Estimating Climate-induced Variability in Carbon and Water Fluxes in the Upper Oyster River Watershed, British Columbia

Forest carbon budgets are commonly estimated using empirical approaches. Such models are not responsive to inter-annual variability in climate, limiting their ability to predict potential carbon cycle responses to climate change. In this study, we have revised a radiation-use efficiency model, Physiological Principals Predicting Growth (3-PG), to be compatible with the carbon pool structure and disturbance scheme of the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) in order to investigate climate-induced variability in forest carbon cycling in British Columbia, Canada. The model was evaluated by comparing simulations with independent estimates of total aboveground biomass (AB), evapotranspiration (ET) and gross primary production (GPP) in stands of Douglas-fir within a 25 square km area within the upper Oyster River watershed on the east coast of Vancouver Island. The model was able to reproduce the salient features of variability in AB, including regeneration and overmature decline, as shown in growth and yield curves. Simulated annual ET was mainly driven by net radiation, but was also positively correlated with summer air temperature. Simulated annual GPP was positively correlated with annual average air temperature and soil water content, stemming from strong relationships during spring and summer, respectively. Inter-annual variability was assessed based on the standard deviation of annual total ET and GPP and compared with eddy covariance (EC) measurements, spanning 1998-2006. Inter-annual variability in simulated ET and GPP was 15 mm and 149 g C m-2, which closely matched EC-based estimates of 13 mm and 115 g C m-2. These results suggest that the model is a promising tool for further analysis of the effects of climate on long-term carbon cycling within the region and comparison with empirical estimates derived from CBM-CFS3.