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The Role of Wood Debris in Forests and Streams

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The Role of Wood Debris in Forests and Streams The Role of Wood Debris in Forests and Streams Frank J. Triska and Kermit Cromack, Jr. In the Pacific Northwest, old-growth WOOD BIOMASS AND ACCUMULATION forests and their associated streams con- tain large quantities of coarse wood debris. As we have indicated, debris accumula- To date, such debris has been considered an tion must be considered over a period of impediment to reforestation and stream about 500 years. Accumulation over such a quality. Consequently, it has been virtu- cycle of secondary succession--from a nearly ally ignored in ecological studies, partly bare forest floor to the debris beneath a because mans need for wood fiber has re- 450-year-old stand of Douglas-fir--is sulted in the removal of debris from for- depicted in Figure 1. The diagram, which ests throughout the world but also because is a composite of data from three sites on the extended period necessary for wood to the H. J. Andrews Forest near Eugene, decay makes it difficult to study nutrient Oregon, is not intended to represent any recycling from such a process. In this particular site. It does, however, reflect paper, we shall attempt to correct that the fact that accumulations of debris are omission by exploring how wood debris is greater in the Pacific Northwest than else- utilized in forest and stream ecosystems. where because of the larger biomass of the Such an exploration is timely in view regions tree boles (Grier and Logan, of the diminishing amount of pristine for- 1977). In most cases, natural catastrophes est. In the Pacific Northwest, the greatest would result in greater accumulations than accumulation of wood debris occurs from depicted here (Franklin and Waring, 1980), natural mortality and blowdown in such for- but those that would result from clearcut- ests. Now that forests are being cut every ting followed by yarding and burning are 80 years instead of standing 250 to 500 adequately portrayed. years (the interval between natural cats- In Figure 1, event 1 represents an in- stropic fires), it is crucial that we crease in wood debris as a result of natural determine the role of wood debris in pris- thinning following canopy closure. The in- tine habitats and then incorporate that crease in wood biomass is slight because knowledge into existing management strate- such debris is finely divided and readily gies for our forests and watersheds. susceptible to decomposition. Event 2 rep- Our exploration will begin with deter- resents a long period of accumulation as mining the amounts of wood debris in various low branches are shed and the canopy in- forest and stream ecosystems and its rates creases in height. Event 3 represents a of accumulation in each. We shall then decrease in wood debris because of a litter examine how debris modifies existing brush fire. If trees were killed by such habitats and creates new ones. Next, we a fire, however, the accumulation of woody shall determine how rapidly coarse wood debris would increase. debris breaks down into its component ele- Event 4 represents windthrow in a large ments and how its carbon and other elements old-growth stand. Event 5 represents the are recycled. Finally, we shall discuss accumulation of large individual trees on what implications these data have for the the forest floor over an extended period. managers of forested watersheds in the Toward the end of succession, even a single Pacific Northwest. tree can introduce a large amount of organic matter. For example, the biomass of a tree 100 cm in d.b.h. equals 10 metric tons. 171 172 FORESTS: FRESH PERSPECTIVES FROM ECOSYSTEM ANALYSIS As the foregoing sequence implies, This distribution results because the deep wood carbon entering the detritus pool of incisions in the basin cause trees to roll both the forest and its stream varies in downhill toward the stream. decomposition rate according to stand age Quantities ranging from 55 to 580 and history. For example, woody debris in metric tons per hectare represent a large young stands is finely divided and highly pool of organic carbon. We now know that susceptible to microbial attack, whereas an such pools of refractory (decomposition- equal amount of large woody debris with its resistant) carbon and other elements pro- low surface-to-volume ratio may have a de- vide a nutrient source throughout secondary composition period of hundreds of years. succession of ecosystems (ONeill et al., Furthermore, the wood component in litter- 1975). Estimated wood debris in old fall is less than 10 percent in young stands Douglas-fir forests (Grier and Logan, 1977; but 70 percent in old-growth forests MacMillan et al., 1977; Waring and Franklin, (Grier and Logan, 1977). 