over or are inaccessible. Moreover, the healing pattern of 

wounds varies greatly, so increment cores taken around scars often produce inaccurate 

estimates of residence time. 

A more reliable, but destructive, method of aging is to saw either a wedge or a complete 

cross section from the scar area; however, sawing injures or kills trees, an intolerable 

effect on long-term research sites. 

Figure 9.- Western hemlock tree 

and seedlings (background) 

growing on a fallen tree that has 

decomposed into a mound on the 

forest floor. 

7 
Seedlings.-The second method of aging a fallen tree is to age the seedlings growing

on it. Aging seedlings is a less reliable method than is aging scars because the lapse

of time between the fall of a tree and the establishment of the oldest seedling is not

known. But this lag can be calculated for a fallen tree by determining ages for both the

scar and the oldest seedling and then subtracting the latter from the former. Several

pairs of these age counts will reveal the mean lag for the stand. Mean lag can vary

markedly from one stand to another.

Age for fallen trees that did not produce scars but do support rooted seedlings can be

calculated by adding the mean lag to the age of the oldest seedling. This method of

aging fallen trees, however, also requires the sacrifice of living trees that may be

important in the development of a stand or in future studies.

Size and Shape of Fallen There are three major sources of coarse woody debris in an old-growth Douglas-fir

Douglas-Fir forest: (1) uprooting of live trees, with or without complete crowns; (2) breakage of crown

and stem of live trees; and (3) breakup and fall of snags (Graham 1982). Because size

of trees and the manner in which woody material comes to be on the ground vary widely,

the resulting pieces of woody debris are heterogeneous in size and shape. Regardless

of its original size, wood passes through the various stages of decay; the smaller it is,

the faster it breaks down and disappears.

Surveys of large, coarse woody debris in old-growth forests show that broken, fallen

trees are typically more abundant than whole fallen trees are (fig. 2).1 To illustrate the

changes in the size and shape of fallen Douglas-fir trees during the decomposition

process, however, we will use a generalized whole, uprooted tree with a complete crown

as an example.

Volume.-The first noticeable decrease in the volume of a fallen tree is in decay class

III (Graham 1982). Bark alone can account for about 20 percent of the volume of a

fallen Douglas-fir (Snell and Max 1981). As the sapwood is consumed and fragmented

by both plants and animals and sloughs off, about 50 percent of the volume is lost,

most of it during the transition from decay class III to decay class IV. This transition is

called the basic fragmentation stage because of the cubical chunks of throughly

brown-rotted heartwood. Fragmentation continues in decay class V, but the wood is

held together by the prolific rooting of plants (fig. 10). Consequently, only 20 percent of

the volume of a decay class V tree is lost; but with the loss of that 20 percent, only a

tenth of the fallen tree’s original volume remains (Graham 1982).

The bark and wood that slough off a rotting fallen tree accumulate alongside the tree.

This material forms a mulch that extends over the ground area influenced by the tree.

Diameter.-The diameter of a fallen Douglas-fir decreases as the outer bark, sapwood,

and heartwood slough off. So most of the overall volume loss is in diameter rather than

in length; in our studies, the transition from decay class III to IV, when the maximum

volume is lost, corresponded to the largest reduction (44 percent) in diameter. By decay

class V, 68 percent of the original diameter has been removed through decomposition

(see footnote 1). Some of the “lost” material is still present as crumbly fragments of

wood and bark on the soil adjacent to the fallen tree, and some is incorporated into the

soil by animal activity. Some has been physically removed by weather or animals, and

some has been used as food by decomposer organisms.

1

 P. Sollins and S. P. Cline. Unpublished data on file at Oregon State

University, School of Forestry, Department of Forest Science,

Corvallis, Oregon 97331.

8
CROSS SECTION 

(CENTIMETERS) 

0 50 

50 

0 50 

DECAY CLASSES 1-111 

0 5 10 

OECA Y CLASS IV 

--hF a :asc _ a •---::: 

0 5 10 

DECAY CLASS V 

0 10 

20 METERS 

20 

Figure 10.-Prolific rooting of 

western hemlock saplings on a 

decay class V fallen tree holds the 

rotten wood together; ground 

cover is Oregon oxalls. 

30 

30 

Figure 11 .-Schematic changes 

in size and shape of fallen 

Douglas-fir trees in the course of 

decomposition. 

A diameter of about 12 inches (30 em) seems to be a critical boundary between slow 

or rapid decay and disintegration of fallen trees. Pieces smaller than that have a higher 

proportion of sapwood to heartwood and disappear more rapidly than pieces of larger 

diameter. This relationship between diameter of fallen trees and speed of decomposition 

has important ramifications in managed forests in which total tree size will be controlled. 

Length.- The length of a fallen Douglas-fir tree decreases over time from the top toward 

the roots because the top has the smallest diameter and decomposes fastest (fig. 11 ). 

Little change in length takes place in decay classes I through Ill. By the time a tree 

reaches decay class IV, its length has been reduced about 15 percent because the top 

has begun to be incorporated into the soil. The largest reduction in length (about 

40 percent) occurs during the transition from decay class IV to V, because the diameter 

decreases through fragmentation. Therefore, only a fraction of the original length of a 

fallen tree remains by the time it reaches class V (see footnote 1 ). 

9 
Substrates as Niches 

10 

Shape.-Viewed from above, an intact, fallen tree changes in shape from a frustum 

(a truncated cone, tapered at the top) in decay classes I through Ill (fig. 5) to nearly 

cylindrical (both ends with a similar diameter) in decay classes IV and V. This change 

in shape can be demonstrated by comparing the ratio of basal diameter to top diameter: 

decay classes I and II = 5:1; decay class Ill = 4.4:1, decay class IV = 2.1 :1; and 

decay class V = 1.6:1 (see footnote 1 ). 

In a cross-sectional view, a fallen tree is circular in decay classes I through Ill. By settling 

and sloughing, it becomes elliptical in decay class IV. The elliptical shape becomes 

exaggerated, approaching a lens shape, in decay class V and later stages. 

Every living conifer is composed of tissues that perform specific functions. When a tree 

dies, the various tissues provide distinguishable substrates that provide different niches. 

The four major tissues of a Douglas-fir tree are outer bark; inner bark or phloem; 

sapwood, the living portion of the xylem; and heartwood, the dead portion of the xylem 

(fig. 12). A class I fallen Douglas-fir is mostly xylem, heartwood (60 to 80 percent) and 

sapwood (5 to 20 percent). The thick, outer bark of an old-growth Douglas-fir may be 

5 to 20 percent of the cross-sectional area, whereas the inner bark is usually less than 

5 percent. 

The outer bark and heartwood of a Douglas-fir tree are physiologically inactive. Outer 

bark forms a physical and chemical barrier between the inner tree and the atmosphere, 

insects, and diseases. Heartwood supports the tree and stores metabolic wastes (Brown 

and others 1949, Hillis 1962). The inner bark and sapwood are physiologically active. 

Inner bark is the growing portion of a tree and is the site of both formation of new cells 

and transportation of food, whereas sapwood transports and stores water and dissolved 

mineral salts (Brown and others 1949). 

Decay rate 18 > sw > HW > 

Soluble carbohydrates 18 > sw > HW > 

Taxifolin content 18 < sw < HW < 

Nitrogen content 18 > 08 > sw > 

Mineral content 18 > 08 > sw > 

Water content sw > 18 > 08 > 

Lignin content 08 > 18 > sw ~ 

Total extractives 08 > 18 > HW > 

Cellulose content HW > sw > 18 > 

C:N ratios HW ~ sw > 18 > 

Density 08 ~ 18 > HW > 

Figure 12.-Relative decay rates and composition of different substrates of fallen 

trees: 08 = outer bark; 18 = inner bark; SW = sapwood; HW = heartwood (adapted 

from Clermont and Schwartz 1948; Gardner and Barton 1960; Graham and Kurth 

1949; Hergert and Kurth 1952; Kurth 1948, 1949; Kurth and Chan 1953; Scheffer 

and Cowling 1966; and this study). 

08 

08 

08 

HW 

HW 

HW 

HW 

sw 

08 

08 

sw 
These parts of the fallen tree provide substrates of differing quality for use by other

organisms. Quality of the substrate depends on physical and chemical properties; the

higher the quality, the faster it disappears through respiration and fragmentation Lambert

and others 1980). Respiration is the enzymatic transformation by decomposer organisms

of organic compounds to carbon dioxide, water, and other simple products. Fragmentation occurs when the substrate is eaten, sloughed, and leached.

The main chemical differences among substrates are: (1) nitrogen content; (2) the

mineral or ash content-phosphorus, potassium, calcium, magnesium; (3) the carbon

matrix-cellulose, lignin, pentosans (Lewis 1950); and (4) the content of other organic

compounds-waxes, pigments, carbohydrates, fats, resins, phenolic compounds

(Graham and Kurth 1949, Hergert and Kurth 1952, Kurth 1948).

Many chemical substances are associated with the carbon matrix but are not chemically

bonded to it. Bark contains more such extractable materials than wood does (Kurth

1948, 1949) (fig. 12); for example, taxifolin, a natural component of Douglas-fir, has

fungicidal properties (Kennedy 1956).

Tissues that were physiologically active in the live Douglas-fir decay most rapidly after

the tree falls because they are higher in quality than inactive tissues are (fig. 12). Inner

bark and sapwood of freshly fallen trees attract the initial decomposers-beetles that

feed on these tissues. By penetrating the protective outer bark, the beetles open the

inner bark and sapwood to invasion by other decomposers. These tissues contain more

soluble carbohydrates, more moisture, and less taxifolin than do the lower quality outer

bark and heartwood (fig. 12). The substrate of poorest quality is the decay-resisting

outer bark, which is low in moisture, carbohydrates, cellulose, and carbon to nitrogen

(C:N) ratio but high in lignin, taxifolin, total extractives, and density (fig. 12).

Regardless of substrate, changes develop during decay of fallen trees: (1) density

decreases; (2) water content increases until decay classes III and IV are reached, at

which time the water content stabilizes; (3) mineral and nitrogen contents increase;

(4) the cellulose content decreases; (5) the relative lignin content increases; and (6) the

C:N ratio decreases (Fogel and Cromack 1977, Foster and Lang 1982, Graham and

Cromack 1982, Grier 1978, Lambert and others 1980, Sollins 1982).

Inner bark disappears fastest because it has the highest substrate quality and the

smallest volume. Both sapwood and outer bark disappear by decay class IV, but for

different reasons. Sapwood elicits high biological activity and disappears because of

insect consumption and microbial breakdown (fig. 13). Outer bark disappears almost

solely by fragmentation and sloughing from the top and sides of a fallen tree. Large

trees are mostly heartwood when they fall and by decay class IV only heartwood remains;

without the initial association with higher quality substrates, such as sapwood and inner

bark, heartwood would undoubtedly decay more slowly (see footnote 1). When decay

reaches the advanced class V stage, the fallen tree appears as a mound on the forest

floor, usually covered with humus and litter or moss and often supporting several to

many hemlocks. Much of the crumbly, brown-rotted heartwood remains in place. That

which has disappeared, however, is not all lost through metabolism of decomposers.

Much of it merges into the humus or becomes incorporated into the soil profile (Denny

and Goodlett 1956; Harvey and others 1978, 1979b; McFee and Stone 1966).

11
Internal Surface Area 

12 

Figure 13.-The inner bark and 

part of the sapwood of this decay 

class II fallen tree have decomposed; the outer bark and 

heartwood are still intact. 

Figure 14.-A fallen Douglas-fir 

tree in decay class IV, with wood· 

separated into the characteristic 

angular blocks known as brown 

cubical rot; this decay stage 

provides superb rooting substrate 

for hemlock seedlings. 

The importance of internal surface areas is that, through such surfaces, a fallen tree 

interacts with its environment. A newly fallen tree, for example, interacts only passively 

with the surrounding forest because its interior is not accessible to plants and most 

animals. But once fungi and bacteria, which are smaller than the wood fibers, gain 

entrance, they slowly dissolve and enter the wood cells. And wood-boring beetles and 

termites chew their way through the wood fibers. Meanwhile, many other organisms, 

such as plant roots, mites, collembolans, amphibians, and small mammals, must await 

the creation of the internal spaces before they can enter. The flow of plant and animal 

populations, air, water, and nutrients between a fallen tree and its surroundings increases 

as the aging process continues. 

Surface area within a fallen tree develops through physical and biological processes. 

A tree cracks and splits when it falls and subsequently dries. Micr<;>bial decomposition 

breaks down the material in the cell wall and further weakens the wood. Wood-boring 

beetle larvae and termites tunnel through the bark and wood; this activity not only 

inoculates the substrates with microbes but also opens the tree to colonization by still 

other microbes and small invertebrates. Wood-rotting fungi produce zones of weakness, 

especially between annual rings, by causing spring wood to decay faster than summer 

wood; and plant roots that penetrate the decayed wood split it as the roots elongate 

and thicken in diameter. Because of all this internal activity, the longer a fallen tree rests 

on the forest floor, the greater the development of its internal surface area. Most internal 

surface area results from biological activity, the cumulative effects of which not only 

increase through time but also act synergistically-insect activity promotes decomposition through microbial activity that encourages the establishment and rooting of plants. 

Most splitting of the sound wood found in decay classes 1-111 in fallen trees is radial as 

the wood dries. Later, the weakening of annual rings by the more rapid decomposition 

of spring wood than summer wood leads to circumferential cracking as well. The blocky 

structure in class IV and V fallen trees results from brown cubical rot (fig. 14). 
.. 

Temperature 

Moisture 

Gross environmental features regulate temperature regimes. Latitude, elevation, aspect, 

and vegetation not only set the maximum and minimum temperature of a site but also 

control the seasonal range in temperature. For fallen trees, the question is twofold: 

(1) What is the relationship between the temperatures of air, tree, and soil? and (2) 

How is this relationship modified through the year by characteristics of the tree itself? 

Summer data (see footnote 1, p. 8 ) indicate the following temperature relationships at 

all elevations sampled: air> fallen tree > soil. The absolute differences were greater 

at low elevations than at midelevations or high elevations; mean winter temperatures 

of the air, a fallen tree, and the soil were nearly identical at each elevation. 

Substrate temperatures within fallen trees of a particular decay class differ little at a 

given time and site, but each decay class responds differently to the temperatures of 

the surrounding air and soil in summer. Measurements (see footnote 1, p. 8 ) in August 

showed the following relationship: air=I=II>III>IV=V>soil. These relationships 

develop because, as decay proceeds, a fallen tree more closely hugs the soil, which 

buffers it against fluctuations in air temperature. Thus, trees in decay classes I and II 

are cylindrical and contact the soil relatively little. Trees in class Ill have sloughed and 

slumped somewhat, and those in classes IV and V are partially to mostly embedded in 

the soil. 

