ORGANIC MATTER DECOMPOSITION IN WESTERN UNITED STATES FORESTS - PDF

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ORGANIC MATTER DECOMPOSITION IN WESTERN UNITED STATES FORESTS Robert L. Edmonds This file was created by scanning the printed publication. Errors identified by the software have been corrected; however,
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ORGANIC MATTER DECOMPOSITION IN WESTERN UNITED STATES FORESTS Robert L. Edmonds This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. ABSTRACT Understanding decomposition processes and the influence of forest management practices on them is crucial to maintaining long-term productivity in western forests. This paper discusses: (1) organic matter accumulations in western forests, including coarse woody debris (CWD), (2) organic matter decomposition rates, including the effects of clearcutting, (3) physical, chemical, and biological factors influencing decomposition rates, and (4) nitrogen dynamics in decomposing substrates. Decomposition rates are much higher in coastal forests (k = yrl for Douglas-fir needles) than in inland forests (k = yrl for pine needles). Decomposition rates for woody substrates are one to two orders of magnitude slower depending on their size. Needle decomposition rates are increased by clearcutting. Nitrogen release from a substrate is related to its decomposition rate and N may be immobilized for a long time in CWD. INTRODUCTION Decomposition is the process whereby litter on the soil surface and belowground roots are broken down to smaller particles (Swift and others 1979). It releases soluble forms of nutrients that are available for plant uptake and provides soil organic matter (Waring and Schlesinger 1985). Understanding decomposition processes and the influence offorest management practices on them is crucial to maintaining the long-term productivity of western forests. Organic matter decomposition also contributes CO 2 to the atmosphere, thus influencing global warming. Fungi and bacteria are the dominant decomposers in coniferous forests (Richards 1987). Small animals, such as mites, fragment fine litter and enhance microbial decomposition. Earthworms, although important in the decomposition process in deciduous forests, are not thought to playa major role in coniferous forests, but they are present (Richards 1987). Insects, especially termites, ants, bark beetles, and wood borers, playa very important role in the decomposition of woody litter by fragmenting wood and introducing fungal decomposers (Harmon and others 1986). Paper presented at the Symposium on Management and Productivity of Western-Montane Forest Soils, Boise, ID, April 10-12, Robert L. Edmonds is Professor of Soil Microbiology and Forest Pathology, CoHege of Forest Resources, University of Washington, Seattle, WA Simple sugars decompose completely to CO 2 and water, but decomposition of the complicated organic substrates in forest ecosystems is not complete. Hard-to-decompose or recalcitrant substances accumulate in the soil as humus, which comprises the soil organic matter so important in maintaining forest productivity. Soil organic matter maintains soil structure, improves soil water balance, and is a long-term source of site nutrients. It is particularly vulnerable to loss through improper forest practices and is important in preventing compaction and erosion. There are many sources of organic matter in western forests, and one of these is coarse woody debris (CWD), such as logs and snags. Determining the importance of CWD in western forests has been the focus of much research in recent years (Harmon and others 1986; Harvey and others 1979, 1981; Larsen and others 1980; Maser and others 1988), and maintaining CWD in western forests is one of the major components of new forestry (Eubanks 1989; Franklin 1989). Coarse woody debris provides: (1) plant habitat through nurse logs, (2) moisture and nutrients for fine roots and mycorrhizae, (3) habitat for animals and birds, (4) pools in streams for fish habitat, (5) sites for nitrogen (N) fixation, and (6) a long-term source of soil organic matter. It also protects against erosion by improving slope and stream stability, maintains species diversity, and helps in maintaining long-term site productivity. Many of these roles change as CWD decomposes. The objectives of this paper are to: (1) present data on organic matter accumulation in western forests, (2) discuss organic matter decomposition rates in western forests including the effects of clearcutting on decomposition rates, (3) discuss the factors influencing organic matter decomposition rates, and (4) examine N dynamics in decomposing substrates. ORGANIC MATTER ACCUMULATIONS The sources of soil organic matter in western forests are fine litterfall (needles, leaves, insect frass, etc.), fine woody litterfall (twigs, branches, cones), coarse woody debris (logs, snags, and stumps), roots (fine and coarse), and soil organisms. The importance of fine roots in contributing organic matter to the soil has only recently been demonstrated. Fine root turnover can be equal to needle litter inputs and in some ecosystems, for example, Pacific silver fir, fine root inputs can be three times aboveground fine litter inputs (Vogt and others 1986). 