Prepared by Mark E. Harmon
Dead trees serve many key functions in ecosystems (Franklin et al. 1987). Since dead trees may persist for centuries (McFee & Stone 1966, Triska & Cromack 1980), they can influence ecosystems as long as living trees. Woody detritus reduces erosion and affects soil development; stores nutrients and water; is a major source of energy and nutrients; serves as a seedbed for plants; and is a major habitat for microbes, invertebrates and vertebrates (Anderson et al. 1978, Davis et al. 1983, Frankland et al. 1982, Franklin et al. 1981, Harmon et al. 1986, Lutz 1940, Swanson & Lienkaemper 1978). Despite these many functions the importance of this material in forest ecosystems has, until recently, been overlooked by ecologists (Harmon & Chen 1991) and forest managers alike (Kirby & Drake 1993, Samuelsson et al. 1994).
Over the last decade there has been an extesive program on understanding the function of dead trees (logs and snags) in Pacific Northwest forests. Past support from NSF to study dead wood dynamics and processes in the Pacific Northwest has come from a two year grant to Schowalter, Lattin, Trappe, Carpenter, and Harmon (BSR-8516590, $425,000), a one year grant to Schowalter, Lattin, Carpenter, Kelsey, Ingham and Harmon (BSR-8717434, $50,000), and a three year grant to Harmon, Jarrell, and Caldwell (BSR-8918643, $450,000). The primary focus of these grants has been a decomposition time series that was established on the Andrews Experimental Forest with LTER (DEB 80-12162, BSR-8514325, BSR-9011663) and US Forest Service funding. This work represents the first integrated and long-term examination of decomposition and nutrient cycling in logs (Harmon 1992). Funding from the first two grants (BSR-8516590, BSR-8717434) allowed us to examine the influence of heterotroph interactions on early decomposition processes. Funding for the third grant (BSR-8918643) allowed us to test the hypothesis that the microelement manganese controls the rate of lignin breakdown in wood. In addition to these studies, our work has also examined the importance of woody detritus to regional (Mexico and Russia) and global C stores. The latter effort is supported, in part, by the National Science Foundation (DEB-9213187, $75,000 to Drs. Harmon & Krankina). To date, significant and new information has been produced from this research; by June 1994 two theses, 12 presentations/poster sessions and 25 publications have resulted (see references followed with * ). There are also 2 manuscripts currently in preparation resulting from the two most recent grants. Finally we have created 8 major databases (TD12, TD14, TD18, TD20, TD21, TD22, TD25, TD26) with 60 files and approximately 20 Mbytes of fully documented data.
Since the long-term log decomposition study established under LTER forms the basis for much of our current understanding it will be described briefly. The experimental design is a split-split-plot and tests the effect of substrate quality and colonization patterns on log decomposition. There are six field installations at the Andrews Forest and at each, twenty 5.5 m long X 50 cm diameter disease-free logs from each of four conifer tree species were placed on the forest floor. These species span a range in decay resistance, Abies amabilis < Tsuga heterophylla < Pseudotsuga menziesii < Thuja plicata (Scheffer & Cowling 1966). Destructive sampling of these logs occurred each of the first 8 years and will continue at longer sampling intervals after 10 years. When logs are sampled, 5 cross-sections are removed at 1 m intervals and examined for changes in density, moisture content, nutrient content, C chemistry (lignin, cellulose, etc.) and decomposer colonization patterns. In addition to the annual sampling for these changes, we are also examining long-term changes in seasonal respiration patterns, water balance, leaching, fragmentation, sporocarp production, insect production, and nitrogen fixation.