Whole fallen tree.-Three trends are visible in the water-holding capacity of a fallen 

tree; the same is true for individual substrates--outer bark (OB), inner bark (I B), sapwood 

(SW), and heartwood (HW): 

1. Water content increases with residence time on the forest floor and with stage of 

decay (fig. 15, sapwood) because of the microbial breakdown of woody substrates that 

produces water and carbon dioxide; also, microbial activity increases as decay advances. 

Simultaneously, the water-holding capacity of woody substrates increases as cell walls 

break down. 

-

1-

z 

w 

0 

a: 

w 

Q. 

-

1-

z 

w 

1-

z 

0 

0 

a: 

350 

300 

250 

200 

150 

~ 100 

v 

~ ........ 

~~ '• SUMMER 

~ 

~ 

Figure 15.-Water content of < 

fallen Douglas-fir trees in winter 3::: 

and summer; each point representsadecayclassfrom I (newly 50 

fallen trees) to V (fallen trees 

decomposed to a mound of brown 

cubical rotted wood). 

13 
2. In decay classes I through III, the mean water content of fallen trees is nearly equal

in summer and winter; but in decay classes IV and V, trees hold more water in winter

than in summer. There are two reasons: (1) Winter usually provides abundant water,

so the complete storage capacity of the woody substrates is used; and (2) evaportranspiration potential is normally low because days are short, temperatures are cool, and

humidity is high.

3. As the overall water-holding capacity of woody substrates increases with stage of

decay, so does the seasonal fluctuation of the water content (fig. 15).

Substrates. -Four trends are visible in the water-holding capacity of the substrates

within a fallen tree:

1. When a tree falls (decay class I), the water content of the substrates is:

IB>SW>OB>HW. Initially, the inner bark and sapwood (both physiologically active

(alive) at the time the tree fell) contain more water than do the outer bark and heartwood

(both physiologically inactive (dead) at the time the tree fell). Nearly all of the living cells

are located in the inner bark and sapwood of a live tree.

2. The water content of all woody substrates increases during the residence time of a

tree and its stage of decay. Again, as with the whole tree, breakdown of woody substrates

by microbial activity produces water and carbon dioxide, and microbial activity increases

as decay proceeds.

3. The water content of substrates increases at different rates within and among

seasons. In winter, for example, water content increases (percent per year):

IB>SW>HW>OB. The water retention capacity depends on a given substrate’s stage

of decay; the more advanced the decay, the more porous the wood and the more water

it retains. In summer, however, water-holding capacity shifts in response to exposure

of the substrate to evapotranspiration. A fallen tree dries because of the dramatic

increase in evapotranspiration brought about by the simultaneous increase in day length

and air temperature and decrease in rainfall and relative humidity. The inner bark, for

example, is protected from excessive drying because inner bark is sandwiched between

the thick outer bark and the relatively moist sapwood And, being the most nutrient-rich

substrate, it supports the highest microbial activity, through which additional water is

produced. Heartwood retains water because of its large volume; thus, although the

exposed outer surface of a tree may dry, the center retains moisture. Microbial activity

continues in the moister areas and produces more water. Outer bark, on the other hand,

is prone to drying not only because it is exposed to direct sunlight and wind but also

because microbial activity is low, so little water is produced. As the outer bark sloughs

off, the sapwood is no longer protected from sun and wind, so it begins to dry out. The

drying of sapwood is speeded up because sapwood is more porous than outer bark;

therefore, the ability of sapwood to retain water against the forces of evapotranspiration

decreases as porosity increases.

4. Seasonal fluctuations of water content increase as the water-holding capacity of the

woody substrate increases (fig. 15).

.
Plant Rooting Woody plants do not root in fallen trees in decay class I. A tree in decay class II may

have plants rooting in its inner bark, which decays fastest of all the substrates; these

roots would have reached the inner bark through fissures in the outer bark. By the time

a tree reaches decay class III, its inner bark is completely decomposed, and the outer

bark and sapwood are often penetrated by roots (in 50 percent of our samples).

Only about a third of the heattwood samples in decay class III trees were colonized by

roots. When a tree reaches decay class IV, however, only heartwood remains, and

nearly all our samples (87 percent) contained roots, Finally, trees in decay class V are

not only completely colonized by roots but are actually held together by them (see

footnote 1, p. 8).

Within a decay class, plant rooting differs among substrates for two reasons:

(1) Substrates decay at different rates, and (2) substrates are not equally accessible to

plant roots. In general, as decay proceeds, plant rooting increases in all substrates

because they become excellent rooting medium-density and hardness decrease, and

water content increases.

Insect Galleries and Frass Wood-boring insects, such as beetles, carpenter ants, and termites, tunnel within fallen

(Excrement) Douglas-fir trees and consume the woody tissues. Their activities are evidenced by their

galleries and tunnels often packed with borings and feces. The collective activity of such

wood-boring insects appears to be the most important factor in initiating early

decomposition of fallen trees-decay classes I-III-for three major reasons.

First, wood is broken down by consumption and digestion by insects and by enzymatic

attack by microbes. Insects, however, can penetrate the wood matrix faster than

microbes can. Second, consumption and digestion of wood also fragment it. As a fallen

tree is fragmented by insect tunnels and galleries, its internal volume is opened to

decomposer plants and animals. The wood matrix is reduced to particles (borings and

feces), and more surface area is availalbe for microbial attack (Crossley 1976). Third,

wood-boring insects serve as vectors for decomposer micro-organisms, such as

intestinal inhabitants that are expelled in the feces (Breznak 1982). They are also

introduced as external associates; fungal spores, for instance, are transported in special

pits and cavities in adult beetles (Francke-Grosmann 1967). Fungal spores and hyphae

are also picked up and carried by insect larvae. In addition, each species of insect that

tunnels in Douglas-fir bark or wood has its attendant predators, parasites, and

scavengers (Deyrup 1981).

Frequency of galleries by decay class of tree.-Studies in the H. J. Andrews

Experimental Forest in western Oregon showed that the percentage of wood samples

with galleries increased as decomposition of the fallen trees proceeded (see footnote

1, p. 8). When the substrates within a decay class were compared for galleries, they

ranked as follows: IB>SW>HW. Outer bark was not included in this comparison because

the nature of the galleries differed. Insects simply chew through, not within, the outer

bark to gain access to the nutritious inner bark and sapwood Inner bark disappeared

before the frequency of galleries reached 100 percent. But all the sapwood samples

had galleries by the time they reached decay class III and the heartwood, by decay

class V.

Effects of galleries on wood properties.-Samples of a substrate with insect galleries

in a fallen tree were softer and wetter and had a lower density, more plant roots, higher

microbial activity, higher nitrogen fixation activity, higher microarthropod populations,

and higher exchangeable and mineralizable nitrogen than did samples from the same

substrate without insect galleries (see footnote 1, p. 8).

15
The Unseen World of Decaying trees comprise considerable accumulations of mass and nutrients in

the Fallen Tree unmanaged, old-growth forest ecosystems (Triska and Cromack 1980). Some of the

largest accumulations occur in the unmanaged forest stands of the Pacific Northwest.

Coarse woody debris can range from 130 to 276 tons per acre (118 to 251 t/ha) in

stands from 100 to more than 1,000 years old (Franklin and Waring 1980). Although

here we are concerned with Douglas-fir, neither decaying wood nor research data are

unique to forests of the Pacific Northwest. McFee and Stone (1966) observed that

decaying wood persisted for more than 100 years in New York, and Falinski (1978)

noted substantial accumulations of coarse woody debris in an old-growth forest in

Poland. Appreciable accumulations of fallen trees also occur in the temperate forests

of the Andes of central Chile, in the course of long-term forest succession (Veblen

1979). These observations evidence the long-term continuity of decaying trees as

structural components in forest ecosystems.

The decomposing wood of a fallen tree serves as a savings account of nutrients and

organic material in the forest soil. Nitrogen, for example, accumulates in decaying trees

and branch wood in forests of the Pacific Northwest and other regions (Graham and

Cromack 1982, Lambert and others 1980, Lang and Forman 1978, MacMillan 1981,

Sollins 1982, Swift 1977b). In old-growth Douglas-fir forests, about as much nitrogen

accumulates in decaying, fallen trees as in the forest floor. Other nutrients, such as

calcium and magnesium, also accumulate in decomposing woody substrates (Cromack

and others 1979, Grier 1978, Rennerfelt and Tamm 1962, Sollins and McCorison 1981,

Sollins and others 1980).

Nitrogen fixation by free-living bacteria is another mechanism of nitrogen accrual

(Cornaby and Waide 1973, Larsen and others 1978, Sharp 1975, Sharp and Millbank

1973, Silvester and others 1982, Triska and others 1982). Although nitrogen fixation in

wood is modest compared with that occurring in other substrates in forests, the

persistence of decaying wood allows small increments of nitrogen to accrue over many

decades.

Further, decomposing wood undergoes changes in other chemical constituents and pH,

as well as in physical structure (Aho 1976, Aho and others 1979, Johannson and

Theander 1974, Shortle and Cowling 1978, Tatter and others 1971). Very old, decayed

wood can even become somewhat humified and leave a long-lasting substrate resistant

to further decay.

Fallen trees harbor a myriad of organisms, from bacteria and actinomycetes to higher

fungi. Of these, only some of the fungi might be noticed by the casual observer as

mushrooms or bracket fungi (fig. 16). These structures, however, are merely the fruiting

bodies produced by mold colonies within the log. Many fungi fruit within the fallen tree,

so they are seen only when the tree is torn apart (fig. 17). Even when a fallen tree is

torn apart, only a fraction of the fungi present are noticed because the fruiting bodies

of most appear only for a small portion of the year. The smaller organisms, not visible

to the unaided eye, are still important components of the system.

16
.l 

Microhabitats and 

Inhabitants 

Figure 16.-Mushrooms of 

lnocybe lanuginosa (Bull.: Fr.) 

Kumm, a mycorrhizal fungus that 

fruits only with roots growing in 

rotten wood. 

Figure 17.-Ceratiomyxa 

fruticu/osa (Mill.) Macbr., a tiny 

fungus that typically fruits in 

cracks within brown cubical rotted 

wood. 

1o.5mm 1 

Fallen trees offer multitudes of both external and internal habitats that change and yet 

persist through the decades. One needs an understanding of the synergistic effects of 

constant small changes within a persistent large structure to appreciate the dynamics 

of a fallen tree and its function in the forest ecosystem. 

Within fallen trees.-The most obvious changes in fallen trees over time are described 

in the classification system shown in table 3. The class I tree, mostly intact and 

undecayed, offers habitat and substrate to only a few primary invaders: bark or 

wood-boring insects, a few fungi, and bacteria. The entry of these organisms opens the 

tree to profound changes that render it amenable to many additional tenants. The 

decayed sapwood enclosed by intact bark in the class II tree provides a soft, fragmented, 

moist environment much of the year. Many organisms inhabiting the class II tree lose 

out as decay progresses to class Ill, when the bark and decayed sapwood slough off. 

Meanwhile, the heartwood becomes increasingly decayed, but the process is much 

slower than in the sapwood so the class IV stage lasts a long time. 

It is in the class IV stage that the fallen tree presents the most diversified habitat and 

hence supports the greatest array of inhabitants. The decayed heartwood is relatively 

stable, so plants that become established on it have time to grow substantial root 

systems. Decayed heartwood splits into chunks; roots grow down the resulting cracks 

as well as along insect channels. Invertebrates-from minute mites to centipedes, 

millipedes, slugs, and snails-find shelter in these openings and passage along them 

(fig. 18). Vertebrates, such as salamanders, shrews, shrew-moles, and voles, find cover 

under debris of sloughed bark and rotten wood alongside the class IV tree; they also 

find the rotten wood on the underside of the tree crumbly enough for digging tunnels 

or burrows (fig. 19). Fungi and other micro-organisms abound on the wood itself as well 

as on the new substrates offered by the feces of animals. 

Gradually, the class IV tree breaks down into increasingly smaller crumbles of rotten 

wood to become class V. The intricate labyrinth of cracks, tunnels, and chambers 

collapses into a mulch like mound that is excellent rooting material (fig. 20). It is, however, 

too homogeneous and unstructured to provide good animal habitat. Because it is now 

composed primarily of residual materials highly resistant to decay, it does not support 

as diverse a community of micro-organisms as it did in the class IV stage. 

17 
18 

Figure 18.-A millipede (Har· 

paphe haydeniana) in a molting 

chamber made of organic debris 

under the loose bark of a class Ill 

fallen Douglas-fir tree. 

Figure 19.-Ciouded salamander, 

an inhabitant of class Ill fallen 

Douglas-fir trees in forest openings (photo courtesy of Greg 

Courtney). 

Figure 20.-Two decay class V 

fallen Douglas-firs in the foreg· 

round are decomposed into 

mounds covered with highlighted 

Oregon oxalis; the background 

fallen tree is in decay class IV. 

Ultimately, the entire tree is incorporated into the soil. It has gone full circle, having 

been formed as a product of soil and of photosynthetically captured carbon, it now 

returns to the soil through release of carbon by decomposition. 

Checklists of plants and animals.-There are few checklists of either plants or animals 

that inhabit fallen Douglas-fir in the Pacific Northwest. No checklist of the micro-organisms 

in fallen trees of western old-growth forests is available; the subject has hardly been 

studied. (Higher fungi have been cataloged for many kinds of rotten wood in Europe: 

Kreisel (1961), Pirk and TOxen (1957), Ricek (1967, 1968), Runge (1975).) Lawton 

( 1971) listed the mosses that occur on rotten wood or stumps in the Pacific Northwest. 

Deyrup (1975, 1976) has done a thorough job with the insects and has identified about 

300 species associated with fallen Douglas-fir. The only published checklist for 

vertebrates that use fallen trees is for northeastern Oregon (Maser and others 1979). 
Nutrient Cycling As carryover structural components of old-growth forests, large snags and large fallen

trees contribute to the internal cycling of nutrients. Such snags and fallen trees also

interact with hydrological cycles and serve as habitat for numerous micro-organisms

and higher plants and animals. For example, in one portion of an old-growth Sitka spruce

forest in the Olympic National Park, fallen trees covered about 12 percent of the forest

floor. A similar proportion of surface area interactions occurs in many old-growth

Douglas-fir forests, which resemble some stands in the Olympic National Park

(Christensen 1977, Elton 1966, Franklin and Waring 1980, Franklin and others 1981,

Grier 1978, Triska and Cromack 1980). Fallen trees in various stages of decay covered

21 percent of the floor of an old-growth Douglas-fir stand in the Cascade Range in

western Oregon (Fogel and others 1973).

