Large-Scale Modeling Shows Little Impact of 20th-Century Changes in Temperature and Fire on the Central Canadian Boreal Forest

Boreal forests are predicted to experience relatively large climate change (IPCC 2001), and the fire dynamics of this region may not be in equilibrium with the changing climate (Flannigan et al. 1998, Kasischke & Stocks 2000). Here we investigate how well a biogeochemical model can simulate observed 20^{th}-century changes in the structure and function of the boreal forest. Biome-BGC was used to simulate a 1000 km x 1000 km section (6-8% of the global boreal forest) of central Canadian forest at 1 km2 resolution. Historical climate, disturbance, and CO2 forcing data were used to drive the model; three dynamic vegetation types (evergreen needleleaf tree, broadleaf deciduous tree, and moss) were employed. Soil type, drainage, and other site conditions were represented as accurately as available data allowed. Assuming that the forest was at a carbon-neutral steady state in 1948, CO2 and disturbance frequency changes had small (<20 g C m-2 yr-1) and opposite effects on forest C balance by 2005; in particular, the C losses from more frequent fires were almost balanced by the C uptake of post-disturbance deciduous forests. Precipitation changes had the largest effects on C balance, with the overall forest shifting from being neutral to a small source of C, while interannual variability increased. Such a shift is smaller than can be measured using current biometric and eddy covariance techniques.