Approaches for reducing uncertainties in regional forest carbon balance

Accurate estimation of regional terrestrial ecosystem carbon (C) balance is critical in formulating national and global adaptation and mitigation strategies in response to global changes. Since the regional C balance cannot be measured directly, it has been estimated using various models. In such studies, errors often exceeded the magnitude of the estimated C balance due to two types of uncertainties: noninclusion of some important factors in the C cycle and the fact that the C balance is a small difference between several large fluxes that can each be determined with only a limited accuracy. In this study, we propose new approaches to reduce these uncertainties and implement them in an Integrated Terrestrial Ecosystem C-budget model (InTEC). To minimize the first type of uncertainties, InTEC considers all the major factors presently known to affect C balance (including climate, atmospheric CO2 concentration, N deposition, and disturbances). To reduce the second type of uncertainties, InTEC estimates the C balance from historical changes in these factors, relative to the preindustrial period. InTEC is built on the basis of widely tested Century C cycling model, Farquhar's leaf photosynthesis model, and age-NPP relationships, and is constrained by N cycling. As a general regional-scale terrestrial ecosystem C budget model, InTEC has so far been applied to Canada's forests [Chen et al., this issue]. The sensitivity analysis showed that these two new approaches reduce the uncertainty in the C balance of Canada's forests substantially.