1980) represents a larger aboveground pool Few estimates exist of the quantity of of organic matter than the entire above- wood debris in forests and their watershed ground biomass of most current eastern streams. One site where data are available deciduous forests (Day and Monk, 1974; from both ecosystems is Watershed 10, a Sollins and Anderson, 1971). 450-year-old stand of Douglas-fir in the Although large wood debris (>10 cm in H. J. Andrews Forest. Biomass estimates of diameter) is more visible, fine wood debris downed logs were made for the forested also represents a substantial input and de- watershed by Grier and Logan (1977) and for composes more rapidly both in streams and the watershed streams by Froehlich et al. on land. Fine wood debris in Watershed 10 (1972). Log debris for the forest was constituted only 13 percent of the biomass estimated at 55 to 580 metric tons per in the streams but decomposed faster, on hectare, whereas that for the stream the average, than coarse wood debris. The channel was estimated at 298 metric tons various litter inputs into the forest floor per hectare. Plotting of these data on a and streams of Watershed 10 are shown in map of standing and downed vegetation on Table 1. Watershed 10 indicates a general decrease in Tables 2 and 3 indicate the biomass of wood debris from stream channel to ridgetop. wood debris in streams and on the forest 240 0.c INDIVIDUAL•.................. .0 c 200 0 0 E 160 (J) 4-WIND STORM 1- Z 0 120 2 > 80 RS 19 3- GROUND FIRE _J X 40 2-BRANCH INPUT TS 1 - TREE THINNING 100 200 300 400 450 YEARS Figure 1. Accumulation of wood debris during the life of a hypothetical stand of Douglas-fir. The solid line indicates the overall increase of wood debris over time; included as reference points are data from the Thompson Site (TS), Washington Reference Stand 19 (RS 19), and Watershed 10 (WS 10), all on the H. J. Andrews Forest near Eugene, Oregon. The dotted line depicts major events (discussed by number in text) that affect debris accumulation. F. J. TRISKA K. CROMACK, JR. 173 floor of various old- and young-growth for- been dendrochronological, primarily the ests. By far the largest amount of wood interpretation of tree scars (MacMillan debris occurs in streams draining old-growth et al., 1977; Swanson et al., 1976) or the redwoods and Douglas-fir/western hemlock. assessment of dated successional sequences Even second-growth Douglas-fir/hemlock has such as fir waves (Sprugel, 1976) in balsam more wood in its streams than do some old- fir forests (Lambert et al., in press). growth sites in other parts of the country, Permanent plots set aside for studies of partly because of the carryover of wood tree growth and mortality afford another debris from the primeval forests. In old- method of dating wood debris. While falling, growth forests in the Great Smoky Mountains trees may scrape against adjacent trees, re- of Tennessee, both spruce/fir and mixed sulting in the removal of bark and the de- hardwood stands have large quantities of composition of callus tissue in annual in- wood debris in their streams. crements in the surviving trees. These in- Comparison of old- and second-growth crements or shock rings permit dating of the forest types indicates that even in primeval events (Shigo and Marx, 1977). Unfortunate- forests, the amount of wood debris in the ly, only a few trees leave such records of streams varied with species composition and their deaths. environmental conditions. Regardless of Dating the accumulation of large debris these two factors, however, the quantity of is especially difficult in third-order or such wood was probably much greater in larger streams. In first- and second-order many primeval forests than can be observed streams, wood remains essentially where it today. It follows that the biota of for- falls. In the larger streams, however, it ested ecosystems and streams draining them tends to be clumped by high water prior to evolved in a system where wood debris play- decomposition, making it especially diffi- ed a far larger role than it does today. cult to date the accumulations or to esti- Thus, wood debris has been removed in many mate decomposition rates or nutrient re- parts of the world before man has fully cycling. One can conclude that wood debris understood its role. tends to exert a greater impact, in terms One of the major difficulties in of amount, on the stream than on the sur- assessing the input rate of wood debris is rounding forest and that this impact the episodic nature of its accumulation. gradually diminishes as one proceeds down- To date, the most successful methods have stream. Table 1. Leaves and coarse and fine wood debris entering the forest floor and watershed stream at Watershed 10, H. J. Andrews Forest, prior to clearcuttingl Leaf litter Wood debris Method of measurement Deciduous Coniferous Fine Coarse g/m2/day Litterfall 0.070 0.346 0.244 Lateral movement .096 .143 .728 Scaling 0.548 Total .166 .489 .972 .548 Percent composition 7.6 22.5 44.7 25.2 1 Unpublished data from F.
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