Fallen trees interact with nutrient cycling processes in a forest through such mechanisms

as litter fall (freshly fallen or slightly decomposed plant material from the canopy),

throughfall (rain or dew that picks up nutrients as it falls through the canopy), nitrogen

fixation, and nutrient uptake by plants associated with the fallen trees. Litter fall and

throughfall are major pathways for the flow of nutrients and energy within forests (Carlisle

and others 1966, Rodin and Bazilevich 1967, Sollins and others 1980); they contribute

nutrients and water to rotten wood. The larger a fallen tree, the more litter it accumulates

on its surface and the more nutrient-rich moisture it intercepts from the canopy. The

moisture gathers nutrients as it passes through the accumulated litter and soaks into

the fallen tree. In addition, a snag may accumulate moisture-carried nutrients and have

a higher nutrient capital when it falls than does a live tree.

Ground contact by fallen trees creates opportunities for various interactions with the

bioticcomponentsof soil and litter. Fungi, for instance, translocate nutrients within the

soil-litter system, as both decomposers and root symbionts. Fungi also immobilize

translocated nutrients and thereby enrich the decomposing wood substrates they inhabit.

In addition, the colonization of decomposing fallen trees by nitrogen-fixing bacteria

permits additional nitrogen accretion within the decaying wood (Cornaby and Waide

1973; Dowding 1976, 1981; Larsen and others 1978; Melin and Nilsson 1950, 1952;

Silvester and others 1982; Sollins and others 1981; Swift 1977a 1977b).

Colonization of decomposing wood by animals helps microbes to enter interior surfaces

of the wood and creates additional openings for entry of water and nutrients; and

penetration of the wood by roots of trees, such as western hemlock, facilitates entry by

mycorrhizal fungi (fig. 21). Western hemlocks colonize rotten wood over many decades

(figs. 9,10, and 14) to insure long-term interactions by root zone processes. Decaying

wood thus serves as a savings account of soil organic materials and nutrients in forest

ecosystems (Christy and others 1982, Graham and Cromack 1982, Kuhnelt 1961,

Lambert and others 1980, Swift and others 1979).

Interactions-Nutrients,

Plants, and Animals

The continuum of a fallen tree is composed of, and driven by, an increasingly complex

network of simultaneously developing minisystems-all interdependent. These

minisystems are: (1) animal-plant-nutrient, (2) plant-nutrient, (3) plant-plant, (4)

animal-plant, (5) animal-animal, and (6) nutrient-plant-animal-nutrient. The cumulative

effect of these systems is far greater than the sum of their parts. Ausmus (1977) stated

the impact simply: ". . . wood decomposition represents a long-term stabilizing force

within the forest ecosystem.”

19
Bark beetle 

20 

Figure 21.-small roots and 

mycorrhizal feeder root tips of 

western hemlock in rotten wood. 

The following simplified scenarios illustrate known facets of each minisystem, and each 

internal minisystem builds on the preceding one. They are continually being added to, 

subtracted from, and variously interwoven (Kimmey and Furniss 1943). Some grossly 

simplified activity levels might be: 

1. Animals that eat the fallen tree, which opens it to the outside and initiates nutrient 

cycling: wood-boring beetles, carpenter ants, termites, and wood-tunneling mites. 

2. Micro-organisms that live on the fallen tree: bacteria, yeasts, and ambrosia fungi. 

3. Animals that eat live micro-organisms that grow on the fallen tree: collembolans, 

mites, and ambrosia beetles. 

4. Live plants, such as western hemlock or huckleberry, that form a specific symbiotic 

(mycorrhizal) association with other live plants (particular fungi), both of which are 

partially or completely rooted in the fallen tree. 

5. Animals that depend on the fungal portion of the mycorrhizal association for their 

food supply: mites, beetles, and the C_alifornia red-backed vole. 

6. Animals that eat live animals: mites, spiders, pseudoscorpions, centipedes, and 

salamanders. 

7. Animals that eat detritus (dead plant and animal material and animal feces): 

earthworms, mites, millipedes, isopods, and earwigs. 

Trees fall on the forest floor in various stages of decomposition, from sound windthrown 

trees to snags to stubs of old snags toppled because they are so badly decayed that 

they can no longer support themselves. Each is unique when it begins its new "life" as 

a fallen tree. Some of the decay organisms living in standing trees in an old-growth 

forest may continue their activity after a tree falls. Others, especially those that were in 

the tops of the standing trees, soon die out after the tree falls and are replaced by 

ground inhabitants. In the following discussions, we describe the events and organisms 

in live trees that were windthrown. Fallen snags or stubs are recruited at more advanced 

stages of decay. 

Animal-plant-nutrient.- Tree bark is such an effective protective barrier that the first 

minisystem to develop in a new, class I fallen tree is the animal-plant-nutrient minisystem. 

For example, a bark beetle (Scolytidae) (an animal) chews through the bark and thus 

connects the outside world with the inside of the tree. As the beetle enters and begins 

to use the tree, it not only introduces fungal spores but also initiates the nutrient cycle 

with its first deposit of bodily waste. 
~~-

Bark beetle in egg gallery 

r 

) 

Flatheaded wood borer 

The character of the available food is decidedly varied in the different parts of a newly 

fallen tree. Proteins are concentrated in the living tissues (phloem-the inner bark and 

cambium); carbohydrates, on the other hand, are concentrated in the dead woody tissue 

(xylem). In addition, the living inner bark and cambium are more easily digested than 

is the sapwood; but the moist sapwood is more digestible than the drier heartwood. So 

each portion of a fallen tree supports a characteristic group of insects adapted to a 

specific set of circumstances. Further, the numbers of a given species are regulated by 

the availability (quantity and quality) of their food supply. Since the inner bark and 

cambium furnish the most nutritious food, this microhabitat is promptly occupied. The 

area of next importance is the sapwood, then the heartwood, and finally the bark 

(Anderson 1960, Graham 1925). 

The insects occupying the live inner bark and cambium must be able to chew or bore 

their way through the dead outer bark and are called borers. There are two groups: 

(1) those that spend all their developmental or immature stages in this substrate and 

(2) those that spend only part of their developmental stages in this substrate (Anderson 

1960, Graham 1925). 

The first group is typified by a bark beetle. For example, the Douglas-fir beetle breeds 

in both live trees and class I fallen trees that result from windthrow. Because these 

beetles depend on the fresh, green tissues of the inner bark and cambium, they must 

develop rapidly before their perishable habitat and food supply dry out or become 

chemically and physically altered by other organisms. 

A female Douglas-fir beetle attacks class I wood in early spring by chewing through the 

outer bark. When she reaches the inner bark and cambium, she chews an egg gallery 

that is usually 2 feet (60 em) or more in length along the axis of the tree. The egg gallery 

is about one-quarter of an inch (6 mm) wide and has small "grooves" on alternate sides; 

10 to 36 eggs are laid together in the grooves. When the eggs hatch, the larvae chew 

lateral feeding galleries through their food supply-the phloem. As the larvae grow, the 

feeding galleries increase in size. Larvae pack their feeding galleries with borings (refuse) 

and frass as they eat. The larvae dine throughout the spring, summer, and autumn. In 

autumn, a larva creates a pupal cell at the end of its feeding gallery where it overwinters 

as a mature larva. Adults overwinter also and begin to emerge in April. The cycle, from 

egg to sexually mature adult, requires about 1 year, and one generation is produced 

annually (Anderson 1960, Chamberlin 1949, Furniss and Carolin 1977, Furniss and Orr 

1970, Graham and Knight 1965). 

The second group is typified by some members of such wood-boring beetle families as 

Buprestidae (flatheaded or metallic wood borers) and Cerambycidae (roundheaded 

wood borers). Some flatheaded wood borers, for example, develop in the inner bark 

and cambium, but others feed there only for a time and then enter the sapwood. Still 

others go directly into sapwood and heartwood. 

21 
22 

Figure 22.- The golden buprestid 

prefers the sapwood and 

heartwood of class I fallen Douglas-fir trees. 

A good example of the family Buprestidae is the golden buprestid (fig. 22). The golden 

buprestid feeds briefly in the cambium but prefers the sapwood and heartwood of class 

I wood. It will, however, inhabit partially to completely seasoned wood. 

A female deposits her eggs in flat masses wedged in crevices in the bark or in cracks 

in exposed wood. The young hatch and immediately start boring into the wood. Their 

oval tunnels increase in size as the larvae grow. The tunnels range in length from 3 to 

15 feet (0.91 to 4.6 m) from where the eggs hatch to where the larvae mature. As they 

feed and grow, the larvae pack their tunnels with borings and frass. The larvae mature 

in 2 or more years, construct pupal cells near the surface of the fallen tree, overwinter, 

and emerge in spring as adults. Before laying eggs, the newly emerged adults feed on 

needles of Douglas-fir, then find a class I fallen tree and start the cycle again. 

Although the normal life cycle of the golden buprestid from egg to mature adult is usually 

2 to 4 years, the length of the cycle is influenced by the quality of a larva's habitat. In 

poor quality habitat, the life cycle may take a decade or two (Anderson 1960, Chamberlin 

1949, Furniss and Carotin 1977). 

Within the family Scolytidae is a group of bark beetles called ambrosia beetles that live 

primarily in the sapwood of class I trees. They are unique among the wood-boring 

insects in that they do not eat the wood, so the borings are cast out of the tunnels where 

they collect on the surface of the bark or wood as light-colored powder. Adult beetles, 

depending on the species, construct a variety of tunnels as follows: (1) an open cavity; 

(2) a long, winding, branched or unbranched cylindrical gallery in which larvae move 

about freely; and (3) a compound tunnel, in which "larval cradles" or small pockets are 

chewed, at right angles, along the main channel (Barras and Perry 1975, FranckeGrosmann 1967, Furniss and Carol in 1977). Ambrosia beetles, especially the females, 

store certain fungi in specialized structures called mycangia. The fungi, called ambrosia 

fungi, are introduced into the beetles' galleries during the beetles' burrowing. Particular 

species of fungi are host specific to certain species of beetles. As the fungi grow, they 

are eaten by both the adults and the larvae. The beetles' requirements, however, are 

very exacting; if moisture conditions in the galleries are unsuitable, the crop of fungi 

either fails and the beetles starve or the crop explodes and the beetles smother in their 

own food. Hence, the stage of decomposition of a fallen tree is critical (Furniss and 

Carotin 1977). 
Ponderous borer

The ponderous borer (family Cerambycidae), the largest western cerambycid, directly

penetrates the sapwood and heartwood. Adults emerge during the summer. After they

mate, a female deposits her eggs in crevices in the bark or in cracks in exposed wood

of class I fallen trees. The eggs hatch and the larvae chew their way into the sapwood

and then deep into the heartwood of the tree. They grow to nearly 3 inches (7.5 cm) in

length by the time they are mature; one generation requires 3 to 7 years to mature. The

oval, meandering galleries of mature larvae are large (1 to 2 inches (2.5-5.0 cm) in

cross section) and are filled with frass and refuse. When they are mature, larvae construct

pupal chambers at the ends of their feeding tunnels and pupate. They emerge as adult

beetles in summer, mate, lay their eggs, and die (Anderson 1960, Chamberlin 1949,

Furniss and Carolin 1977).

Another penetrator of fallen trees is the carpenter ant. The abundance of dead wood

in an old-growth forest provides numerous sites for queen carpenter ants to establish

new colonies, but as these sites deteriorate, the queens die, and the weaker colonies

subsequently become decadent and also die. Although fallen trees, stumps, and the

bases of snags serve as initial nesting sites, they are not permanent. “Permanent”

nesting sites are provided by living trees. The reproductive success of carpenter ant

colonies is related to the age of the colonies, and large (old-growth) trees provide

permanent nesting sites. When a nesting site finally deteriorates, a strong ant colony

can move to another site and withstand catastrophes that would eliminate a weak colony

(Sanders 1970).

In brief, the carpenter ant’s life history is as follows: the young males and females leave

the nest in early spring and, being winged at that time, disperse in all directions. The

air is sometimes filled with these flying ants that emerge simultaneously from many

nests in their mating flight (Graham and Knight 1965).

The males die shortly after mating, but the young, mated females may go into old,

established colonies to replace decrepit queens or each may establish a new colony.

In the latter case, a young queen seeks a small cavity (for example, in a fallen tree)

and constructs her brood cell by completely enclosing the cavity, leaving neither exit

nor entrance. In this sense, members of the genus Camponotus differ from ants of other

genera in that a queen works alone in founding a colony (Graham and Knight 1965).

Once the brood cell is complete, the young queen breaks off her wings; they are no

longer of use. And after sealing herself in, she does not feed again until her first brood

of young is mature. She lays a few eggs that hatch in about 10 days. The newly hatched

larvae are fed a secretion from the queen’s salivary glands. The larvae complete their

development, spin their cocoons, pupate, and emerge as adults about 30 days after

hatching. The only food the first brood has during their development is the material from

the queen’s salivary glands. Although this first brood of workers is small, they take over

the work of the nest as soon as they mature (Furniss and Carolin 1977, Graham and

Knight 1965).

The workers cut approximately parallel, concentric galleries that run longitudinally

through the wood, primarily where it is soft from decay. These galleries are continually

increased to accommodate the enlarging colony. The wood is not eaten, as it is by

beetles, but is cast out through openings cut to the outside, sometimes called windows.

Excavation of the galleries causes piles of wood fiber or sawdust to accumulate below

the access holes. Food secured by the workers is also brought into the colony though

these openings (Furniss and Carolin 1977, Graham and Knight 1965).

23
The workers feed the queen, care for the eggs she lays and for the larvae that hatch

from the eggs, and feed the larvae secretions from their mouths. The “nurses” continually

move both larvae and pupae to the most favorable places within a colony. When young

adults are fully developed, the nurses assist them from their cocoons and treat them

with the “greatest consideration” (Graham and Knight 1965).

At home in shady, wet places, carpenter ants are catholic in diet. Their menu includes

animal food, such as caterpillars of certain butterflies, and sweets, such as honeydew

secreted by aphids. Carpenter ants have even been known to shelter aphid eggs in

their nests during winter and to carry them out in the spring and place them on plants

to develop (Furniss and Carolin 1977). With this exchange of materials, the ants establish

a purposeful, physical link between the inside and the outside of a fallen tree.

By the time a fallen tree reaches late class II, it has a relatively high moisture content,

but the wood is still sound. At this stage of decomposition, it is suitable for habitation

.a

..’

