Above-and Belowground Carbon Allocation in Post-Fire Lodgepole Pine Forests: Effects of Tree Density and Stand Age

Abstract

Validating the different components of the carbon (C) budget in forest ecosystems is essential for developing allocation rules that allow accurate predictions of global C pools and fluxes. In addition, a better understanding of the effects of natural disturbances on C cycling is critical - particularly in light of changes in disturbance regimes that may occur with alterations in global climate. This study investigates the indirect effects of fire on C cycling in lodgepole pine (Pinus contorta var. latifolia Engelm. ex Wats.) stands in Yellowstone National Park by examining above- and belowground C pools, fluxes and allocation patterns in post-fire stands that vary in tree density and stand age (four forest types: low (<1000 trees/ha), moderate (7,000-40,000 trees/ha), and high tree densities (>50,000 trees/ha) in 13-yr­old stands; and ~110-yr-old mature stands). Above­ and belowground biomass were estimated to investigate the effect of tree density on biomass partitioning in young stands. The effect of tree density on soil-surface C02 efflux (Fs) and microbial biomass C (Cmic) in young stands was also examined, and data from mature stands were used as a proxy of pre-fire conditions to estimate the extent of ecosystem recovery 13 years after a stand replacing fire. Finally, the effects of tree density and stand age on ecosystem C pools, fluxes and allocation patterns were investigated. Partitioning of biomass to belowground increased with tree density in young stands primarily as a result of inherent differences associated with tree size, not competition. Fs and Cmic increased with tree density in young stands and with stand age, and both Fs and Cmic were correlated exclusively with biotic variables. These findings support recent studies demonstrating the prevailing importance of plants in controlling overall rates of Fs, and suggest that increased decomposition of older, recalcitrant soil C pools is relatively unimportant following fire. Fire, through influences on tree density and stand age, has important and lasting effects on the magnitude of C pools and fluxes in lodgepole pine ecosystems. However, results presented here suggest that overall C allocation patterns following fire are independent of tree density and stand age.