Forest Disturbances and Climate Feedbacks in a Mixedwood Boreal Forest

Boreal forests play a critical role in global climate via important biophysical and biogeochemical feedbacks. Large-scale disturbances, particularly fire and harvesting, significantly affect these feedbacks by altering the surface and stand attributes, and can impact boreal forests’ role in the global climate system. Surface- and stand-attribute-driven feedbacks change rapidly in early successional stages, making them challenging to model. As the frequency and intensity of disturbances in boreal forests are predicted to increase, a vast landscape with proportionally more young forests is likely to result. Understanding these feedbacks during early stand development is thus more critical than ever before. Scarcity of data on key biophysical (e.g., albedo, soil temperature) and biogeochemical (e.g., soil greenhouse gas fluxes) processes during early stand development has been noted, particularly in mixedwood boreal forests. Using a series of micrometeorological towers in fire and harvesting chronosequences of a mixedwood boreal forest of northwestern Ontario, we studied combined effects of vegetation cover and climate warming on the surface soil (~2 cm depth) temperature in post-disturbance stands, and the patterns and drivers of surface albedo and soil CO2 and CH4 fluxes during early stand development stages in post-fire and post-harvest stands. A proxy-year analysis indicated that surface soil temperature in winter and spring was lower in a warm year compared to a baseline year, and the magnitude of this difference varied with vegetation cover (Chapter 2). Albedo differences between post-fire and post-harvest stands were most pronounced during winter and spring and primarily driven by stand age and species composition (Chapter 3). We also found that CO2 effluxes were lower in post-fire stands compared to post-harvest stands; post-fire stands were never a source, but some young post-harvest stands were a net source of CH4 . The magnitude in flux difference between post-fire and post-harvest stands varied with stand age and was affected by environmental variables such as soil temperature, moisture, pH, and litter depth (Chapter 4). These findings are critical for understanding dynamics in soil temperature, albedo, and soil carbon fluxes during early successional stages and useful for climate-smart boreal forest management.Ph.D