4

by the Pacificdampwood termite, which sooner or later colonizes most large fallen trees

in the coastal forest (Furniss and Carolin 1977).

Colonies of Pacific dampwood termites contain reproductively active individuals (primary

.‘4 reproductives), soldiers (that are sterile), and nymphs; but they do not have workers.

-.’ \

The winged, reproductively active individuals, the most familiar form, sally forth on warm

i

evenings in August, September, and October to mate and form new colonies. Tevis

(1953) aptly described the termites’ appearance: ". . . on a particular quiet, sultry

Termite afternoon . . . shortly before sundown the forest became alive with the wing-rattling

of termites streaming from slit-like holes in damp logs and taking to flight.”

Typically, colonies are formed by swarms of primary reproductives. Such individuals

appear in established colonies and accumulate for a short period before departing on

their nuptial, colonizing flights. During the predeparture period, the primary reproductives

are sexually immature. Their sexual maturity culminates as they take wing, in about

equal numbers of males and females. Termites are weak fliers and seldom travel far

without the assistance of a breeze. They are also vulnerable to predators, such as bats.

The survivors, on completing their flights, cast their wings. When a survivor encounters

an individual of the opposite sex that has also cast its wings, the pair walks in

tandem-the male following the female-as they seek a fallen tree in which to excavate

a small nuptial chamber and copulate (Imms 1960, Whitaker and others 1977).

Individuals produced during the early stages of colony development are sterile; winged,

reproductive individuals develop when a colony is about 4 years old and contains 400

to 500 individuals. Individual termites are long lived (several years), an important factor

in permitting a close relationship between successive generations and an important

element in the termite’s social organization (Imms 1960).

Pacific dampwood termites feed on the rotting wood in which they live. They even appear

to be attracted to a specific fallen tree when it is ready for their habitation; its readiness

depends on the presence of certain acids and aldehydes produced by particular

tree-inhabiting fungi. These substances initially attract termites to a fallen tree and then

attract them within the tree to the wood that is appropriate for consumption (Becker 1971).

24
A termite can digest the wood it eats because of a mutually beneficial, three-way

relationship between the individual termite, cellulose-digesting Protozoa, and

nitrogen-fixing bacteria that live in its gut. The Protozoadigest the cellulose in the wood

and convert it to a form that the termite can use as food. In turn, the termite’s body

provides an anaerobic chamber that is replete with food for the Protozoa. Wood particles

eaten by the termite pass into the chamber and are engulfed by the Protozoa that

ferment the cellulose. Major products of fermentation are carbon dioxide, hydrogen, and

acetic acid. The acetic acid is absorbed through the wall of the termite’s hindgut and is

oxidized as energy (Breznak 1975).

Nitrogen is important in the termite’s diet, especially because it is required by the

cellulose-digesting Protozoa. Wood, the primary ingredient of the diet, is low in nitrogen,

although termites consume mostly wood colonized by fungi that probably supply vitamins

and some nitrogen (Esenther and Kirk 1974). Whenever the nitrogen intake by a termite

is deficient, the nitrogen-fixing bacteria in a termite’s gut make up the difference. The

nitrogen-fixing activity varies inversely with the amount of nitrogen contained in a

termite’s diet. The nitrogen-fixing response of the bacteria to changes in dietary nitrogen

is rapid. The nitrogen-fixing system is efficient and is potentially capable of quickly

supplying a termite with usable nitrogen should its dietary level suddenly drop

(Breznak 1975).

The Pacific dampwood termite lives in a series of galleries, eaten in the wood, without

any external sign of a colony. Even so, by the time a colony has run its course, it has

greatly altered the interior microhabitats of a fallen tree. The abandoned galleries form

a major network of passages used by other animals and through which plant roots find

easy access and ready nutrition.

Some mites also eat the well-decomposed class IV wood (Hartenstein 1962, Luxton

1972). A particular group of organisms in fallen trees, immature wood-tunneling mites,

preferentially feeds on the fecal material of such organisms as wood borers. These

immature mites probably consume wood that has already been chewed and partially

digested until they develop an adult’s capability of chewing and digesting wood first-hand

and can excavate their own tunnels (Wallwork 1967, 1976).

Plant-nutrient.-The succession of plants on fallen trees is mediated by changes in

nutrient availability and physical properties over time. Three broad phases can be

defined: initial, optimal, and final (Kreisel 1961; Ricek 1967, 1968). Early invaders

prepare the tree for later colonization by altering its physical and chemical properties

during the initial phase. The altered tree provides the best substrate for a wide array of

organisms during the optimal phase. Ultimately, the depletion of nutrients and physical

deterioration of the wood during the optimal phase diminish its value for many organisms,

so fewer species inhabit the final phase.

25
The initial phase encompasses class I and early class II fallen trees, those with bark

intact and heartwood still sound. The earliest use of a newly recruited tree is likely to

be by heart rot or butt rot fungi and associated micro-organisms that inhabited it before

it fell. Other organisms quickly enter where its interior is exposed at breaks or splits in

the bark. When beetles chew through the bark, they create additional ports of entry.

The beetles also carry spores of decomposition fungi that thrive in the beetle galleries,

which are excellent incubators filled with nourishment for the fungus and protected by

the bark from drying and from temperature extremes. These earliest invaders are

opportunistic scavengers-fungi and bacteria that join the insects in exploiting the readily

available carbohydrates of cambium, phloem, and sapwood of a recently fallen trunk.

As the most easily extracted nutrients are depleted, these early invaders sporulate. The

spores are then carried off by the emerging new generation of insects reared from eggs

laid after the tree fell.

Meanwhile, free-living, nitrogen-fixing bacteria may be active at low levels in the wood

to add to the nitrogen pool available for growth of the wood decay fungi (Aho and others

1974, Larsen and others 1978, Silvester and others 1982). Other nitrogen is added by

rain falling through the old-growth canopy, which is rich in lichens that fix nitrogen (Sollins

and others 1980); and much of the nitrogen extracted by initial decomposers is still

present in their dead cells or in frass, ready to be recycled.

Later in this initial successional phase, the early scavengers are succeeded by fungi

more competent in decomposing cellulose and lignin in the sapwood This process

moves relatively fast, as long as the bark hangs together on the fallen tree to preserve

the incubator environment within. Furrows in the bark on the upper side fill with leaf

litter and provide sites for several years for seeds to germinate. Where the bark is intact,

seedlings generally die during summer drought. If a seedling’s roots find a crack or hole

in the bark and grow into the decomposed layer between bark and wood, however, it

may find enough moisture to survive the summer. Western hemlock, huckleberry, and

salal commonly become established this way on fallen trees, as do spruces (Christy

and others 1982, Eichrodt 1969, Gobl 1968, Minore 1972, Mork 1927). Seedling top

growth is generally poor at this successional phase because nutrients are limiting and

the roots may not have contacted the mycorrhizal fungi needed for nutrient acquisition

(see “Plant-plant”). Root growth, however, may be extensive but it is trapped in the zone

of decomposition between bark and heartwood.

A transition from the initial stage to the optimal stage of plant succession on a fallen

tree occurs in late class II and class III stages; sloughing of bark is a major event in

this transition. When the bark sloughs off, it may take with it some of the plants that

have grown through it to form roots in the decaying sapwood Plants that remain will

have much of their root systems exposed to air and sun. As sapwood deteriorates and

sloughs off, additional plants are removed with it.

As the tree enters class IV (most bark and sapwood removed), it also enters the optimal

stage of plant succession. The heartwood is much decayed, usually as brown cubical

rot. Wood thus rotted becomes spongy and tends to separate into angular chunks, and

the intervening cracks provide interior surfaces for the fruiting of fungi and for the growth

of roots. Insect tunnels provide additional passages for root growth. Mineral content of

the fallen tree at this stage may exceed the original content because minerals have

been added by litter fall from the canopy and by throughfall of rain, have been brought

I

J

26
in by animals, or have been translocated from underlying soil by fungi or roots. Nitrogen

may be added by similar means and by biological fixation (Larsen and others 1978,

1982; Silvester and others 1982). These circumstances provide an excellent rooting

medium for plants. A great variety of fungi, both decomposers and symbionts, thrive in

the complex of niches within the fallen tree. Lichens, mosses, and liverworts become

established on the surface and stabilize it after the sapwood sloughs off. Hemlocks and

other plants become established on the upper surface of class IV trees, but their roots

grow through it into underlying soil as well as along its length. Nutrients may be acquired

from the tree itself or from the soil.

The optimal stage may last for many decades, but in time the fallen tree enters the final,

more depauperate stage of class V. It loses structure and shape and becomes

increasingly low and covered with forest floor material that buries the lichens, mosses,

and liverworts. Roots of overstory trees and of trees that became established in earlier

stages now permeate it and bind it together. New plants seldom become established

because of the humus cover and the intense competition of roots already there. Relatively

few decomposers remain because only the lignins, the most resistant to decay, are left.

Mycorrhizal fungi seem to predominate and are the primary fruiters. The moisture content

of such material remains high compared with that of soil during summer drought, and

roots and fungi can grow actively in it long after most roots in nearby, dry soil have

become dormant or desiccated (Harvey and others 1978, 1979a, 1979b; Place 1950).

Plant-plant.- The earliest interactions between plants in the newly recruited, class I

fallen tree are between micro-organisms, the initially invading scavengers. Competition

for the readily available carbon, nitrogen, and minerals is simple: The first to arrive gets

the most. More subtle interactions may also develop between specific organisms; for

example, some fungi produce antibiotics that suppress certain bacteria, or volatiles,

such as ethylene, that may inhibit some organisms and stimulate others (Graham and

Linderman 1980, Marx 1973). Free-living, nitrogen-fixing bacteria may provide some of

the nitrogen required by wood-rotting fungi (Aho and others 1974).

As fallen trees progress from decay class I to class II, the scavengers are replaced by

competitors with the enzyme systems needed to decompose the more complex

compounds in wood. The fungi involved in this activity are often mutually antagonistic,

so that a given part of the tree may be occupied by only one fungus that excludes others

by physical or chemical means (Li 1981, Nelson 1967).

The penetration of plant roots through openings in the bark of an advanced class II tree

introduces new possibilities for fungal or microbial colonization because the roots offer

carbon and nitrogen compounds quite different from those of the wood. Some of these

compounds may be secreted from the roots to selectively promote growth of certain

micro-organisms (Slankis and others 1964, Smith 1969). Some fungi are capable of

penetrating feeder roots to tap the nutrients within. Fungi that damage or kill roots in

the process are pathogens. Certain fungi, however, grow on and in plant feeder roots

as symbionts that not only acquire photosynthetic products from the host root but also

provide nutrients, vitamins, and growth regulators to the host. These symbiotic,

fungus-root structures (mycorrhizae) are extremely important in colonization of rotten

wood by higher plants (fig. 21).

27
Most plants that grow on rotten wood in old-growth Douglas-fir forests require

mycorrhizae to survive; without the mycorrhizal fungi, the host plant cannot acquire

adequate nutrients. Western hemlock, spruces, and the Ericaceae are all dependent

on mycorrhizae. Hemlock seedlings may survive a year or more without mycorrhizae,

but growth is minimal until the mycorrhizal fungi colonize the roots (Christy and others

1982). The fungi must first be introduced into the fallen tree as spores or resting cells;

they are transported there by insects, small mammals (Maser and others 1981), wind,

rain, or a combination thereof. In a class II tree, the spores need to get through the

same openings in the bark used by the plants. They are most likely to be moved through

the openings by water or as “hitchhikers” on insects or other arthropods.

Some plants on rotten wood form mycorrhizae when the fungi are available but can

also grow without them. Many ferns appear to be in this category (Boullard 1979). A

few other plant families, such as the sedge and rush families, rarely form mycorrhizae.

None of these are as abundant as the mycorrhizal plants on rotten wood in Douglas-fir

forests, perhaps because they are less competitive for the nutrients in rotten wood:

Many of the nutrients are in organically bound forms that can be cracked only through

action of enzymes not produced by green plants. Mycorrhizal fungi, in contrast, are able

to decompose some of these compounds and to translocate the released nutrients to

the host (Todd 1979).

As a fallen tree progresses through decay class III, the diversity of plants and associated

mycorrhizal fungi may increase, especially if roots grow down through the rotted sapwood

into the underlying soil. Sloughing of bark, however, suddenly exposes roots in decayed

sapwood to freezing in winter and drying in summer, and many of the fine roots die. As

the decayed sapwood sloughs off, plants rooted in it may slough off with it. Even if their

roots penetrate partially rotted heartwood underneath, they fall over and often die.

The optimal stage of plant development is attained in the class IV tree; plant roots and

mycorrhizae grow through cracks and channels of the brown cubical rotted heartwood.

Numbers of species of mycorrhizal and decomposer fungi increase, as does the potential

variety of interactions among the fungi and between the fungi and higher plants. The

variety of mycorrhizal fungi further increases as lichens and mosses become established

on the upper surface of the fallen tree (Gobl 1968). Moss communities capture and

accumulate many nutrients from rain, and these nutrients can be cycled into higher

plants by the mycorrhizal fungi (Binkley and Graham 1981, Nadkarni 1981). At the same

time, fungi and roots that bridge rotten wood to underlying soil may transfer soil nutrients

to the fallen tree. Asfungal cells or fine roots within the wood die, the minerals acquired

from the soil add to the nutrient capital of the tree. These tissues can then be decomposed

and their nutrients recycled into further growth of wood-inhabiting organisms.

The class IV tree gradually becomes well permeated with roots as it decays into class

V. The well-rotted heartwood provides a moist substrate in which mycorrhizal fungi and

roots can continue to interact into the summer, well beyond the time that fungi and fine

roots have become dormant or dead from drought in the upper layers of mineral soil

(Harvey and others 1978, 1979a; Place 1950).

28
7

i

Relatively few species of mycorrhizal fungi seem able to grow in rotten wood compared

with the number that grow in soil (Gobl 1968, Kropp and Trappe 1982) perhaps because

most mycorrhizal fungi do not produce the enzymes needed to decompose the tannins

and other phenolic compounds in wood (Giltrap 1982). Some of the mycorrhizal fungi

that inhabit both mineral soil and rotten wood develop much more strongly in the wood

than in the soil (Trappe 1962, 1965) and some appear to be restricted to rotten wood

(Kropp and Trappe 1982). Most mycorrhizal fungi in rotten wood produce mushrooms

or truffles as their spore-bearing, reproductive bodies, but they can do this only if they

are in symbiotic association with host roots. This particular result of a fungus-plant

interaction occurs mostly in class IV to V trees and is an important feature of the

animal-plant interactions discussed in the following sections.

