Exotic pine forestation shifts carbon accumulation to litter detritus and wood along a broad precipitation gradient in Patagonia, Argentina

Abstract

Carbon dioxide emissions to the atmosphere from human activity continue to increase, and accordingly, strategies of biological carbon (C) sequestration in terrestrial ecosystems have been proposed. Forestation projects have garnered wide public support, and research has focused principally on how soil C storage is affected. Nevertheless, our mechanistic understanding of how forestation, particularly with exotic woody species, affects ecosystem processes is not well understood. We took advantage of a land-use change in Patagonia, Argentina, that involved the simultaneous planting of a single conifer species (Pinus ponderosa) along a broad precipitation gradient [250–2200 mm mean annual precipitation (MAP)], replacing natural ecosystems from semi-arid steppe to broadleaf forest. Comparing C fluxes and stocks in five paired natural and planted forest sites during three consecutive years demonstrated that aboveground net primary production (ANPP) was consistently greater in forested areas along the gradient, while litter decomposition markedly decreased. Dramatic increases in leaf litter detritus, coupled with increased aboveground woody biomass, contributed to identical levels of C accumulation in pine plantations from 250 mm to 1350 mm MAP, without significantly detectable differences in surface soil C. The replacement of intact forest in the most humid site resulted in large decreases in vegetation C pools. The implications for ecosystem C cycling suggest that inhibition of C turnover, along with the aboveground woody growth, are key variables contributing to the observed patterns of C accumulation from exotic pine forestation along this precipitation gradient. Given the transient nature of these C stocks, vulnerable to loss as CO2 due to climatic or anthropogenic disturbances, these changes may not contribute to long-term C sequestration in these ecosystems. The conversion of natural ecosystems as a management tool for C mitigation should include a consideration of the realized sequestration potential but also the unintended consequences for changes in both C inputs and C turnover that determine the ecosystem C balance, as well as potential effects on biodiversity and long-term ecosystem functioning.