Forest Detritus Under Changing Climate and Disturbance Senarios chaired by Christopher Woodall and Mark Harmon

What Drives Decomposition Rates of Coarse Woody Debris (CWD)?

Steffen Herrmann et al. — Thu, 25 Jun 2009 09:00:00 PDT

Currently increasing efforts are made to manage CWD as a habitat component and a carbon store in forest ecosystems. For this a basic understanding of patterns and rates of dead wood decomposition in different forests is crucial. The decomposition rate of CWD is mainly dependent on climatic (wood temperature, wood moisture) and substrate specific (tree species, decay stage, diameter) variables. Here, we analysed the influence of these factors using a combined approach. 1) We assessed the decay rate of Fagus sylvatica, Picea abies and Pinus sylvestris in three diameter classes (10-20 cm, 20-40 cm, >40 cm) along a climatic/altitudinal gradient (temperature, precipitation) retrospectively in the field. 2) We analysed under controlled conditions the effect of varying wood temperature (5, 10 and 20 ∞C) and moisture (three steps) on the current respirational carbon loss of CWD of Fagus sylvatica, Picea abies and Pinus sylvestris in relation to decay stage (1, 3 and 5 related to a 5-class decay classification system). 3) We measured CWD respiration continuously over one year in the field on a F. sylvatica and P. abies log and analysed the effect of substrate specific (tree species, decay stage, diameter), micro-climatic (wood moisture and temperature) as well as environmental variables (ground contact or suspended). A highly significant effect of wood temperature and moisture on respirational carbon loss regardless of decay stage was observed under controlled conditions as well as in the field. In both cases the respirational C loss of F. sylvatica CWD was about twice that of P. abies. Suggestions will be provided, how C loss from CWD might be represented in decomposition and ecosystem C models.

Dead Wood in High-Boreal Labrador Black Spruce Forests – Buried And Forgotten?

U. Hagemann et al. — Thu, 25 Jun 2009 08:40:00 PDT

Dead wood (DW), and in particular woody debris (WD), is an important component of the forest C cycle. In cool and wet climates, where microbial activity is restricted and moss growth is vigorous, large amounts of WD can be buried, i.e. overgrown by moss. Abundance, size, and decay class of DW buried in the organic layer were assessed in 15 stands of black spruce (Picea mariana (Mill.) BSP) in Labrador: 3 old-growth stands, and 12 stands regrown following clearcut harvesting (1970-72, 1989, and 2005) or wildfire (1985). Field measurements were based on Line Intersect Sampling and the Canadian National Forest Inventory Ground-plot Protocol. Harvested, burned, and old-growth sites contained 5.8ñ9.6, 4.7, and 18.2ñ37.3 Mg C ha-1 of buried dead wood (BW), respectively. Old-growth BW-C stocks largely exceeded total aboveground DW-C stocks (12.0 Mg C ha-1), indicating accumulation and/or preservation over long time periods. Stand-replacing fires, the predominant regional natural disturbance, burn only a portion of the organic layer and thus wood buried in it, potentially not interrupting the accumulation of BW over several stand generations. BW in old-growth sites was mainly in decay class 4 and 5, but decay class 2 and 3 BW contributed ~30% of total BW-C. A considerable portion of WD is hence buried before reaching more advanced stages of decay. Following burial, decay rates likely slow down considerably due to cold and moist conditions. BW accumulation appears to depend on a combination of climate (e.g., temperature, precipitation), micro-topography (e.g., drainage), ground vegetation (e.g., moss growth), and stand disturbance history (e.g., fire intensity and return interval). Excluding BW from DW inventories in cool and moist coniferous forests with a vigorous moss layer and long fire-return intervals such as found in high-boreal Labrador or coastal Scandinavia can result in massive underestimates of DW-C stocks.