Animal-plant.- The introduction of fungal spores into a recently windthrown, class I

tree by bark beetles and wood borers initiates a sequence of complex animal-plant

interactions that continues until the tree disappears altogether. Once established and

growing under the bark, the fungal colonies are grazed by a variety of invertebrates

(McMillen 1976, Petersen 1971, Russell 1979). Many decomposer fungi sporulate within

fallen trees and depend on insects for dispersal of spores. Thus, the young bark beetles,

emerging from under the bark where they began life as eggs, take with them spores of

the associated fungi. Similar relationships between insects and fungi probably occur

through all decay stages of fallen trees. Insects eat the fungi and disperse the spores.

Many of the fungal grazers are food for predators, so the animal-plant interactions are

a prelude to animal-animal interactions.

As the fallen tree progresses through decay classes II and III, the zone of decomposition

beneath the bark provides shelter to additional invertebrates and substrate for roots and

mycorrhizae. The mycorrhizal fungi are thus introduced to the system as an additional

food source to grazers, such as nematodes (Riffle 1971) aphids (Zak 1965) and

collembolans (Warnock and others 1982). Slippage of the bark, and eventually of

decayed sapwood removes that favorable environment and the organisms within it from

the top and sides of the tree; that material, however, does not disappear. Most of it

accumulates loosely alongside the log to provide a new habitat favorable to many of

the same organisms as before, plus larger animals, such as slugs, snails, salamanders,

and small mammals.

Various mites, insects, slugs, and snails feed on the higher plants that become

established on rotten wood. These plants also provide cover for the animals, as do the

lichens, mosses, and liverworts that colonize fallen trees in decay class IV. Wood-boring

beetles, termites, and carpenter ants produce channels in the heartwood that provide

passageways for roots. The fruiting bodies of the mycorrhizal fungi, produced from

energy supplied by the host plant, can also be a major source of food for insects (Fogel

and Peck 1975); other arthropods (Buller 1922); and small mammals, such as the

California red-backed vole (Fogel and Trappe 1978; Maser and others 1978a 1978b;

Ure and Maser 1982).

29
I

:

Collembolan

I Oribatid mite I

Specific groups of animals exemplify these generalities. Collembolans are tiny, wingless

insects, most of which jump by means of a springlike appendage on the abdomenhence, the common name “springtail.” Collembolans living in the soil can be divided

into two groups: (1) those that live in the surface layer and feed on fungi, algae, lichens,

and decaying vegetation; and (2) those that live deeper in the soil and graze on

micro-organisms (Edwards and others 1970, Furniss and Carolin 1977, Zachariae 1962).

Both groups probably also exist in fallen trees. Those that feed on fungi, algae, lichens,

and detritis could inhabit the protected areas in the exterior bark of a class I tree as

their food supply begins to accumulate. As the tree’s interior is made available, first by

inner bark feeders and then by wood borers, collembolans gain a new dimension in

habitats and niches. By the time a decomposing tree reaches decay class III, insect

tunnels have penetrated all levels and the entire tree is “open” so collembolans can live

deep within it and graze on such micro-organisms as bacteria.

Because collembolans occupy essentially any atmospheric habitat, they are extremely

versatile. The greatest number and variety of species are found in the soil or in contact

with it, especially where organic materials abound. Most species are susceptible to

desiccation; but in damp environments, species that normally would be restricted to the

soil or to litter may occur on the surface of the ground or even in standing trees

(Christiansen and Bellinger 1980).

Development of collembolans is not necessarily continuous; growth is interrupted in

cold seasons. Development of eggs and immature stages may also be interrupted by

dry conditions. Dry periods are survived by a form of diapause, a suspension of growth

or development. In a few genera, diapause is even associated with a regressive

modification of the mouthparts and digestive tract, and sometimes also with striking

external modifications (Christiansen and Bellinger 1980).

Collembolans molt throughout life, from 2 times to over 50 times. Growth apparently

continues throughout an individual’s life, but at a decreasing rate after sexual maturity.

Sexual activity of adults appears to be restricted to alternate instars (Christiansen and

Bellinger 1980).

As with collembolans, the mite fauna diversifies within a fallen tree over time. In fact,

the mite fauna in rotten wood may be surprisingly complex. As stated by Krantz (1978,

p. 57), “The remarkable diversity in morphology . . . is more than equalled by the variety

of behavioral characteristics . . .Specializations in habitat often are paralleled by

specializations in structure.”

Mites can be divided initially into two groups-free living and parasitic. The free-living

forms exhibit a variety of food habits.

Small-plant eaters include mites that feed on bacteria, algae, fungi, and lichens. Such

mites occur in the soil and the litter of the forest floor, and they can inhabit the outer

bark of a class I fallen tree as it becomes suitable. As dead plant and animal materials

accumulate on a fallen tree, mites (which relish putrid material) become part of the

community. The strategy of feeding on dead organisms has been even further refined

by the mites; some feed on the decaying material of higher plants (large-plant eaters),

whereas others are considered to be general plant eaters, consuming fungi, bacteria,

and algae, as well as decaying material from higher plants. The general plant eaters

often depend on fungi to soften, decay, and prepare the plant tissues. As organic

materials increase, an array of fungi becomes established, and mites that prefer fungi

for food appear on the scene (Krantz 1978, Luxton 1972).

30
Mites immigrate into a fallen tree as other organisms, such as wood-boring beetles,

penetrate it and modify its habitability. As a fallen tree is colonized by bacteria, fungi,

and yeasts, mites that dine on these organisms also take up residence. As these

organisms live and die, mites that feed on dead tissues join the community and add to

its diversity. Our observations suggest that the mite fauna really begins to flourish as a

fallen tree approaches class IV. Finally, as the fauna increases and diversifies, the fecal

output generates still another level in the partitioning of food resources available to the

mite community.

Even in death, mites and other invertebrates interact with plants. Their empty

exoskeletons serve as incubators for the spores formed by certain mycorrhizal fungi

(Rabitin and Rhodes 1982).

A larger inhabitant of fallen trees is the common earwig. The earwig is mainly nocturnal

and spends the day in cracks and crevices and under the bark of large fallen trees, as

well as in other places. Although reputed to feed on tender foliage and dead or decaying

vegetable matter, it also devours both living and dead insects (Borror and Delong 1964,

Imms 1960, Langston and Powell 1975).

Unlike most insects, the common earwig cares for its eggs and young. After depositing

a group of eggs in the soil in winter or early spring, the female rests over them much

as a hen does her chicks. The newly hatched earwigs remain under and around their

parent until they are able to fend for themselves. Common earwigs become adults in

summer, and having wings and being capable of flight, they can disperse. There appears

to be one generation per year (Imms 1960, Langston and Powell 1975).

Common earwig Although some common earwigs may be primary tenants of large fallen trees, others

may be secondary or part-time tenants. Nevertheless, they are an important part of the

food chain, as will be seen in the next section.

The California red-backed vole is the quintessential example of interactions within fallen

trees (fig. 23). The vole heavily uses them for cover; we have found that most class II

through class V old-growth Douglas-fir trees have vole tunnels underneath. The vole

eats mostly fungi and includes lichens but prefers truffles (Ure and Maser 1982). Some

truffles, such as Rhizopogon vinicolor fruit mostly in rotten wood, where their mycelium

forms the tuberculate mycorrhizae of conifers (Trappe 1965, Zak 1971) (figs. 24 and

25). The truffle spores are dispersed by animals that eat the truffle (Trappe and Maser

1976). Thus, there is a tight cycle of interdependence: the vole needs the truffle for

food; the truffle depends on the vole for dispersal of spores and on a mycorrhizal tree

host for energy; the tree requires mycorrhizal fungi for uptake of nutrients and provides

the rotten wood needed by the vole for cover. Moreover, since both voles and

Rhizopogon vivicolor specialize in rotten wood as habitat, the vole disperses the

Rhizopogon spores to the kind of substrate in which the fungus thrives.

3 1
Red bellied checkered beetle 

32 

Figure 24.-Fruiting bodies of 

Rhizopogon vlnicolor A. H. Smith: 

left and right, surface views; 

center, cross sections. This 

fungus typically forms mycorrhizae 

with Douglas-fir roots in rotten 

wood. 

Figure 23.-The California redbacked vole depends on decayed 

fallen trees as habitat (photo, 

courtesy of D. Ure). 

Figure 25.-Mycorrhizae of 

Douglas-fir formed with the fungus 

Rhizopogon vinicolor. 

Animal-animai.-As a class I fallen tree is penetrated by the wood-boring beetles and 

they begin to thrive within it, Nature's system of checks and balances is also activated. 

At first this system is composed primarily of predaceous beetles in the families Cleridae 

(checkered beetles) and Trogositidae (no common name). The redbellied checkered 

beetle, for example, is an important predator of the bark beetle in Douglas-fir trees 

(Cowan and Nagel 1965). Adult red bellied checkered beetles prey on adult bark beetles, 

and their larvae prey on the larvae of bark beetles. There is one generation of redbellied 

checkered beetles annually in Oregon (Furniss and Carolin 1977). 

As the diversity of fauna within a fallen tree increases, so do the number and variety 

of predators. Among the smallest predators are the predaceous mites. Predaceous 

mites are common near the surface of the soil and in mosses, humus, rotten wood, and 

animal waste products. They prey on small arthropods, such as collembolans, on 

arthropod eggs, on small roundworms, and occasionally on each other. Predaceous 

mites are commonly long legged and fast, and they have strong mouthparts for capturing 

and chewing their prey (Krantz 1978, Luxton 1972). 

The next level of predation may be pseudoscorpions. These little creatures are related 

to spiders. They look like miniature scorpions with pinchers but without tails and stingers. 
Pseudoscorpions move in a slow walk, but they can climb and can walk upside down

on the ceilings of small caverns within rotten wood; they move backward much faster

than forward. They construct silken nests in which they rest. A pseudoscorpion not

occupied in spinning silk often sits in its nest with its pinchers (technically called

pedipalps) and the anterior of its body protruding through the opening. From this position,

it can grab a passing collembolan or mite to eat. Some species of pseudoscorpions

seem to prefer small flies, small beetles and their larvae, ants, and even small

earthworms (Comstock 1948, Weygoldt 1969). The pseudoscorpion keeps its nest clean,

discarding prey remains as far away from its nest as possible without actually leaving

the nest. An individual may leave its nest in search of food or a mate; when it cannot

find its way back, it makes a new nest or occupies an empty one (Weygoldt 1969).

L

Pseudoscorpion

A female pseudoscorpion carries its eggs and developing embryos in a brood sac

attached at its genital opening. The eggs are small, with little yolk, so the embryos are

nourished by a nutritive fluid produced by the female. Most species reproduce in spring

or summer and may have several broods per season (Weygoldt 1969).

Folding-door spider The next order of predaceous magnitude is probably spiders, of which the Pacific

folding-door spider is the largest spider inhabiting fallen trees. In general, spiders

undergo little metamorphosis or change during development; when hatched, they look

like miniature adults. Legs lost during development are usually regenerated. Although

spider eggs hatch soon after they are laid, spiderlings that hatch in the autumn remain

in the brood sac until the next spring. All spiders are predaceous; they eat mainly insects.

A spider normally kills its prey by injecting poison into the captured prey with its “fangs”

(Borror and DeLong 1964, Comstock 1948).

The folding-door spider’s abode is a tube constructed in an existing crack within the

outer layer of a class III or IV fallen tree with many cracks and crevices. The horizontal

tube is completely lined with silk. To close its tube, a spider grasps the rim on opposite

sides and pulls it in toward the middle. Except when capturing prey, a female seldom

leaves her tube, but a male wanders in search of a mate (Levi and others 1968).

From our observations, Pacific folding-door spiders prey on whatever they can catch

and subdue. Although we have occasionally found evidence of food refuse around the

entrance of a spider’s tube, it was usually scattered as we dissected a fallen tree and

was not identifiable.

Another group of predators is the centipedes, of which Scolopocryptops sexspinosa

(Say) (no common name) is the largest found in fallen Douglas-firs. Centipedes have

one pair of legs per segment. Centipedes overwinter as adults in a protected place,

such as within rotten wood. They lay their eggs in the spring and early summer. Some

species have sticky eggs that a female hides with debris, but a female S. sexspinosa

usually coils around her eggs to protect them and may periodically lick them to keep

them clean (Borror, and DeLong 1964, Levi and others 1968).

Centipedes are predaceous. They feed on spiders, insects, and other small animals.

All centipedes have poison jaws with which they paralyze their prey. Some species,

such as S. sexspinosa, have such stong poison jaws that they are effective even against

such predators as birds and large insects (Borror and DeLong 1964, Levi and others

1968, Maser and Hooven 1974). Centipedes figure prominently as predators in trees in

decay classes III, IV, and V.

33
As the bark becomes loose on a late class II fallen tree, lungless salamanders (family

Plethodontidae) join the internal community. Three species of salamanders are

associated, as predators, with rotten wood in western Oregon: Oregon slender

salamander, Oregon salamander, and clouded salamander.

Oregon slender salamanders are endemic to the northern half of the Cascade Range

in western Oregon (Stebbins 1966). They are most often associated with trees in decay

classes III to V, either under intact bark or in termite channels deep within (Stebbins

1954). Females lay about 8 to 11 eggs in June. Large, moist, cool fallen trees are

important to the Oregon slender salamander in the heat of summer because they, and

other species of slender salamanders, seem to be particularly prone to fatality from heat

stress (Maiorana 1977). They may actually need a suitable fallen tree for only a few

weeks in summer, but without it during that time they could die.

Although we know of no specific data on the food habits of the Oregon slender

salamander, a similar species that occurs in extreme southwestern Oregon and in

northwestern California, the California slender salamander, will serve as an example.

Both species are about the same size and inhabit forested areas. The major items in

the diet of California slender salamanders are collembolans and mites, followed by such

groups as flies, spiders, and small snails (Bury and Martin 1973). Evidence suggests

that the California slender salamander even selects collembolans (specifically family

Sminthuridae) and mites (specifically family Oribatidae) over other types of prey

(Maiorana 1978). These small, slender salamanders are well suited to a role of predator

within the narrow confines of wood-boring beetle and termite galleries in classes III

through V fallen trees.