118 The amount of dead organic matter accumulating in an ecosystem is the balance among litter and root inputs, decomposition, and the effects of fire. Organic matter accumulations in western U.S. forests are among the highest in the World's forests (Cole and Rapp 1981). There is considerable variability, however, in organic matter accumulations in this zone, with highest accumulations in productive wetter and cooler coastal areas and lowest accumulations in less productive hotter and drier inland areas (table 1). A large proportion of this accumulation is in CWD and soil organic matter. Highest CWD accumulations occur in old-growth forests in the western Olympic Mountains ( 500 Mg/ha) (Agee and Huff 1987) with lesser amounts in old-growth forests in the western Oregon and Washington Cascades (averaging Mg/ha) (Spies and others 1988). Fire is the major disturbance in western forests and 600-1,000 Mg/ha ofcwd can be found immediately after a catastrophic wildfire in old-growth forests (Spies and others 1988). This CWD will decompose with time, and new accumulations will begin at about age 50 years. Lowest amounts of CWD «100 Mg/ha) tend to occur 100 to 200 years after the fire, after which the amount of CWD will increase again. Windstorms can also add large amounts of coarse woody debris in areas close to the coast. Forest management has changed organic matter accumulations, particularly with respect to CWD (Harmon and others 1990; Spies and others 1988). Spies and others (1988) suggest forest management activities greatly reduce the amount of CWD below minimums typically encountered under natural ecosystem dynamics. For most of the managed rotation CWD biomass is 30 Mg/ha. This is supported by data from second rotation forests in the Puget Sound area where surface CWD biomass is only around 30 Mglha (R. L. Edmonds, unpublished data). In third rotation forests CWD biomass is further reduced to 10 Mg/ha. Stumps left after harvest only account for about 3 Mglha in third-rotation forests, but coarse woody roots contribute significantly more (around 30 Mg/ha) Table 1-Organic accumulations (Mg/ha) in mature western U.S. forests Location Litter and humus Fine roots Coarse woody debris logs snags coarse roots Soil 1Johnson and others (1982). 2Grier and Logan (1977). 3fahey (1983). 4Fahey and others (1985). SVogt and others (1986). sharmon and others (1986). 7Spies and others (1988). 8Agee and Huff (1987). 9Pearson and others (1984). Coastal (Douglas-fir/hemlock) ,6,7, (live) Inland (pine) 3, , (live) (R. L. Edmonds, unpublished data). This loss of a longterm source of organic matter is of considerable concern. Soil organic matter is also likely to be reduced with forest management (Harmon and others 1990), although few studies have been conducted to examine this. The optimum level of organic matter in soils to maintain productivity in western forests is not known. However, organic matter removal on poor Douglas-fir sites tended to have a greater effect on reducing productivity than on good sites (Bigger 1988). The forest floor, roots, and fine woody litter become increasingly important contributors to soil organic matter as the intensity of forest management increases and the contribution oflarge woody litter decreases. DETERMINATION OF DECOMPOSITION RATES Organic matter decomposition rates are usually determined by examining substrate dry weight or mass loss, changes in specific gravity, or carbon dioxide evolution from a substrate. Substrate mass loss with time is typically used to determine decomposition rates of fine litter (needles, leaves, twigs, cones, bark, small branches, etc.), whereas specific gravity change is usually used for determining CWD decomposition rates. Although the specific gravity oflarge boles can be determined relatively easily, it is more difficult to determine how long a bole has been on the ground unless the bole is a result of a known blowdown. Typically, logs that have been on the ground for a long time are aged by examining adjacent living trees for scars and aging the scars using an increment borer (Sollins and others 1987). The age of trees growing on downed logs can also be determined. Both fragmentation and respiration losses have to be taken into account for boles and snags. Carbon dioxide evolution is not typically used for determining decomposition rates of specific substrates, because it is such a dynamic measure and it is hard to integrate over a long time period. It is also difficult to separate substrate respiration from root respiration, and forestfloor CO 2 evolution is not always well related to decomposition rates (Vogt and others 1980). Typically, decomposition rates are expressed as k values or fractional loss rates rather than just changes in mass or specific gravity with time. The unit of k is typically yr10r per year. A simple negative exponential curve can be used to express decomposition and the k value can be determined from the equation X / Xo = e-kt, where Xo = initial dry mass or specific gravity and X = mass or specific gravity at time t (years) (Olson 1963). Typically, k values using this equation decrease with time because the rate of decomposition is more rapid early in the process and slows with time (Edmonds 1984; Yavitt and Fahey 1982). More complex models have also been developed (for example, Bunnell and Tait 1974; Means and others 1982; Melillo and others 1989). The k values for CWD commonly have a fragmentation component (k ) Cra and a respiration component (k ) or (k. ) (Harmon and resp mm others 1986). 