Coarse Woody Debris Dynamics and Nutrient Cycling in Black Spruce Forests of North-Western Quebec

J. Jacobs et al. — Thu, 25 Jun 2009 09:20:00 PDT

Dead wood is an important component of forest ecosystems. It has been demonstrated to be a crucial component to support biodiversity and has the potential to play a role within nutrient cycles of boreal forests. Coarse woody debris (CWD) is typically abundant in natural forests, however, it is poor in nutrients and typically contributes a minor fraction of nutrients annually produced in boreal forests. Recent studies have shown CWD has the ability to provide a large proportion of nitrogen, calcium, phosphorus required for live tree growth. Black spruce forests are typically the most nutrient limited of all boreal ecosystems which may elevate the importance of CWD. The black spruce forests of the claybelt region in Quebec and Ontario are prone to paludification. This is a process where litter accumulates, moss growth prevents regeneration, an increase in the water table and a decrease in the rate of litter decomposition. Our study objectives are to 1) determine the volume and decay stage of CWD across a successional gradient of black spruce stands, 2) determine the decay rate of CWD along the successional gradient, 3) determine the rate or nutrient release from CWD and 4) examine the role of CWD in black spruce ecosystems. We studied a chronosequence of 10 black spruce stands from 54 to 710 years old. In each stand we sampled cross sections of CWD and determining the time since death, amount of decay and nutrient content. Our preliminary analysis demonstrates that maximum volumes of CWD and highest decay rates are reached approximately 100 years after a stand replacing disturbance. Nutrient analysis is in progress and results will be presented. CWD is important for conserving biodiversity and has the potential to play critical roles in forest nutrient dynamics, making it an important consideration under ecosystem based management strategies.

Carbon Flux of Down Woody Materials in Forests of the North Central United States

C. Woodall — Thu, 25 Jun 2009 10:30:00 PDT

Down woody materials (DWM) are a detrital forest ecosystem component that offset approximately 1 percent of annual CO2 emissions in the US. Across large-scales, DWM carbon (C) flux has only been simulated based on forest stand attributes (e.g., stand age and forest type). In order to provide an initial assessment of DWM C flux across large-scales, the annual change in fine and coarse woody C stocks and other attributes (e.g., coarse woody volume change) was assessed for forests in the north central United States. Using DWM inventory data from the USDA Forest Service’s Forest Inventory and Analysis program, it was found that DWM C stocks decreased across the study region at an average annual rate of 0.3 tonnes/ha (emission). Flux rates varied both in their amount and status (emission/sequestration) by forest types, latitude, and DWM component size. The decay of large fine and coarse woody debris without recruitment of freshly fallen pieces may explain the loss of DWM and emission of C. Given the differences in sample designs, early implementation of change estimation algorithms, and relatively low sample size, numerous future research directions are suggested.

Potential Future Dead Wood Dynamics in a Multi-Ownership Physiographic Province

R. S.H. Kennedy et al. — Thu, 25 Jun 2009 09:40:00 PDT

Dead wood is important to the processes, structural complexity, and biodiversity of forested ecosystems. Forest management may have unforeseen consequences to dead wood via the interaction of proposed activities with the legacy of past management, natural disturbance, and site productivity. We assessed the potential effects of future forest management for a 300-year period across a large (ca. 23,000 km2) forested region that contains numerous ownerships and land management strategies. To do this, we used an ecological gap model (ZELIG), a dead wood decomposition dynamics model (CWDM), live and dead wood data from a physiographic province-wide plot database, and ownership- and land-allocation-specific management prescriptions. Dead wood amounts were projected to increase over the simulation period across the region, primarily because conservation-oriented management approaches utilized on federal lands increased the volume of large logs and snags and number of large snags on federal lands. Large snags and logs decreased on forest industry lands as legacy dead wood derived from historical natural disturbance events was not replaced through management. The results of this study provide an estimate of maximum potential amounts of dead wood in the forests of the Coastal Province of Oregon, USA, under current policies, climate, and forest management. In cases where present day amounts of dead wood may be lower than the historical range of variability, conservation-oriented policies which are designed to maintain or increase dead wood amounts, such as the Northwest Forest Plan, may have a strong positive influence on dead wood abundance in parts of a region that are also under intensive management.

