Modeling Coarse Woody Debris Decomposition in Forests

Dead wood comprises nearly 20 percent of the forest biomass in the United States, ranging from several to hundreds of Mg per hectare; hence, it is an important component of the forest carbon (C) pool. Coarse woody debris (CWD) is the main woody detrital component due to its size and relatively slow decomposition rate. CWD decomposition (CWDD) can influence C and nutrient cycling in forest ecosystems due to its interactions with the soil. Accordingly, a tool for simulating CWDD is important for assessing forest C pools in response to changing abiotic conditions as well as management regimes. A process based C model has been developed to assess CWDD in forests that can be run independently or coupled to a forest C model to assess CWDD. The annual mass attenuation of CWD from the simulation follows an exponential decay that is mediated by climate, decomposer community, soil properties, wood type and size. A simulation using 70 climate datasets across 16 to 65 degrees latitude showed that the decay constant (k) increased with an increment in annual mean air temperature and precipitation, and that the k decreased with an increment in latitude. Snow cover influenced the decay rates of downed CWD due to the insulating effect of the snowpack. There were substantial differences in the decay rates of downed CWD in the presence of termites, and little effect on standing CWD. The difference in the decay rates in boreal and high mountain areas between standing and downed dead CWD was statistically insignificant, as decay was mediated primarily by fungi. However, the difference in decay rates in other areas between downed and standing CWD was due primarily to differences in CWD moisture content and temperature, when termites are absent. The decay rates from the model were not constant for downed and standing CWD because of temporal changes in temperature and precipitation. This model testing appears to effectively reflect decomposition processes, and is poised for validation with field measurements.