Litter and dead wood dynamics in ponderosa pine forests: Application of a simple compartment model.
Hall, Sonia*,1, Burke, Ingrid1, Hobbs, N2, 1 Graduate Degree Program in Ecology, Fort Collins, CO, USA2 Natural Resources Ecology Laboratory, Fort Collins, CO
ABSTRACT- Detritus in fire prone forests can be a large carbon pool and can control fire behavior, whose frequency can in turn control the detritus' long-term dynamics. Forests where fire has been excluded for over a century are ideal for studying the dynamics of detritus (litter and dead wood of different sizes). Existing chronosequence studies qualitatively describe the trends in detritus following disturbance, or use simulation models to quantify its accumulation under specific disturbance regimes. Our objective was to understand the long-term patterns of accumulation of detritus after fire, the differences in dynamics between pools, and the timing of maximum pool sizes relative to historical fire intervals. We developed a continuous time model of detrital turnover based on three pools (aboveground live tree biomass, standing dead, and detritus). We modeled net primary productivity with a Gompertz function, and all other fluxes (mortality, fall and decomposition) as constant proportions of their source pools. We estimated model parameters for each type of detritus using data from 31 sites in the ponderosa pine forests in northern Colorado, ranging in time since fire from 1 to 159 years. The model explained 39-80% of the variability. The litter pool followed an S-shaped curve, leveling off after 120 years. All woody pools had an initial peak at 7 to 42 years after fire, then decreased and leveled, or reached a minimum and increased again. The model reproduced these different dynamics, suggesting that the difference in pattern do not reflect the occurrence of different processes. They likely reflect differential fire effects on needles and woody material. The estimated rate constants for fall were similar to published values, though those for mortality and decomposition were lower and higher, respectively. The extreme pool sizes occurred in time frames that are within the range of historical fire intervals in these forests, supporting the mixed severity fire regime hypothesis.
Key words: long-term dynamics, litter, dead wood, time since fire
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