Heterotrophic respiration from the decomposition of organic soil and litter (non-living distinguishable plant material) comprises one of the largest fluxes of carbon on earth, second only to net primary production, and is roughly six times the magnitude of the flux from fossil fuels. Field-based methods have been helpful in constraining estimates of litter production, a large fraction of heterotrophic respiration, but simulation models are necessary to quantify how feedbacks from climate and vegetation dynamics affect litter production. However, scaling from forests to globe has proved problematic. Recently, efforts have been made to solve scaling issues in process-based ecosystem models by representing spatial heterogeneity in ecosystem production through the use of age-structured models. We introduced a form of ecosystem demography into the Lund-Potsdam-Jena (LPJ) dynamic global vegetation model using a time-since-disturbance discretization of forest stands to track successional dynamics that incorporate feedbacks from changes in the availability of important resources (space, light, water, nutrients). Our goal was to evaluate (i) the effect of forest stand-age on modeled litter production, and (ii) the impact of land use change and climate on litter production in context of age-structured forests.
Results/Conclusions
We performed idealized simulations of land use change at the scale of a grid-cell (0.5°), across six biomes. First, primary forest was converted to pasture by deforestation (5% of grid-cell yr-1 for 5 years), followed by simultaneous creation of secondary forest by abandonment of pasture and creation of new pasture by deforestation (both in equal magnitudes at 5% of grid-cell yr-1 for 5 years), and a final creation of secondary forest by abandonment of all pasture. The idealized simulation resulted in secondary forests spanning age-0 to age-6, and one primary forest (oldest). The simulation used transient climate (1901-2013) from CRU-NCEP as inputs. We found a 5-fold increase in litter production after the creation of secondary forests, being larger in younger versus older forests, which resulted from rapid establishment of trees and grasses, followed by self-thinning of plant density due to competitive exclusion. During years with lower precipitation, tree mortality reduced stand density to a greater degree in older forests than in younger stands. The reduced stand density caused a sequence of rapid establishment and competitive exclusion that drove litter production, which was higher in older versus younger stands. Our study revealed that the dynamics of litter production changes considerably when representing ecosystem heterogeneity by an age-structured model.