Biophysics of disturbances in the Canadian boreal forest
The boreal forest is the world’s largest biome spreading approximately 1.43 billion ha in a circumpolar complex of northern Eurasia and North America (Kasischke et al. 1995). In Canada, it occupies approximately 300 million ha (Stewart et al. 1997). Because of its large extent, this forest is a major economic sector of the boreal nations, harbors biodiversity, and affects global climate significantly. At the same time, this forest is more sensitive to and will be more affected by climate change than any other forests (Houghton et al. 1996). Therefore, it is very important to understand how boreal forests affect the climate and vice versa.
Stand initiation in the boreal forest is mainly triggered either by natural disturbances (fire, insect, wind throw) or by logging. Hitherto, a number of studies have addressed how the changing climate will affect productivity (Boisvenue & Running 2006) and the intensity of fire (Stocks et al. 1998) and insect (Volney & Fleming 2000) of this forest. However, logging and fire receive attention only recently.
Changes in land cover by fire or logging affect biogeophysical and biogeochemical processes that can affect regional or global climate. Using Global Climate Model (GCM) simulations a number of studies, for example Betts (2000), Govindasamy et al. (2001), Bala et al. (2007), Li et al. (2015), concluded that deforestation in high latitude actually cools the climate as opposed to conventional wisdom. This is because the conifer-dominated forests receive higher net radiation than other type of forests around the world. As deforestation opens the canopy, winter snow (high albedo) reduces net radiation at the surface layer leading to a net cooling.
Similarly, fire also opens canopy and, although there is a positive radiative forcing (RF) immediately after fire, which eventually dampens within a few years, reduces the amount of available net radiation at the surface. Integrating net radiation with the effects of greenhouse gas and aerosol, Randerson et al. (2006) showed that the net effect of fire in the boreal forest is cooling.
These models, however, are very much idealized and rely mainly on satellite-based indirect measurements. In addition to the limitations arising from sensor geometry, satellite-based measurements lack necessary temporal resolution to address diurnal variations of biophysical parameters in GCMs.
Although both radiative (for example, albedo) and non-radiative forcings (for example, evapotranspiration efficiency, surface roughness) govern net forcing after disturbances, most of the GCMs ignore the later (Davin & de Noblet-Ducoudre 2010; Bright et al. 2015). Using data from field measurements and considering both radiative and non-radiative forcings, some studies found very weak evidence of net cooling from deforestation in the boreal region (Lee et al. 2011; Bright et al. 2015).
Stand structure (for example, LAI, height) and composition (for example, percentage of conifer tree) are dynamic through the course of stand development. These structural and compositional features of a forest dictate both the radiative and non-radiative forcings. We did not find any GCMs considering stand age as the driver of forcings. Instead, the models used ‘biome replacement’ approach (replacing boreal forest with grassland to calculate RF) and a single-age stand with mature conifer trees. This approach seriously underestimates albedo from the boreal forest (Nelson et al. 2007). Additionally, the scales at which forestry activities (usually a few hectare) and a GCM (grid size usually 1 Km2) operate are different. Upscaling local perturbations to the GCM grids, adds uncertainty to the estimation of net forcings (Bonan 2008; Lee et al. 2011; Bright et al. 2015).
Even though some studies (for example, Randerson et al. 2006) addressed the biogeochemical effects of fire and logging on the net forcings from boreal forest, they are mainly confined to CO2 and aerosol. None of the GCMs include the most important of non-CO2 forcings: CH4. Incorporation of the non-CO2 forcings in estimation of net cooling/warming, might give us a different insight from the boreal forest and climate change perspectives.
Since the boreal forest is very important for Canada and for the globe, it is imperative to have a deeper understanding of the effects of disturbances on the radiative and non-radiative forcings to devise climate friendly management policy. To address a few of the aforementioned gaps, in this study I set up a series of micrometeorological towers in the post-fire and logging chronosequence sites in the boreal forest of northwestern Ontario, Canada.