Another salamander, the Oregon salamander, although thought to primarily inhabit

rodent burrows in forested areas (Stebbins 1954), is frequently found under pieces of

bark that sloughed off large, fallen Douglas-firs. It is also found within classes III through

V trees, particularly during cold or dry weather (Stebbins 1966). Individuals are usually

solitary except when they are breeding or are associated with young. Egg clutches,

averaging 11 or 12 eggs, are often deposited under the bark or within fallen, rotting

Douglas-firs. One female with a clutch of 12 eggs was found on June 30, 1982, in a

very wet class IV Douglas-fir in an old-growth Douglas-fir stand on Mary’s Peak, Benton

County, Oregon.

The two most important foods of the Oregon salamander are collembolans and spiders,

followed by isopods (sowbugs), millipedes, and adult beetles (Bury and Martin 1973).

The third salamander is the clouded salamander (fig. 19). It frequents rotten wood,

particularly Douglas-fir in late classes II through IV (Stebbins 1966). These salamanders

are often found under the loose bark of large fallen trees in spaces excavated by

wood-eating insects (Fitch 1936). In fact, young clouded salamanders show a striking

affinity for bark (McKenzie and Storm 1970). According to Stebbins (1954) they are

especially abundant in well-illuminated openings in a forest. In addition, the clouded

salamander is the most arboreal member of the genus and has been found 20 feet

(6.1 m) up in standing trees (Stebbins 1954).

Eggs laid in late spring or early summer under bark and in cavities in rotten wood are

guarded by the female. The eggs may be attached separately by their stalks but close

together, or they may have their stalks twisted around one another and be attached to

a common point on the ceiling or wall of the nesting chamber (Stebbins 1954).
I lsopod or sowbug 1

‘:

,%”

:‘,

j/

/

/

Snout beetle

Bury and Martin (1973) and Storm and Aller (1947) listed ants as the most important

food of clouded salamanders. We also found ants to be an important food item, but

only in summer.2 The major food of the adult clouded salamanders in winter, spring,

and fall were isopods and beetles, particularly snout beetles. Isopods, ants, beetles,

and common earwigs were important in summer, when foods were eaten in greater

diversity. Important foods for small juvenile salamanders (as large as three-fourths inch

(19 mm) in snout-vent length) were mites, collembolans, flies, and very small beetles.

Largerjuveniles (more than three-fourths inch (20 mm) in snout-vent length) consumed,

in order of importance, flies, isopods, beetles, mites, and centipedes in winter; beetles,

ants, and isopods in spring; ants and beetles in summer; and isopods, beetles, and

ants in fall (see footnote 2). Storm and Aller (1947) also found termites, probably the

Pacific dampwood termite, in the stomachs of some individuals, which indicated that

the salamanders had been feeding within a class III to class IV fallen tree.

The final level of predation within large, rotten, fallen Douglas-firs in classes III through

V is probably that of small mammals, such as shrews and shrew-moles.

Shrews are small, with short legs, tiny eyes, and long, pointed noses. Although they

cannot see well, their senses of touch, smell, and hearing are acute. Thecommon shrew

in western Oregon Douglas-fir forests is the Trowbridge shrew. This small, “nervous”

mammal is abundant around fallen trees, particularly classes III and IV, that are well

settled on the forest floor and have been in place long enough to act as the shrew’s

grocery. The Trowbridge shrew has the most catholic diet of all western Oregon shrews.

It eats at least 47 types of food, the most important of which are centipedes, spiders,

internal organs of invertebrates (probably mostly beetles), slugs, and snails. In addition,

it shows a definite affinity for fallen trees, as do some of its prey (Maser and others

1981, Terry 1981, Whitaker and Maser 1976).

The American shrew-mole is a tiny mole. As are other moles’ ears, the shrew-mole’s

ears are merely holes near the shoulders and are not visible because of the dense fur.

It has minute eyes nearly concealed by fur, and broad front feet with stout claws adapted

for digging (Maser and others 1981).

These small moles spend much time burrowing in the surface soil-litter layer and along

and under classes II and III fallen trees. When trees reach classes IV and V, shrew-moles

also burrow within them. Their close tie with fallen trees in old-growth forests is probably

reflected in their diet (Maser and others 1981, Terry 1981); for example, in a study by

Whitaker and others (1979), the three foods eaten with the highest frequency were

earthworms (81.8 percent), centipedes (54.5 percent), and flies (36.4 percent).

The shrew-mole is ideally equipped to forage in and around fallen trees because its

nose is extremely sensitive to touch; it is much like a blindman’s cane. In almost constant

motion, it quickly identifies any object it contacts. Further, this mole’s small size,

adaptions for digging, and herculean strength make it an efficient, burrowing predator

within and beneath rotten wood (Maser and others 1981).

2

J. O. Whitaker, Jr., and C. Maser. Unpublished data on file at

Indiana State University, Department of Life Sciences, Terre Haute,

Indiana 47809.

35
Nutrient-plant-animal-nutrient.- A tree begins life with nutrients from the soil and ends

life with nutrients that diffuse into the forest floor and become parts of other trees. It is

appropriate, therefore, to select an element (nitrogen) that is necessary for the growth

and reproduction of all organisms within a forest and to use it to represent full cycle the

minisystems within a fallen tree.

As a tree grows, nitrogen is incorporated into the wood-cell structure. After it dies,

processes of decomposition begin to break down the structure of the wood and ultimately

to recycle the nitrogen.

During decomposition, micro-organisms (such as fungi and bacteria) incorporate

nitrogen from the wood into their own cellular structures as they digest the carbon from

the wood substrates. As the microbes decompose wood, the carbon-to-nitrogen ratio

gradually shifts until nitrogen becomes relatively more available for plant uptake. As

microbial colonization and decay increase, animals varying in size from small mites and

collembolans to large beetle larvae feed on microbial tissues enriched in nitrogen from

digestion of wood; and vertebrates, from California red-backed voles to black-tailed

deer, may obtain some of their protein nitrogen from decaying trees by feeding on fungal

fruiting bodies, such as truffles and mushrooms (Fogel and Trappe 1978, Maser and

others 1978a, Trappe and Maser 1978).

As decomposition proceeds in a fallen tree, other sources of nitrogen are added in the

form of leaf litter and other litter components that fall on the surface of the tree. As these

decay, their nitrogen becomes available to enrich the wood; and as rainwater-rich in

nutrients from passing through the tree canopy and surface litter-accumulates on the

fallen tree’s upper surface, some nitrogen is leached into the wood.

Microbes that colonize a fallen tree from the ground surface provide additional nitrogen,

as do animals that take up residence in the tree or eliminate their metabolic waste

products on its upper surface. As decay proceeds, plant roots penetrate the fallen tree’s

surface and tap the rotting wood as a source of available nitrogen, other nutrients,

and water.

A fallen tree disappears gradually through the decades, and its nitrogen capital is

returned to the forest. Weathering processes, such as freezing and thawing, and animal

activities contribute to the disintegration and disappearance of the tree. Some residue

may remain for centuries in the forest as it slowly becomes incorporated into the organic

portion of the soil (fig. 20).

Biotic Succession

Within and Around

Fallen Trees

As a fallen tree decomposes, it creates a gradually changing myriad of internal and

external habitats (Maser and others 1979). Plant and animal communities within a fallen

tree are very different from those outside, but both progress through a series of orderly

changes. As a fallen tree decomposes, its internal structure becomes simpler, whereas

the structure of the plant community surrounding the fallen tree becomes more complex

(figs. 26 and 27).

36
EXTERNAL SUCCESSION 

GRASS-FORB/ 

SHAUB·SEEDLING 

INTERNAL SUCCESSION 

DECAY CLASS I 

tj 

POLE-SAPLING 

DECAY CLASS II 

- ~ 

Figure 27.-Fallen trees progress 

through two simultaneous successional processes-internal and 

external (modified from Maser and 

others 1979). 

...b. , 

YOUNG FOREST 

OECAVCLASS Ill 

...-. 

1----' 

Figure 26.-Piant and animal 

communities are very different 

inside this class Ill Douglas-fir 

than they are outside. 

I MATURE FOREST OLD GROWTH 

DECAY CLASS IV DECAY CLASSV 

~ 

l__....w--- f--" . -·· ...... -· 

Internal succession in a fallen tree is related to the following factors: (1 )the tree species 

and its inherent decay-resistant chemical properties; (2) its size-the larger it is, the 

longer it lasts; (3) what killed the tree; (4) whether it originated as a tree or a snag; 

(5) the microclimate around it; (6) its placement on the ground; and (7) the biotic 

community peculiar to it. 

Internal succession is also influenced by temperature, moisture, and stage of decay. A 

class I fallen tree, for example, has many readily available nutrients that support 

opportunistic colonizers. As decay proceeds, its moisture-holding capacity increases, 

but nutrients become less available because either they have been used or they remain 

locked in the more decay-resistant compounds of the wood. Ultimately, the rapidly 

growing opportunists are succeeded by organisms with more sophisticated enzyme 

systems, and decay continues. 

37 
External succession is related to the changes that take place in the plant community

surrounding a fallen tree. A fallen tree is a connector between the successional stages

of a community; it provides continuity of habitat from the previous forest through

subsequent successional stages. A large fallen tree therefore provides a physical link-a

nutrient savings account-through time and across successional stages. Because of its

persistence, a fallen tree provides a long-term, stable structure on which some animal

(both invertebrate and vertebrate) populations appear to depend for survival.

External succession is influenced by the same factors as internal succession, with the

additional influence of light. Consider, for instance, a class IV or V tree that supports a

lush community of mosses, liverworts, hemlocks, and other flowering plants. As the

canopy closes over the opening created by the original falling of the tree, light becomes

limiting to the growth of green plants. If, at this point, a nearby tree falls, the environment

can change immediately and strikingly. Greater solar radiation increases the amount of

light but may also raise the daytime temperature of the fallen tree. In turn, nighttime

temperature may be lower because of the increased heat that reradiates to the

atmosphere. In addition, more rain and snow reach the ground.

Stream Characteristics Fallen trees and other large pieces of wood significantly shape the energy flow, nutrient

and Fallen Trees dynamics, and structure of biota in streams of the Douglas-fir region (fig. 28). Streams

in old-growth forests contain large quantities of organic debris: 220 to 770 tons per acre

(200 to 700 t/ha). Large, organic debris shapes a stream channel by damming it, which

creates ponds that trap sediments, or by obstructing it, which redirects waterflow that

creates meanders and pools (Anderson and others 1978, Franklin and others 1981,

Naiman and Sedell 1979, Sedell and others 1982b, Swanson and others 1976, Triska

and Cromack 1980, Triska and others 1982).

Large pieces of wood in streams provide a diversity of spawning and rearing habitats

for salmonids. In the smallest (first order) streams, for example, over 50 percent of the

habitat is related to presence of large wood (fig. 29) and about 25 percent is created

and maintained by wood in larger (third order) streams (Anderson and Sedell 1979,

Franklin and others 1981, Swanson and Lienkaemper 1978, Triska and others 1982).

Most large wood is randomly spaced in small streams (first and second order) because

flow volume is insufficient to transport large trees downstream. Intermediate streams

(third to fifth order) have less wood. Big chunkstypically accumulate where the channel

is obstructed by immobile dead trees or channel constrictions; such obstructions provide

sites for collection of small to intermediate size debris that moves downstream at high

flows. Most debris comes to rest on the flood plains or on the outsides of bends of larger

streams or rivers (sixth to eighth order). But even in big rivers, historical records show

that large pieces of wood contribute significantly to in-channel structures that trap

sediment, pond water, and create side channels and sloughs (Franklin and others 1981,

Naiman and Sedell 1979, Sedell and Luchessa 1982, Swanson and others 1978,

Swanson and Lienkaemper 1978).

The food base or energy supply of a stream in an old-growth Douglas-fir forest is primarily

litter from the adjacent forest combined with algae produced in reaches of streams

exposed to light. Pristine streams retain much of the forest litter. Fallen trees in a stream

form “stairsteps” that allow over 70 percent of the litter to be retained long enough to

be biologically processed by stream organisms. Similar relationships exist for pristine

streams in the Canadian boreal forest (Naiman 1982, Sedell and others 1975, Triska

and others 1982).
A Historical Sketch of 

Woody Debris in 

Northwest Waterways 

Figure 28.-Note how the fallen 

tree (foreground) channels the 

water against the rootwad (left) 

that, in tum, buffers the streambank from the current. 

Figure 29.- The stable, fallen tree 

forms an obstruction that dissipates some of the stream's energy 

and creates a pool used by 

salmon ids. 

The influence of the forest as a source of energy and as a channel structure diminishes 

as a stream gets larger. Edges of an unmanaged stream, however, are dominated by 

forest vegetation, and fallen trees create and maintain side channels and small backwater 

areas that are prime sites for deposition of organic materials and rearing of coho salmon 

(Sed ell and others 1982a). Further, old-growth forests typically have a mix of herbaceous 

and shrubby plants and areas of various sizes exposed to sunlight; this combination 

provides a stream with a mix of coniferous and deciduous leaf litter as well as patches 

of algae. Such diversity of habitat and energy provides an interactive aquatic system 

with a stable, rich mix of both invertebrate and vertebrate species. 

Most early descriptions of Northwest streams and rivers are recorded in British and 

United States army journals. They tell, for example, of valleys so wet that early travel 

was along the edges of the hills and not along the valley bottoms (Dicken and Dicken 

1979). The Tualatin Valley was described in British army journals as "mostly water 

connected by swamps" (Ogden 1961 , p. 122). Much of this flooding was a result of 

beaver activity and accumulated sediment, fallen trees, and living vegetation in the 

channels. Because the valley bottoms had accumulated fine silt and organic material 

of alluvial origin, the land was fertile, and the task of draining it for farming began early 

in Oregon and Washington. 

Oregon State Agricultural College soils scientist, I. A. Williams (1914, p. 13), wrote of 

the condition of Willamette Valley streams in 191 0: 

Many of the smaller streams ... through these flat sections of the valley flow 

sluggishly and frequently overflow their banks during periods of heavy winter 

rainfall. ... Most of these have sufficient grade to carry even more water 

than ordinarily comes to them; seldom less than 3, and usually more, feet of 

fall per mile. The annual overflow is caused from the obstructing of the channel 

by the growth of trees and the extension of their roots, the dams thrown 

across the channels by beavers and the consequent accumulation of 

sediment and other debris, etc .... It is a common condition, however, and 

usually all that is necessary is a clearing out and opening up of the clogged 

channel of the stream to afford entire relief ... to the farmer .... 