Mass Loss. As part of our LTER sponsored research (DEB 80-12162, BSR-8514325, BSR-9011663) we have measured log mass loss each year. Our initial work has confirmed that mass loss is a nonlinear process, which can only be understood if the proportion, decay resistance, and colonization pattern of the major tissues comprising a dead tree are considered (Carpenter et al. 1988). Annual changes in log density also indicated a significant lag-time is caused by slow colonization. After one year, log tissue density was not significantly different from initial values (Kelsey & Harmon 1989). After eight years, inner bark and sapwood density decreased up to 23-63%, whereas outer bark and heartwood density remained about the same (Harmon & Sexton 1995). The effect of colonization was also observed in our in situ respiration studies; yearly maximum respiration rate increased the first 5 years, and then decreased. High concentrations of phenols, tannins and pentane soluble organics (terpenes) appeared to be inhibiting decay in outer bark and in Pseudotsuga and Thuja heartwood (Kelsey & Harmon 1989), which accounts for the post 5-year decrease.
Lignin Degradation. As part of our third grant (BSR-8918643) we have been investigating the factors controlling lignin decomposition, which are crucial for predicting long-term C accumulations in ecosystems. Degradation of wood lignin is ecologically interesting, because in some situations lignin is not degraded (brown-rot), whereas in others it is completely degraded (white-rot). We had hypothesized that the importance of these two types of decay might be controlled by the abundance of the microelement Mn. Preliminary field surveys conducted at Andrews, Coweeta, Harvard Forest, and Luquillo LTERs, and Fraser Experimental Forest indicate that coniferous forests (generally with lower Mn content) have more brown rot than eastern deciduous or moist tropical forests. Moreover, in our work at Andrews and Fraser Experimental Forests Abies which is high in Mn was more prone to white rot than Pseudotsuga or Pinus which are low in Mn. Other observations, however, raise questions about this hypothesis. Adding Mn to Naematoloma, the major white-rot fungus at Andrews, did not increase lignin degradation in wood chips indicate that of this species is not increased. Moreover, examination of lignin degradation using proximate C analysis (Ryan et al. 1990), indicates lignin degradation in sapwood appears similar for Thuja, a low Mn species and Abies, a species high in Mn. One explanation for our mixed results is that some white-rot fungi species have coevolved with low Mn tree species. For example, Oxyporus is a white-rot that only grows on Thuja and is quite efficient in degrading lignin despite the low Mn found in this wood.
Water Balance. Seasonal and successional changes in the water balance of logs and the moisture content of individual layers (e.g., outer bark) has been measured during the first seven years (Harmon & Sexton 1995). After one year of decomposition, 38-47% of the canopy throughfall landing upon logs ran off the surface, 29-34% leached out the bottom, and 21-30% was absorbed and/or evaporated. After seven years of decomposition, leaching flows increased 1.7 times while the run-off flows decreased five -fold. Overall, these results indicate that 2-6% of the stand level canopy throughfall is absorbed and then evaporated from logs. This result is given added significance because past hydrologic models in the Pacific Northwest have assumed logs do not intercept throughfall! To put these numbers in context the canopy intercepts 12% of the precipitation (Rothacher 1963).
Nutrient Cycling. We are currently assembling a complete budget for major nutrients (N, P, K, Ca, Mg, Mn, Zn) during the course of log decomposition. In addition to quantifying changes in nutrient concentrations, fluxes associated with leaching, fragmentation, insect emigration, and nitrogen fixation are being measured. Examination of specific nutrient fluxes in logs has revealed previously unmeasured and important pathways that may explain why changes in nutrient concentrations are not matching results from past work. Given the high proportion of polymeric C in logs, we were very surprised to find leaching was a pathway of nitrogen loss during the first 7 years. Cumulatively, logs leached 0.2-0.3% and 0.7-1.5% of the initial C and N, respectively, over the first 7 years (Harmon et al. 1994). The export of nutrients via fungal sporocarps (fruiting bodies) was a major, unmeasured pathway of loss in logs (Harmon et al.1994). Past work suggests fungi immobilize nutrients within logs, however, as the sporocarps form outside the log and fall to the forest floor, they represent a net export. In the first 7 years, sporocarps transferred 0.9-2.9% and 1.9-6.6% of the initial N and P stored in logs to the forest floor, respectively. N exports from logs by leaching and fungi are small, but the fact they occur at all raises serious questions about past conceptual models of the nitrogen dynamics of woody detritus. After all, these losses are occurring when logs have a C:N ratio exceeding 500:1! In addition to examining nutrient losses, we are examining inputs via nitrogen fixation. Long-term temporal patterns of nitrogen fixation are more complex than the linear increase previously reported (Griffiths et al. 1993). We found a non-linear pattern of change with an early peak at 18 months followed by a decline until 30 months, which is then followed by steady linear increase until at least 72 months.
