Document: JOH-3-63-32

Land-use history and carbon storage in forest ecosystems.

CASPERSEN, J.* and S.PACALA

Princeton, NJ 08540, USA ¹

Abstract:
Evidence from a variety of methods has led to a consensus that there is a large carbon sink in the northern hemisphere. However, there is little consensus concerning the processes responsible for this sink. Numerous experimental studies suggest that growth enhancement through carbon dioxide and nitrogen fertilization is insufficient to account for the magnitude of the carbon sink. Thus, there has been increasing focus on land-use changes as an alternative explanation for the sink, particulary land-use changes in forested regions with a history of widespread agricultural abandonment. Estimating the relative contribution of regrowth and enhancement to the northern hemisphere carbon sink and predicting future carbon dioxide concentrations requires an assessment of how the history of land abandonment affects current rates of carbon accummulation and how long it will continue to do so. We have used the Forest Inventory and Analysis (FIA) dataset to quantify how aboveground net ecosystem production (ANEP) changes during stand development and evaluate the mechanisms driving changes in ANEP. Our analysis shows that ANEP peaks rapidly (within 10-20 years) and then gradually declines until stand biomass equilibrates (within 200-500 years). Due to the disparate time-scales of the increasing and decreasing phases of ANEP, the rate of decline largely determines the ANEP of secondary forests and how long it will take forests to recover from past clearance. Yet, there is little agreement about what mechanisms govern the rate at which ANEP declines as stands develop. One demographic hypothesis is that ANEP declines as the total amount of carbon lost through tree mortality increases as trees get bigger. Thus, forest biomass equilibrates when a stand reaches a stable size distribution. Three other ecosystem-level hypothesis suggest that tree growth declines as stands develop, due to increased plant respiration, decreased nutrient cycling or decreased stomatal conductance. Our analysis shows that the decline of ANEP is driven largely by demographic changes in carbon loss rather than changes in uptake (tree growth). The results of our analyses have important implications for efforts to model the terrestrial carbon cycle and predict the trajectory of atmospheric carbon dioxide.

Keywords: Forest, Carbon, FIA, land-use

This abstract is being presented at: 3:15 PM in session:
Oral Session #52: Carbon Storage in Ecosystems.