119 (!) 100 v' PHASE 1- Leaching. microbial colonization «80 W a: 60 PHASE 2 - Cellulose loss en en 40 PHASE 3 - Stable phase. «lignin loss I-- \ 20 W () a: 0 W a TIME (months) Figure 1-Phases of decomposition in red pine needles in Massachusetts. From Melillo and others (1989). 80 (!) «W a: en en «I-- W () a: w lit' PHASE 1 - Lag PHASE 2 - Fragmentation. leaching. microbial mineralization PHASE 3 - Microbial mineralization, further fragmentation a o 100 TIME (years) Figure 2-Phases of decomposition of Douglas-fir logs in western Oregon and Washington. Data from Sollins and others (1987). 200 THE DECOMPOSITION PROCESS Decomposition offine litter, such as needles or leaves, occurs in three phases as shown in figure 1 for red pine in the eastern U.S. (Melillo and others 1989). In the first phase, during the first few months, the labile or fast fraction (sugars and starch, etc.) is lost by rapid microbial assimilation or leaching. Harmon and others (1990) noted that leached litter in coastal Washington decomposed slower than unleached litter. The second phase is dominated by loss of structural or slow carbon, which is primarily cell wall polymers such as cellulose. The third phase is the stable or metastable phase in which there is a very slow decrease in mass. This phase is dominated by lignin decomposition. Mellillo and others (1989) have examined decomposition along a decay continuum from plant litter to soil organic matter and feel that litter mass loss can best be modeled using a two-phase model: an initial phase of constant mass loss and a very slow loss dominated by a degradation of lignocellulose (acid-soluble sugars plus acid-insoluble C compounds). As the decaying litter enters the second phase, the ratio of lignin to lignin plus cellulose (the lignocellulose index-lci, or the fraction oflignin in the lignocellulose complex) approaches 0.7. Beyond this the LCI increases only slowly throughout the decay continuum indicating that acid insoluble materials (lignin) dominate decay in the later stages. For red pine needles the second phase began when the organic matter remaining was about 20 percent; it ranged between 15 and 30 percent for other litter materials (Melillo and others 1989). Woody litter goes through a similar process but has at least one additional stage as shown in figure 2. Before any weight loss or change in specific gravity occurs there is a lag phase (phase 1), which is usually related to the size of the substrate (larger woody substrates usually have a longer lag time). In phase 2, logs begin to weather and fragment. Leaching losses and microbial activity occur. After the period of active fragmentation there is a period of rapid microbial mineralization (phase 3). Table 2-Decay classification for Douglas-fir boles (from Maser and others 1988) Characteristics Decay class of fallen trees II III IV V Bark Intact Intact Trace Absent Absent Twigs Present Absent Absent Absent Absent Texture Intact Intact to Hard, large Small, soft, Soft and partly soft pieces blocky pieces powdery Shape Round Round Round Round to oval Oval Color of wood Original color Original color Original color Light brown to Red brown reddish brown to dark brown Portion of tree Tree elevated Tree elevated on Tree sagging All of tree on All of tree on ground on support support points; near ground ground on ground points sagging slightly Invading roots None None In sapwood In heartwood In heartwood 120 is followed by the stable phase, which is dominated by lignin decomposition (phase 4). Most coniferous logs at this stage consist of a mass of crumbly brown cubical rot. Maser and others (1988) have described five visual decay classes for Douglas-fir logs as shown in table 2 involving the presence or absence of bark, twigs and roots, texture, shape, color, etc. Note that roots do not begin to invade logs until they reach decay class III. DECOMPOSITION RATES IN WESTERN FORESTS The study of organic matter decomposition in western forests is of relatively recent origin. Few studies were conducted before the 1970's, and most of these involved examining bole deterioration, particularly after windstorms (Shea and Johnson 1962) or insect epidemics (Wright and Harvey 1967). They did not determine actual decomposition rates. Interest in studying organic matter decomposition rates in western forests from an ecological