Coarse woody debris decomposition assessment tool: Model development and sensitivity analysis

Abstract

Coarse woody debris (CWD) is an important component in forests, hosting a variety of organisms that have critical roles in nutrient cycling and carbon (C) storage. We developed a process-based model using literature, field observations, and expert knowledge to assess woody debris decomposition in forests and the movement of wood C into the soil and atmosphere. The sensitivity analysis was conducted against the primary ecological drivers (wood properties and ambient conditions) used as model inputs. The analysis used eighty-nine climate datasets from North America, from tropical (14.2° N) to boreal (65.0° N) zones, with large ranges in annual mean temperature (26.5°C in tropical to -11.8°C in boreal), annual precipitation (6,143 to 181 mm), annual snowfall (0 to 612 kg m-2), and altitude (3 to 2,824 m above mean see level). The sensitivity analysis showed that CWD decomposition was strongly affected by climate, geographical location and altitude, which together regulate the activity of both microbial and invertebrate wood-decomposers. CWD decomposition rate increased with increments in temperature and precipitation, but decreased with increases in latitude and altitude. CWD decomposition was also sensitive to wood size, density, position (standing vs downed), and tree species. The sensitivity analysis showed that fungi are the most important decomposers of woody debris, accounting for over 50% mass loss in nearly all climatic zones in North America. The model includes invertebrate decomposers, focusing mostly on termites, which can have an important role in CWD decomposition in tropical and some subtropical regions. The role of termites in woody debris decomposition varied widely, between 0 and 40%, from temperate areas to tropical regions. Woody debris decomposition rates simulated for eighty-nine locations in North America were within the published range of woody debris decomposition rates for regions in northern hemisphere from 1.6° N to 68.3° N and in Australia.

Coarse Woody Debris Decomposition Assessment Tool: Model validation and application

“…The efficiency (E = 0.87) of model calibration for the calculated mass loss using the disks collected in situ showed that the model performance was excellent (E�0.75), and reflects a difference in model performance efficiency for model calibration against data obtained from different methods (see Discussion below). These performance metrics from model calibration step indicate that the model is applicable for assessing CWD decomposition across the range conditions on the EFs, and it is in line with design objectives [29].…”

“…The model has also been tested with respect to wood properties, including species group (hardwood and softwood), wood density (0.1 to 1.0 g cm -3 ) and size (4.0 to 45 cm in diameter). The results from the model assessment showed that CWD decomposition was highly sensitive to climatic factors, elevation, CWD properties and position (standing vs downed) [29]. Accordingly, the assessment affirms that CWDDAT should be a potentially effective tool for estimating CWD dynamics in forests.…”

“…Similarly, testing across the highly variable boreal zone is warranted [5,48,49]. Overall, this work demonstrates that the CWDDAT [29] is a useful tool for assessing CWD decomposition.…”

“…When considering the role of CWD decomposition with respect to the various forest C pools, environmental fluxes across habitats, and a changing climate, it is necessary to use a mechanistic model that reflects inherent biogeochemical processes. The CWDDAT (Coarse Woody Debris Decomposition Assessment Tool) has been developed to simulate CWD decomposition in forests by incorporating organic matter decomposition processes and ecological drivers regulating CWD mass loss as well as the fate of the wood C for climatic conditions that range from the tropics to the boreal zone [29].…”

“…Here we report on: (1) model calibration using observations from four sites to affirm whether or not the model performs within the design objectives [29]; (2) model validation using observations from five sites to assess whether or not the model performs as well with different observational inputs as it did under testing and calibration conditions, and (3) model application by assessing decomposition of CWD left by Hurricane Hugo on the Santee Experimental Forest (Santee EF) in South Carolina in 1989. To achieve the first two objectives the observations from nine sites on the FACE (Free Air Carbon Dioxide Enrichment) Wood Decomposition Experiment (FWDE) are used to provide decomposition response data (6 years) for three tree species.…”