Modeling Leaching as a Decomposition Process in Humid Montane Forests

Forests in the White Mountain region (New Hampshire, USA) range from northern hardwoods and pine at low elevations to spruce–fir at middle elevations, subalpine fir, and krummholz on mountain peaks. We studied landscape-scale effects of gradients in forest cover type, litter chemistry, rates of litter input (including fine woody litter), temperature, and moisture on forest-floor pools and input–output fluxes of C and N. We coupled a new model of decomposition processes to a Geographic Information System (GIS) and applied it in 10-ha cells in the White Mountain National Forest. We sought to synthesize current understanding of forest-floor leachate qualities and quantities in humid forests with measured changes in carbon fractions (“lignin,” “cellulose,” and “extractives”) in decomposing litter. We included N dynamics as transfers of N among C fractions, including the production of dissolved organic N in leachate from the forest floor. We calibrated fluxes of dissolved organic C and N (DOC and DON) to measurements from two stands at the Harvard Forest, Massachusetts, USA. We calibrated rates of humus decay to two sites at the Hubbard Brook Experimental Forest. By including leaching as a mechanism of mass loss from the forest floor, we were able to calculate CO2 mineralization fluxes by difference. Predicted masses of forest floor were higher and peaked over a broader elevational range in spruce–fir forest than in hardwoods, due primarily to a slower decay rate of humus in coniferous forests. DOC and DON fluxes were higher in coniferous forests, while CO2 fluxes were higher in hardwood forests. Predicted residence times for C in the forest floor were longer, and organic N concentrations were higher, in spruce–fir forests than in hardwood forests. Along a generalized elevational ecotone from hardwood to spruce–fir forests in the region, predicted mass and N in the forest floor increased and DOC and DON fluxes increased, while CO2 fluxes decreased.