viewpoint increased after the initiation of the Coniferous Forest Biome Program of the International Biological Program (IBP) in the 1970's (Edmonds 1982). A number of studies were initiated in western Oregon and Washington at that time, mostly focusing on needle decomposition (for example, Edmonds 1979, 1980, 1984; Fogel and Cromack 1977). The IBP also stimulated interest in studying decomposition rates of coarse woody debris, and a number of studies were conducted starting in the late 1970's (Graham and Cromack 1982; Grier 1978; Harmon and others 1986 Sollins 1982; Sallins and others 1987; Spies and othes 1988). Decomposition rates of needles, roots, branches, bark, and wood in western forests, including inland sites, are summarized in table 3. Douglas-fir was taken to be a representative species for coastal sites, while data for pines are presented for inland si tes. There are differences, however, in decomposition rates among different species occurring in a local area. For example, in the Washington Cascades k values for low-elevation red alder Douglas-fir, western hemlock, and high-elevation Pacific ' silver fir are 0.45, 0.44, 0.38, and 0.30 yr\ respectively, based on 2 years of decomposition (Edmonds 1980). Overall decomposition rates for needles are much higher in coastal Douglas-fir (k = yrl) than in pine types in California (k = 0.05 yrl), Arizona (k = 0.14 y-l), and Wyoming (k = 0.14 yrl), due to moister conditions in coastal forests. This is also illustrated in figure 3, which shows isolines of first-year needle and leaf decomposition rates in the U.S. The predicted rates are higher than actual rates because first-year rates are usually very rapid. Note that decomposition rates in the West are in general considerably slower than those in the eastern U.S., especially the southeastern U.S. This largely reflects different climatic regimes. It is considerably warmer and wetter in the southeastern U.S. than it is in the West and summers tend to be dry in the West even in coastal areas. The factors controlling decomposition rates are discussed in more detail in the following section. Table 3-Typical average decomposition rates of needles, branches, roots, bark, and wood in western U.S. forests Substrate Needles Branches Roots fine coarse Bark Wood (diam., cm) 24 (log) 37 (log) 65 (log) 0.44 (WA) (OR) (WA)B 0.18 (OR) (OR) (WA) (WA) (WA, OR) (WA, OR)13 Coastal (Douglas-fir) 18ased on single exponential model of Olson (1963). 23/k (Olson 1963). 3Edmonds (1980a). 4Stohlgren (1988a). 5fogel and Cromack (1977). 6Klemmedson and others (1985). 7Vavin and Fahey (1986). 8f:dmonds (1987). 9J=ogel and Hunt (1979). lovavin and Fahey (1982). lledmonds and Eglitis (1989). 12Harmon and others (1986). 13Spies and others (1988) (includes both fragmentation and respiration). Inland (pines) TI TI 95% decay (yr)2 k (yr-l) 95% decay (yr) (CA) (A) (WYf (WY) (Wy) - - - ', \ \ \, 60 -=-= km a 100.JOO 500 Figure 3-lsopleths of percent needle and leaf litter produced annually that will decompose in the first year. k values (yr1) are shown for the western U.S. Adapted from Meentemeyer (1978a). are discussed in more detail in the following section. Decomposition rates also change with stand age, with the fastest rate occurring near the time of canopy closure (Edmonds 1979). There are few data on fine root decomposition rates in the West, but the data that do exist indicate that rates are slightly slower or similar to those for foliage (table 3). Berg (1984) also noted this for Scots pine fine roots. Coarse woody roots in Wyoming decomposed more slowly (k = yr-l) than fine roots (k = 0.05 yr-l). This k value for coarse roots, however, was calculated after years of decomposition. When calculated after years it decreased to yrl (Yavitt and Fahey 1982). Coarse root wood appears to decay very slowly in the later stages of decomposition. Yavitt and Fahey (1982) noted there was still a large amount of root wood at their sites in Wyoming 100 years after tree death. Bark also decomposes slowly, and Douglas-fir bark decomposes at a similar rate (k = 0.03 yr-l) to small logs (table 3). Considerably more decomposition data exist for surface woody litter. Wood decomposes one to two orders of magni tude slower than needles depending on size (table 3). Branches decompose at a faster rate than logs, and small logs decompose at a faster rate than large logs (table 3). Edmonds and Eglitis (1989), in an exception to this rule, however, noted that small Douglas-fir logs (average diameter 24 cm) decomposed at a slower rate than medium-sized logs (average diameter 37 cm). Small logs were not as easily attacked by wood-boring insects, which apparently spread wood-rotting fungi. This was only a 10-year study, however, during the initial period of decom posi tion. Large Douglas-fir logs in coastal forests may exist on the forest floor for more than 500 years based on a simple k value (table 3). Spies and others (1988), however, suggested that using kresp values to calculate the longevi
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