39 
Descriptions of streams in the Puget Sound lowland and the Willamette Valley were

similar. Most consisted of a network of sloughs, islands, beaver ponds, and driftwood

dams with no main channel. The Skagit River lowlands encompass about 198 square

miles (512 km2), of which over 50 square miles (128 km2) were marsh, sloughs, and

wet grass meadows. U.S. Army Corps of Engineer maps for 1875-91 for the lower

Nooksack and Snohomish Rivers in Washington show large areas of sloughs, swamps,

and grass marshes (Reports of the Secretary of War 1875-99). All the coastal Oregon

valleys contained marshy areas and numerous sloughs. The interaction of the streams

and their flood plains in the lowlands of both States was great before they were cleared

and channeled by pioneer farmers (U.S. Congress, House 1848).

The channels of fast, turbulent rivers and low-gradient rivers, regardless of alluvial or

bedrock control, were influenced by large amounts of wood. The lower Siuslaw River

and lower North Fork Siuslaw River were so filled with fallen trees that trappers were

unable to explore much of these river systems in 1826 (Ogden 1961). The Willamette

River flowed in five separate channels between Corvallis and Eugene in 1870.

Reports of the Secretary of War (1875-99) state that the “obstacles were . . . great above

Corvallis” and the riverbanks were heavily timbered for half a mile (0.8 km) on either

side. Over 5,500 drifted, dead trees were pulled from a 50-mile (80-km) stretch of river

in a 10-year period. The trees ranged from 5 to 9 feet (1.5 to 2.8 m) in diameter and

from 90 to 120 feet (27.7 to 37 m) in length. The river was also confined to one channel

by engineering activities. In both Oregon and Washington, other rivers were completely

blocked by driftwood in the lower, main channels. The Skagit River, for example, had

a driftwood jam that was three-fourths of a mile (1.2 km) long and one-fourth of a mile

(0.4 km) wide. The Stillaguamish River was closed by six driftwood jams from the head

of tidewater to river mile 17 (km 27.2). Drifted, dead trees were so numerous, large,

and deeply imbedded in the bottom that a steam “snag boat” had to operate for 6 months

to open a channel 100 feet (30 m) wide.

Driftwood jams in high-gradient river systems were often located where the channel

gradient abruptly decreased. Morse (1883, p. 9) described the South Fork

Nooksack River:

. . . we came to a place where the river, during freshets had ground sluiced

all the earth away from the roots of the trees, and down some 6 feet to the

gravel. This covered a region of country a mile in width by five in length.

Overgrown yellow fir timber had once covered most of that section. If the

river below there was only clear of jams that place would be a paradise of

hand loggers. On the gravel lay many million feet of sound fir timber, which

only needed to be junked up during the summer and the winter freshets

would float the logs down to the sea. Immediately below this place, the jams

first extend clear across the river, and for the next 20 miles there is a jam

across the river nearly every mile.

From the above scenario, it is obvious that large, woody debris was an important factor

in the early river systems of the Pacific Northwest (figs. 30 and 31). Human objectives,

other than watershed management, however, dictated clearing the rivers; so present

practices downplay the ecological role of large, woody debris in modern river systems.

40
Vegetation of Streamsides 

and Gravel Bars 

Figure 30.-Logs in a small 

stream awaiting enough water to 

be floated to the mill (photo from 

USDA Forest Service historical 

files). 

Figure 31.-A splash dam at 

Austin Place, Hamilton Creek, 

Oregon, August 16, 1907. Such 

dams were used to regulate the 

flow of water to float logs down a 

stream to the mill. (Historical 

photo, courtesy of the Homer 

Museum, Oregon State 

University.) 

Figure 32.-Note the large pieces 

of wood on the flood plain, especially in the lower right comer 

where the stable wood has formed 

a protected site that has allowed 

...... ..::._...._......_.....,~----' the alder to grow. 

Large, fallen trees have both positive and negative effects on live vegetation that borders 

watercourses. Trees carried by floodwaters can severely batter live plants on a flood 

plain, but this is normally restricted to a narrow belt along the immediate channel. 

Stabilized, large pieces of wood, on the other hand, provide protected sites where alder 

and other species of plants can become established (fig. 32). 

Once established, live vegetation begins to stabilize a stream channel. Such features 

as a gravel bar also become stabilized and enriched with fine sediments and organic 

materials as plant root systems develop and the stems resist the flow of water and 

reduce its velocity. 

Fallen trees protect thickets of vegetation on exposed channel bars. Alders growing in 

bordering areas not protected by down trees sustain heavy, repeated pruning by floating 

woody debris and moving bed loads (Swanson and Lienkaemper 1982). The down trees 

that protect the outer edge of a thicket and those in the thicket itself create local 

environments of quiet water where fine sediments and organic debris are deposited 

during high flows. This process, coupled with the production of leaf and woody litter by 

the stand, results in soil development and growth of the stand. The large, down trees 

thus help a stand to reach the stage of structural development that allows it to better 

withstand floods. 

41 
Fallen trees on gravel bars also provide sites where some stream-transported species

of hardwoods and shrubs can reroot and grow.

Restabilization of streams after major floods, debris torrents, or massive landslides is

accelerated by the presence of large, woody debris along and within a channel. Swanson

and Lienkaemper (1978) found that after a fire an aquatic habitat was maintained by

the large, woody debris (supplied to the stream by the prefire forest) while the postfire

forest was developing. In many instances, however, salvage logging of streamsides

destabilizes the structure of the channel and thereby the quality of the habitat.

Decomposition and

Nutrients in Streams

Small streams that drain heavily forested watersheds depend on organic materials from

the adjacent forest as a source of carbon and nutrients for biological processes. Energy

flow and nutrient cycling are measured by calculating the budgets of carbon and nitrogen

in a stream. By examining the internal transfer of organic materials and nutrients, we

can gain significant insights on the ability of a small stream to process incoming sources

of energy. Sedell and others (1974) and Triska and others (1982) studied a small stream

in the western Cascade Range of Oregon. They found that 85 percent of the organic

material in the stream was large, fallen trees or branches from large trees. Of the tree

leaves and needles that fell into the stream during the year, only 10 to 20 percent were

transported downstream. The remainder were either stored in the channel or used as

food by microbes. Microbial respiration accounted for about 55 percent of the carbon

produced. There also was a large amount of fine detritus that was partly formed as a

byproduct of the decomposition of large pieces of wood.

Large wood was the primary structural feature of small streams, and it dominated the

carbon budget. The structural characteristics of large wood allowed leaves and needles

to be retained in a stream long enough to be used as food by microbes and invertebrates.

Without the wood, such material would be rapidly transported downstream, and the

stream system would not be as efficient in processing organic material. The ability of a

small stream to process organic material is also important because the stream provides

preconditioned food to biotic communities in larger streams and rivers.

Retention of organic material is important to a stream ecosystem in that large quantities

of stored organic material buffer the annual fluctuations in energy flow. During periods

of drought, for example, enough moisture and waterlogged material remain available in

a stream to provide habitat and food for aquatic organisms; during major floods when

large amounts of organic material are transported downstream or washed up on the

banks, enough remains to continue to provide habitat and food for aquatic organisms.

It is important to understand that carbon, nitrogen, and all the other materials that leave

a watershed either pass through or accumulate in the stream environment, which

encompasses less than 1 percent of the watershed area. Such a concentration of

nutrients, the capacity of a stream to store organic material, and the efficiency to process

it depend on the number and quality of fallen trees in the stream.

42
Figure 33.-Some of these fallen 

trees will become waterlogged; 

others will be submerged only 

during high water. Both circumstances affect the rate of 

decomposition. 

Decomposition and nutrient cycling.-Trees that fall into streams decompose at 

different rates and in different patterns than those that fall on the ground. Decomposition 

in water is slower than on land because waterlogging prevents deep diffusion of oxygen 

into the wood; the fungi and invertebrates that cause the most rapid decomposition of 

fallen trees on land are strongly aerobic (Triska and Cromack 1980). Waterlogged parts 

of fallen trees tend to decompose in thin surface layers, about one-fourth inch (0.5 em) 

thick. As the decomposed surface is grazed or abraded, oxygen can penetrate farther 

into the underlying wood, which in turn becomes substrate for the decomposers. If only 

part of a fallen tree is in constant contact with water, that part decomposes slowly but 

the exposed part may decompose quite rapidly because neither a low level of oxygen 

nor extremely high or low moisture content limits decomposer activity. Trees that fall in 

very small streams may contact water only during the rainy season when the stream is 

flowing at its highest, and lack of moisture the rest of the year may slow their 

decomposition (fig. 33). 

As decomposition of large pieces of wood advances, the concentration of essential 

nutrients, such as nitrogen and phosphorous, increases. Nitrogen increases primarily 

through biotic use of the carbon and through fixation of nitrogen. Nitrogen-fixing 

micro-organisms use both the wood and the bark of a fallen tree. Although bark decays 

more slowly than wood, the tannins in Douglas-fir bark are not effective in reducing 

nitrogen fixation (Baker and others 1983). Nitrogen fixation on fallen trees in streams 

accounts for 5 to 1 0 percent of the annual nitrogen supply to the stream (Triska and 

others 1982;. 

43 
Aquatic Invertebrates

Midge adult

Dragonfly adult

The continuum of animal associations on woody debris in aquatic systems varies from

being restricted to the wood to using it opportunistically. The sequence of colonists

parallels the stage of wood decay. New wood entering a stream is used primarily as

habitat, although some species of midges (Chironomidae) tunnel in the cambium and

phloem. The wood is then colonized by a community of algae and microbes that provides

food for a group of insects, functionally called grazers or collectors. Although this type

of feeding does not significantly affect the structure of the wood, the colonization of the

superficial layer of the wood by fungi softens it enough that it may be abraded and

ingested by invertebrates that scrape their food off surfaces. Most important, however,

the wood becomes suitable for obligate wood grazers and the more generalized wood

shredders, such as caddisflies (Trichoptera) and stoneflies (Plecopteia), that ingest the

wood infested by fungi. These activities result in a more sculptured surface texture that

in turn provides habitat for many organisms. Decades of fungal colonization and growth

soften the wood and allow oxygen to penetrate. Invertebrates that bore into the internal

matrix of a fallen tree speed the decomposition process by consuming the wood and

by transporting microbes into the tree. In the final phase of decay, detritivores-such

as annelid worms (earthworms)-penetrate the material; continued decomposition then

resembles that in soil and damp, terrestrial habitats (Anderson and others 1978, Dudley

and Anderson 1982).

In contrast to sound wood that enters a small stream as described above, some wood

is already conditioned by fungi and other terrestrial organisms before it enters the water.

Such preconditioning shortens the aquatic decomposition process by allowing more

rapid internal colonization by aquatic microbes and invertebrates. Decomposition is also

faster in larger streams during periods of high water because the physical abrasion

removes softened tissue as the wood is transported downstream or deposited on a

flood plain or the outside of a bend in the stream. Only in small headwater streams or

in backwaters of larger streams can rotting wood retain enough structural integrity to

provide a substrate for aquatic invertebrates during the final stages of decomposition.

Invertebrate use of wood substrates. -Quality and texture of wood are important in

determining the kinds of organisms that will colonize it. The species of wood, degree

to which it is waterlogged, and decay class all affect the quality. The extent of colonization

by terrestrial fungi and wood-boring insects also influences the attractiveness of the

wood once it enters the water because such activity is closely associated with decay

class. Dudley and Anderson (1982) found that about two-thirds of the obligate

wood-using organisms were generally in soft wood; one-third in grooved, textured wood;

and less than 10 percent in solid, smooth wood. In contrast, the facultative organisms

occurred as follows: 20 percent on solid, smooth wood; 60 percent on grooved wood;

and 20 percent on or in soft wood.

Aquatic invertebrates are also functionally classified by how they use wood substrates:

(1) boring or tunneling; (2) ingesting of wood by grazing, scraping, or rasping; (3) scraping

algal communities growing on wood; (4) attaching to the wood or hiding in its grooves;

and (5) preying. Classification based on function are interrelated with the texture of the

wood, which partly explains why a higher incidence of facultative organisms are

associated with smooth, firm wood and obligate organisms with soft wood. Smooth

wood surfaces are suitable for attachment and for grazing the microbial film, whereas

soft wood is more easily penetrated by borers and contains fungal mycelia as a source

of nutrients (Anderson and others 1978).

44
I Cranefly adult I

Many of the surface-associated invertebrates are more opportunistic in their selections

of feeding sites and habitat than are the internal associates. Invertebrates that use the

surface probably do so for protection from the stream current, suspended sediments,

and predation but not for obtaining food. The many grooves, crevices, and loose pieces

of bark on a well-conditioned fallen tree held the majority of invertebrates collected by

Dudley and Anderson (1982).

Borers include some families of semiterrestrial or semiaquatic beetles (Coleoptera) and

some caddisflies that hollow out twigs for cases in which they live and others that tunnel

into soft wood to pupate. Fly (Diptera) larvae, however, are among the dominant borers,

both in abundance and species richness. The depth to which a species (insect or

microbe) can penetrate the wood is probably restricted by oxygen gradients that account

for the fact that galleries are located just under the surface of the wood.

Boring activity exposes new surfaces of sound wood to microbial inoculum and

colonization. When such activity is associated with some of the obligate species of fly

larvae, a visible zone of stained wood often radiates outward from the larval galleries;

the staining is caused by fungi that may be symbionts carried by the fly larvae. Existing

galleries of terrestrial beetles increase fungal activity after the wood is submerged in

water, and they are also prime habitat for cranefly (Tipulidae) larvae.

\

i \

Mayfly adult

In essence, feeding activities of borers, grazers, scrapers, and raspers result in continual

utilization and decomposition of wood. The amount of wood ingested depends not only

on the method of feeding and the rate of consumption but also on the firmness of the

wood and the amount of fungal penetration into its superficial layer. Firm but grooved

or textured wood generally has a soft, stained layer one-eighth to three-sixteenths inch

(2 to 5 mm) deep that has been colonized by fungal mycelia. Grazers and shredders

(beetles, caddisflies, and some stoneflies) exploit this area enriched by fungi, which

contains about five times as much nitrogen as occurs in nonenriched wood.3 Scrapers

(mayflies, Ephemeroptera) and raspers (snails, Mollusca) also ingest the soft layer and

the periphyton-an assemblage of minute organisms attached to surfaces submerged

in water.

Many species associated with thoroughly decomposed wood are detritus feeders that

just happen to be in the wood instead of in some other soft, organic material, such as

leaves. The feeding and burrowing activities of the detritivores reduce the size of particles

and cause mineralization of the woody material.

Net-spinning caddisflies frequently use textured, wood surfaces to attach their nets and

to conceal themselves. Fallen trees also direct streamflow in a manner that provides

net-spinning caddisfly larvae with ideal sites, both of surface structure and water velocity,

for attaching their nets and filtering food from the water. In one study, for example,

Dudley and Anderson (1982) found densities of 120 caddisfly (Hydropsyche sp.-no

common name) larval nets per 12 square inches (100 cm2) of wood.

