Forests of the northeastern US are experiencing rapid shifts in tree species composition, largely due to invasions of non-native insects and diseases that attack host species and cause their replacement by competing species. We are exploring how these species changes will affect forest carbon (C) and nitrogen (N) cycling using a new forest ecosystem model (called Spe-CN) that is parameterized for individual tree species. The model simulates C and N pools in vegetation, downed wood, litter, humus and active and passive SOM. Species differentiation decreases as litter moves through the decay sequence. Litter pools are separated by tree species, humus pools are separated into coniferous vs. deciduous fractions, and there is no species differentiation in the SOM pools. Decomposition constants (k) are specified for lignin-associated and non-lignin-associated fractions of fresh litter, coniferous and deciduous humus pools and the active and passive SOM pools. This presentation will focus on which decomposition-related parameters control the accumulation of C and N in the forest floor, and how our knowledge (or lack thereof) of these parameters influences our ability to model the effects of species change.
Results/Conclusions
The model predicts that pools of C and N in the forest floor can change markedly as tree species composition changes, with the direction, magnitude, and timing of the change dependent on the characteristics of the declining tree species and their replacements. However, we find that decomposition rates of fresh litter have little impact on forest floor C and N accumulation. (One caveat is that few data are available for root decomposition, so rates are not well constrained.) Rather, what matters most for forest floor C accumulation is the later stage of decomposition, when the organic matter from the litter becomes humus in the soil. We know very little about how humus decomposition rates vary in stands dominated by different species, or about factors controlling that variation. We also have limited knowledge of the controls on microbial immobilization of N released from decomposing humus, which is important for N accumulation in the forest floor. To improve modeling of the effects of species change on C and N dynamics, we suggest that additional leaf litter bag studies will have only marginal value; instead we need empirical studies of factors regulating root decomposition and rates of humus decomposition and N mineralization.