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.

There are three major sources of coarse woody debris in an old-growth 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.

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Figure 10.-Prolific rooting of western hemlock saplings on a decay class V fallen tree holds the rotten wood together; ground cover is Oregon oxalis.

A diameter of about 12 inches (30 cm) 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 III.  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 (about40 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).

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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 III (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:l; decay class III = 4.4:1, decay class IV = 2.1:1; and decay class V = 1.6:1 (see footnote 1).

In across-sectional view, a fallen tree is circular in decay classes I through III.  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.

Substrates as Niches   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).

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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).

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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.  Microbial 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 I-III 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).

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Temperature

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 III have sloughed and slumped somewhat, and those in classes IV and V are partially to mostly embedded in the soil.

Moisture

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 (IB), 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.

Figure 15.-Water content of fallen Douglas-fir trees in winter and summer; each point represents a decay class from I (newly fallen trees) to V (fallen trees decomposed to a mound of brown cubical rotted wood).

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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).

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Plant Rooting  hereiam

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 heartwood 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
(Excrement)

Wood-boring insects, such as beetles, carpenter ants, and termites, tunnel within fallen 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 available 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).

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