Simulating impacts of water stress on woody biomass in the southern boreal region of western Canada using a dynamic vegetation model

Drought-related dieback of aspen-dominated woodland has been a persistent and possibly increasing phenomenon over recent decades in the southern boreal forests of western Canada. The Integrated BIosphere Simulator (IBIS) dynamic vegetation model was modified for Canadian ecosystems (hence “Can-IBIS”) and used to simulate effects of water stress on the woody biomass of aspen-dominated stands in the boreal mixedwood regions of Saskatchewan and Alberta. The modified model was evaluated using eddy-covariance measurements of CO2 and water vapor fluxes made at a forested site and a grassland site located in the study region. The tested model captured 74% of the variation in biomass growth trajectories at 13 boreal and 12 parkland field study sites; the mean difference between simulated and observed values was approximately 1100gCm−2. Under the combined influences of climatic variation and increasing atmospheric CO2 from 2000 to 2008, simulated values of net biomass growth were 544 and 240gCm−2 for the boreal and parkland study sites, respectively. Drought-induced biomass losses at the drier sites (in both boreal and parkland regions) were simulated to be 100–350gCm−2, corresponding to an annual modeled mortality rate of 5–7% during severe drought years. These results were consistent with field measurements and other statistical studies. Changes in biomass over the nine-year period varied with geographical location and corresponded to spatial variation in monthly values of the self-calibrated Palmer Drought Severity Index. We conclude that Can-IBIS can be used to investigate annual impacts of water stress on woody biomass growth, although cumulative physiological effects of multi-year droughts on tree mortality would benefit from improved simulation of subgrid-scale (soil texture-driven) processes. In particular, two areas for further development are: (1) calibration based on the results of soil surveys at fine spatial/temporal scales; and (2) biophysical experiments to refine the representation of water stress constraints on biomass turnover.