Estimating Species Loss Caused by Reductions in Coarse Woody Debris in Eastern Boreal Mixedwood Stands

T. Work et al. — Thu, 25 Jun 2009 10:50:00 PDT

In eastern boreal forests and elsewhere, use of residual forest biomass for bioenergy has been proposed and in some cases mandated. While advocated as ‘green’ energy, the long-term consequences of biomass harvesting on forest productivity and biodiversity are the subject of intensive research. One likely consequence of biomass harvesting is the reduction of residual coarse woody material (CWM) throughout forest stands. Saproxylic organisms, which require deadwood as either a habitat or resource, will likely be affected by these reductions. In this context, it is necessary to have a means to estimate the overall effect of reductions in CWM on species richness in order to develop ecologically relevant standards for biomass harvesting and to assess the feasibility of biomass harvesting in perspectives of sustainable forest management. Here we demonstrate an approach to estimating species loss as a function of reduced volumes using incidence based rarefaction. We used saproxylic diptera (flies) collected from in situ emergence cages placed on aspen and spruce CWM from a variety of decompositional stages in eastern boreal mixedwood forests as an example of this approach. We extrapolated incidence-based rarefaction curves based on 216 species and morph-species to observed CWM volumes in our study sites. These curves are then used to estimate species loss under different levels of biomass harvest. Two factors, species turnover among individual downed logs (inter-log variability) and the variability of species along the length of downed logs (intra-log variability) emerged as important factors in our approach. Intra-log variability was estimated empirically from subsequent sampling of log sections taken along the length of downed aspen and spruce logs. This approach can be easily adapted to other biodiversity studies and regions. It also provides a means to rapidly evaluate impacts of biomass harvesting that can be used in conjunction with empirical studies that manipulate CWM.

Log Decomposition Dynamics in Interior Alaska

J. Yarie et al. — Thu, 25 Jun 2009 11:10:00 PDT

Logs on and in the forest floor represent a potential large pool of carbon in forest ecosystems. The decomposition of the logs results in the release of stored carbon back into the atmosphere. It is currently thought that this release will take a substantial period of time however; these changes have not been measured in a large number of species in forested regions. From 1994 to 1996, a log decomposition monitoring study was started in a series of sites established in the successional turning points, using ecosystems dominated by alder, balsam poplar and white spruce on floodplain locations and aspen, birch and white spruce in upland locations. This study was set up using green logs from within the forest ecosystems. Fifteen logs were placed on the forest floor in each of six replicate sites to be sampled at years 0, 2, 5, 10, 15, 20, 25, 30 and subsequent 10 year intervals till year 100. The initial ten year results show large differences in the decomposition rates between the species. Currently the species with the highest decomposition rate is alder on floodplain sites, which has lost 62.5% of its total mass (wood and bark) in 10 years. The lowest rate was for white spruce on floodplain sites that lost 29.5%, or birch in upland sites which has lost 30.6% of its total mass in 10 years. This represents a loss of 59.4%, 29.5% and 28.2% of the carbon in the floodplain alder, white spruce and upland birch, respectively.

Woody Detritus as a Control of Forest Carbon Budgets

M. E. Harmon — Thu, 25 Jun 2009 11:30:00 PDT

It is becoming increasingly apparent that the role of woody detritus in forest carbon dynamics can no longer be ignored. Not only is woody detritus a major carbon store in forests, but its importance increases with disturbances. The factors controlling the production and decomposition of this material have been identified. However, how these factors interact and how the balance of these interactions changes from place to place has not been appreciated. The amount and nature of the woody detritus created and left by disturbance profoundly influences the pattern of net ecosystem carbon balance (NECB) over succession. The more woody detritus left by disturbances, the longer NECB remains negative. Since there is a feedback between the accumulation in live wood versus the losses from woody detritus, the average of NECB over succession approaches zero unless the nature of the disturbance regime changes. With increasing rates of disturbance being observed in North America, it is highly likely that losses from woody detritus decomposition will cause a shift from a positive NECB (sink) to a negative NECB over the next few decades. The timing and magnitude of this shift is highly uncertain due to the stochastic nature of disturbance and our very poor understanding of woody detritus decomposition.