Modeling the Effects of Wildfire on the Canadian Boreal Forest

Changes in climate, atmospheric CO2 concentration and fire regimes have been occurring for decades in the boreal forest, with future climate change likely to increase fire frequency — the primary disturbance agent in most boreal forests. Previous attempts to assess quantitatively the effect of changing environmental conditions on the net boreal forest carbon balance have not taken into account the competition between different vegetation types on a large scale. We used a spatially explicit version of Biome-BGC with three competing vascular and non-vascular vegetation types to examine the effects of climate, CO2 concentrations and fire disturbance on net biome production, net primary production and vegetation dominance in 100 Mha of Canadian boreal forest (Figure 3-4). We found that:

1. the carbon balance of this region was driven by changes in fire disturbance from 1948 to 2005. Increased wildfire disturbance (D, red line) during this period caused the boreal forest to change from a weak carbon sink to a weak carbon source (Figure 1). The combined simulated effect of climate change, meteorology, and wildfire disturbance (MDC, black line) also simulated a change from a carbon sink to carbon source for the region.
2. Inter-annual variation in meteorology (M, blue line) affected the variability, but not the mean, of the landscape carbon balance, with precipitation exerting a more significant effect than temperature.
3. Increased fire frequency in the late twentieth century increased deciduous trees and mosses production at the expense of coniferous trees.
4. Poorly drained black spruce ecosystems decreased the variability of the landscape carbon balance, which suggests that increased climate and hydrological changes have the potential to affect disproportionately the carbon dynamics of these areas.

The charts below illustrate examples of output from the spatially explicit version of Biome-BGC used in these simulations.

The chart below depicts the difference in net ecosystem production between a simulation using historical disturbance, meteorological, and carbon dioxide data (MDC, 1948-2005) and one using mid-century disturbance, meteorological, and carbon dioxide (Base, 1948-1967).

The map below shows the carbon balance of central Canada in 2005 with yellow-red-green-blue indicating the continuum between carbon sources and sinks. The boreal forest is shown in light green over an NDVI composite image. White circles indicate 1989 wildfires and the subsequent re-growth.

The effect of increased fire frequency in the late twentieth century on deciduous trees and moss production at the expense of coniferous trees (finding #3 above) is illustrated in the map below – a map of evergreen needleleaf, primarily black spruce, net primary production for a 1 million km2 region of boreal forest in central Canada. Regions shaded blue denote lower values of NPP than those shaded orange or red. The large blue circles are areas burned by wildfire in the last 15 years. Evergreen needleaf productivity is low in these regions due to stand replacement with deciduous species such as aspen following fire.