Regional and Global Studies. During the most recent funding period, our intersite collaborative efforts have increased. This stems from a need to expand beyond detailed process studies within the Pacific Northwest. Thus far, collaborative studies examining the input, decay, and stores in coarse woody debris have been conducted in Colorado (Moir et al. in prep), China (Harmon & Chen 1991), Mexico (Whigham et al. 1992, Harmon et al. in press), and Russia (Krankina & Harmon 1994, Krankina & Harmon, in press). In reviewing the role of tree death as a detrital input in the global C cycle, Harmon et al. (1991, 1993) found 4-17 Pg C year-1 is added globally to detrital pools by dying trees. At these rates of input, woody detritus may store between 80 and 300 Pg C, indicating current budgets have missed as much as 15% of the total terrestrial detrital stores and may have underestimated total surface (leaf+woody) detritus by a factor of 2 to 5 (Harmon et al. 1993). Our work has also been important in determining the C balance in North America. Both the US EPA (Turner et al. 1993) and Forestry Canada (Kutz et al. 1992) relied heavily on our databases.
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Harcombe, P.A., M.E. Harmon, and S.E. Greene. 1990. Changes in biomass and production over 53 years in a coastal Picea sitchensis-Tsuga heterophylla forest approaching maturity. Can. J. For. Res. 20:1602-1610.*
Harmon, M.E. 1989. Effects of bark fragmentation on plant succession on conifer logs in the Picea-Tsuga forests of Olympic National Park, Washington, USA. Amer. Mid. Nat. 121:112-124.*
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Harmon, M.E. 1992. Fire influences on coarse woody debris. p. 11 In Fire in Pacific Northwest Ecosystems: Exploring Emerging Issues. January 21-23, 1992, Oregon State University Portland, OR.*
Harmon, M.E. Long-term studies of log decomposition. Ecological Society of America. Davis, California, August 1988.*
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Harmon, M.E. 1987. Influence of litter-humus accumulation and canopy openness on Picea sitchensis and Tsuga heterophylla seedlings growing on logs. Can. J. For. Res. 17:1475-1479.*
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Harmon, M.E., K. Cromack, Jr., and B.G. Smith. 1987. Coarse woody debris in mixed-conifer forests, Sequoia National park, California. Can. J. For. Res. 17:1265-1272.*
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Swanson, F.J. and G.W. Lienkaemper. 1978. Physical consequences of large organic debris in Pacific Northwest streams. USDA For. Serv. Gen. Tech. Rep. PNW-69.
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Turner, D.P., G.J. Koerper, M.E. Harmon, and J.J. Lee. in press. A carbon budget for forests of the conterminous United States. Ecological Applications. *
Turner, D.P., J.J. Lee, G.J. Koerper, and J.R. Barker. 1993. The forest sector carbon budget of the United States: Carbon pools and flux under alternative policy options. US EPA Environmental Research Laboratory, Corvallis, Oregon.*
Whigham, D.F., I. Olmsted, E.C. Cano, and M.E. Harmon. 1991. The impact of Hurricane Gilbert on trees, litterfall, and woody detritus in a dry tropical forest in the northeastern Yucatan Peninsula. Biotropica 23:434-441.*
Zhong H. and T.D. Schowalter. 1989. Conifer bole utilization by wood-boring beetles in western Oregon. Canadian Journal of Forest Research 19:943-947.*
Zhong, H. 1988. Host selection and wood channelization by beetles in conifer logs in western Oregon. MS Thesis, Oregon State University.*