3

K. Cromack, Jr. Unpublished data on file at Oregon State University,

Department of Forest Science, Corvallis, Oregon 97331.

45
Stoneflv adult

Fish Habitat

Another use of wood in streams is for pupation. Many caddisfly larvae pupate on or in

wood; and several families of flies, especially crane flies, bore into soft, saturated wood

to pupate. In addition, many species of insects use partially submerged wood to crawl

out of the water so they can emerge as terrestrial adults. Stoneflies and some species

of mayflies and dragonflies exemplify taxa that seem to prefer wood to mineral substrates

for emergence as terrestrial adults.

The important role of large, woody debris in creating and maintaining spawning and

rearing habitat for fish has been recognized and documented within the last 10 years

(Swanson and Lienkaemper 1978, Swanson and others 1976). But we are only now

beginning to really appreciate the seasonal differences in function provided to salmonids

by woody debris.

Summer.-Several streams were examined in western Washington so that the

population biomass of salmonids in streams flowing through old-growth forests could

be compared with that in recentlyclearcut areas (Bisson and Sedell, in press). Although

the total salmonid biomass increased in a clearcut, the species richness declined to a

population of steelhead trout, the majority less than 1 year old. Coho salmon and

cutthroat trout, between 1 and 3 years old, were proportionately less abundant in the

clearcuts. Bisson and Sedell (in press) related the shifts in composition of species and

age groups to changes in the habitat caused by the cutting of old-growth trees and by

the removal of large, stable, woody debris from the stream channel. The amount of

stable debris declined and the amount of unstable debris increased after passage of

the 1976 Washington Forest Practices Act that mandated immediate removal of debris

after logging. Pool size appeared to decrease and riffle size to increase as a result of

clearcutting and channel clearing. The frequency (number per kilometer or mile) of both

pools and riffles appeared to decline in clearcuts, which suggested that the normal

stairstep stream profiles had been altered to a more even gradient.

Pools and backwaters are used by coho salmon and large cutthroat trout; in fact, pool

volume is directly related to coho biomass in coastal streams in Oregon. In addition,

large, stable, woody debris is preferred as protective cover by young coho salmon,

yearling steelhead, and older cutthroat trout, particularly in high-gradient river systems

(Bustard and Narver 1975a, 1975b; Everest and Meehan 1981; June 1981; Lister and

Genoe 1979; Nickelson and others 1979; Osborn 1981; Sedell and others 1982a, 1982b)

(fig. 34).

Winter. -Most species of salmonids exhibit shifts in preference between summer and

winter habitat. Large, stable, woody debris is important to the selection of winter habitat

by coho salmon, steelhead trout, and cutthroat trout. The presence of large, woody

debris enhances the use of different habitats within pools. Pools are preferred by all

species at base streamflows during the winter. The level of preference, however, is

determined by the quality of the pool and the abundance of woody debris; the more

woody debris, the greater the use of the pool. Further, large, stable debris tends to

attract fish more to pools along the edge of a stream than to pools in the middle of

a channel.
Management Impacts 

on Fallen Trees 

On Land 

Figure 34.--Large, stable, woody 

debris increases the habitat value 

of a pool for salmonids. 

Figure 35.-5almonlds use the 

stable wood on the flood plain as 

protective cover during winter 

floods. 

When winter storms cause streams to flood, the pool-riffle sequences or stairstep stream 

profile is transformed into a continuous, high-velocity torrent, and there is little protection 

for salmonids from moving bedload sediment or swift, turbulent waters. During these 

unpredictable but brief periods, slack water refuges are associated almost exclusively 

with such large, woody debris as fallen trees and with standing vegetation in riparian 

flood plains (fig. 35). All three species of salmon ids enter these debris-enhanced refuges 

until streamflows subside. Large, woody debris may also expand the feeding space by 

creating or enhancing the amount of slow-moving or standing water in organic-rich flood 

plains, even along the smallest streams (Bustard and Narver 1975a, 1975b). 

The Douglas-fir forests of the Pacific Northwest evolved with a continuing deposition of 

fallen trees to the forest floor and to streams. Fallen trees may periodically have been 

partially destroyed by fire but were ultimately replaced during development of successive 

stands (Harris and others 1982, Swanson and Lienkaemper 1978). 

What will happen to the Douglas-fir ecosystem when fallen trees are no longer added, 

as will be the case under intensive forest management with increased utilization of wood 

fiber? And what will happen under short rotation management, when large trees are no 

longer produced? 

These questions-and others that we do not yet know to ask~an be answered only 

through careful research. Until such research has been done, we only speculate. Our 

speculation in turn can be based only on what we know about the functions of fallen 

trees. These functions relate to the size, shape, placement, porosity, and cohesiveness 

of a fallen tree; its suite of functions is altered when any one of these factors is changed 

(Fellin 1980). We can thus begin to consider how stand management will affect 

each function. 

Trees in an unmanaged forest generally fall randomly, although a storm or catastrophic 

event, such as the explosion of Mount St. Helens, sometimes causes all the trees to 

fall in one direction over a substantial area. Diversity results from the common random 

placement; some fallen trees lie along a slope, others across it. Trees that fell two 

centuries ago are mixed with those that fell two decades ago, 2 years ago, and 2 days 

ago. Some fell on the ground, some across others; some are whole, some broken. The 

resultant diversity provides a myriad of habitats in both time and space. 

47 
In Streams

Machine entry in a stand reduces diversity because heavy equipment fragments and

scatters class IV and V rotten wood. Class I and II trees may be salvaged or cut for

firewood; class III trees that hang together and are not removed as salvage or firewood

tend to become bunched or aligned along the direction of skidding. Habitat diversity

declines to a fraction of what had been available (Eckholm 1975); probably fewer kinds

of organisms can thrive. Further, because woody substrates serve as long-term soil

organic material and nutrient reservoirs, increasingly intensive timber management,

coupled with shorter rotations, could significantly alter the role of decaying wood in the

nutrient cycling processes.

A forest manager can take steps to maintain the present diversity of fallen trees on a

site. Machine trails can be designed to minimize the scattering of class IV and V trees.

Wood utilization standards can be adjusted to leave a good representation of younger

classes. Some trees-both defective and sound trees for built-in diversity-an be left

to become fallen trees of the future. Where necessary, burning of slash can be designed

and timed to minimize destruction of woody residue. These are all feasible practices,

but they will cost money. Hence, the forest manager needs to know how a stand will

benefit from the fallen trees over the long run.

When fallen trees and other large pieces of wood are removed from stream channels

and riparian zones by massive debris torrents or excessive postlogging channel cleanup,

carrying capacity for salmonids in winter is reduced (Bisson and Sedell, in press;

Swanson and Lienkaemper 1978; Toews and Moore 1982). In turn, the smolt yield of

anadromous salmonids is reduced the next spring. Sedell and others (1982b) examined

the long-term history of debris management. They concluded that the systematic removal

of fallen trees and debris jams for navigation and transportation of logs in the late 19th

and early 20th centuries destroyed a major structural component of fish habitat in

intermediate to large rivers.

Enhancement or mitigation efforts designed to improve fish passage by removing debris

jams and logging slash have resulted in declines in wintering populations of salmonids

in small streams4 (Bryant 1982, Lestelle 1978). The decline in the summer rearing

capacity of a stream for coho salmon is a direct result of less large wood that provides

cover and forms pools in streams (Bisson and Sedell, in press).

4

Elliott, S. T. Ecology of rearing fish. Annual performance report,

study OIB. Alaska Department of Fish and Game; 1979; 19: 39-52.

Hall, James D.; Baker, Calvin O. Biological impacts of organic debris

in Pacific Northwest streams: Proceedings of workshop 1; 1975

September; Corvallis, OR. Oregon State University; 13 p.

48
Research Needs Recent and current research in old-growth forest is revealing much about the roles and

qualities of fallen trees. Understanding this information may allow use of fallen trees as

sensitive barometers of the “habitat health” of a stand (Harris and others 1982). To learn

how the functions of fallen trees differ in an old-growth stand, a clearcut, and a young

stand, we need to compare the characteristics and inhabitants of fallen trees in such

areas. The physical qualities of a fallen tree-moisture, temperature, nutrient content,

pH--are likely to change markedly with stand removal, regeneration, and regrowth. We

need to learn how the plants and animals change with such alterations within and around

a fallen tree. We especially need to know much more about the fallen tree-soil interface,

probably the single most important habitat niche for the survival of organisms in

drastically altered stands. To fully interpret the long-term significance of fallen trees and

the old-growth forests from which they come, we need to learn more about their

contribution to the forest ecosystem as a whole and to the quality of the soil in particular.

Research on fallen trees in streams needs to address the quality and quantity of the

woody debris that is required in riparian zones to maintain or enhance the aquatic

ecosystem. Because saturated wood decomposes slowly, it accumulates on the bottom

of a stream to become a site of nitrogen fixation and nitrogen, to form habitat, and to

help capture and hold leaf litter that in turn is a source of energy for the stream ecosystem.

How many live trees are needed (and of what diameter) to provide a future source of

fallen trees in a stream for structure, nutrient cycling, and fish habitat? Although most

fisheries biologists have focused their attention on wood in streams, of equal importance

is the large wood (quality and quantity) along channel margins and in overflow areas

because this is the winter habitat of the salmonids.

In summation, we must not sacrifice the options of future generations on the altar of

cost-effectiveness through decisions based on insufficient data. It is the professional

charge of researchers to obtain the needed data and of managers to apply it.

Epilog When thinking of and dealing with diversity in a forest, conventional vision

focuses on structure and habitat. Diversity, however, has another

dimension-one that is only now being perceived: function. The basic

components of structural and functional diversity are inseparably interwoven

in a forest. A broadened philosophical view of management-a forest versus

a commodity-is necessary if certain structurally related functions, such as

retention of water and cycling of nutrients in large, fallen trees, are to be

options in managed forests of the future.

(Maser and Trappe 1984)

49
Common and

Scientific Names

Plants:

Alder

Douglas-fir

Huckleberry

Oregon oxalis

Salal

Sitka spruce

Western hemlock

Alnus spp.

Pseudotsuga menziesii (Mirb.) Franco

Vaccinium spp.

Oxalis oregana Nutt. ex T. and G.

Gaultheria shallon Pursh

Picea sitchensis (Bong.) Carr.

Tsuga heterophylla (Raf.) Sarg.

Invertebrates:

Carpenter ant

Common earwig

Douglas-fir beetle

Golden buprestid

Pacific dampwood termite

Pacific folding-door spider

Ponderous borer

Redbellied checkered beetle

Vertebrates:

Beaver

Black-tailed deer

California red-backed vole

California slender salamander

Clouded salamander

Coho salmon

Cutthroat trout

Oregon salamander

Oregon slender salamander

Shrew-mole

Steelhead trout

Trowbridge shrew

Camponotus spp.

Forficula auricularia L.

Dendroctonus pseudotsugae Hopkins

Buprestis aurulenta L.

Zootermopsis angusticollis (Hagen)

Antrodiaetus pacificus (Simon)

frgates spiculaus (Le Conte)

Enoclerus sphegeus F.

Castor canadensis Kuhl

Odocoileus hemionus Rafinesque

Clethrionomys californicus (Merriam)

Batrachoseps attenuatus (Eschscholtz)

Aneides ferreus Cope

Oncorhynchus kisutch (Walbaum)

Salmo clarki Richardson

Ensatina eschscholtzi Gray

Batrachoseps wrighti (Bishop)

Neurotrichus gibbsi (Baird)

Salmo gairdneri Richardson

Sorex trowbridgei Baird

Acknowledgments The following people critically read and improved the manuscript: Michael Castellano,

Jerry F. Franklin, Donald K. Grayson C. Y. Li, Randy Molina, and Robert F. Tarrant.

Pen and ink illustrations were prepared by Paula Reid. Phyllis Taylor-Hill typed the

various drafts. The research was partially funded by the National Science Foundation

Grant DEB 80-04562. We are grateful for the help.

This paper is contribution No. 1 of the cooperative project, “The Fallen Tree-An

Extension of the Live Tree,” that involves the U.S. Department of the Interior, Bureau

of Land Management; U.S. Department of Agriculture, Forest Service, Pacific Northwest

Forest and Range Experiment Station; Oregon State University, Department of Forest

Science; U.S. Department of Agriculture, Agricultural Research Service; and Oregon

Department of Fish and Wildlife.

The interdisciplinary nature of the work reported in this paper evokes the fond memory

of our friend, colleague, and teacher, Gerry S. Strickler. Gerry’s life of research was

dedicated to understanding how different organisms in wildlands interact with each other

and their environment. And he left us with a greater appreciation of the importance of

cause-and-effect relationships between humans and forests.

50
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56
Maser, Chris; lhlppe, James M., tech. eds. The seen and unseen world of 

the fallen tree. Gen. Tech. Rep. PNW-164. Portland, OR: U.S. Department 

of Agriculture, Forest Service, Pacific Northwest Forest and Range 

ExperimentStation;1984. 56 p.lncooperationwith: U.S. Department of the 

Interior, Bureau of Land Management. 

Large, fallen trees in various stages of decay contribute much-needed diversity 

to terrestrial and aquatic habitats in western forests. When most biological 

activity in soil is limited by low moisture availability in summer, the fallen tree-soil 

interface offers a relatively cool, moist habitat for animals and a substrate for 

microbial and root activity. Intensified utilization and management can deprive 

future forests of large, fallen trees. The impact of this loss on habitat diversity 

and on long-term forest productivity must be determined because managers 

need sound information on which to base resource management decisions. 

Keywords: Fallen trees, decay (wood), decomposition, old-growth stands, 

Douglas-fir, Pseudotsuga menzlesil, mycorrhlzae, soil moisture. 
The Forest Service of the U.S. Department of 

Agriculture is dedicated to the principle of multiple 

use management of the Nation's forest resources 

for sustained yields of wood, water, forage, wildlife, 

and recreation. Through forestry research, 

cooperation with the States and private forest 

owners, and management of the National Forests 

and National Grasslands, it strives - as directed by 

Congress - to provide increasingly greater service 

to a growing Nation. 

The U.S. Department of Agriculture is an Equal 

Opportunity Employer. Applicants for all Department 

programs will be given equal consideration without 

regard to age, race, color, sex, religion, or national 

origin. 

Pacific Northwest Forest and Range 

Experiment Station 

809 N.E. Sixth Avenue 

P.O Box3890 

Portland, Oregon 97208 

<:i US GOVERNMENT PAINTING OFFICE 1984·794·737