Characterizing Fire-Induced Forest Structure and Aboveground Biomass Changes in Boreal Forests Using Multitemporal Lidar and Landsat

Wildfire is the dominant stand-replacing disturbance regime in Canadian boreal forests. An accurate quantification of postfire changes in forest structure and aboveground biomass density (AGBD) provides a means to understand the magnitudes of ecosystem changes through wildfires and related linkages with global climate. While multispectral remote sensing has been extensively utilized for burn severity assessment, its capacity for postfire forest structure and AGBD change monitoring has been more limited to date. This study evaluates the interactions among burn severity, forest structure, and fire-return intervals for two representative sites in the western Canadian boreal forest. We adopted burn severity measurements from Landsat to characterize the heterogeneity of wildfire effects, while vertical forest structure information from Lidar was utilized to inform on realized forest changes and carbon fluxes associated with fire. Dominant trees in biomass-rich stands showed higher tolerance to low- and moderate-severity wildfires, while understory vegetation in these same stands showed a severity-invariant response to wildfires indicated by high vegetation mortality regardless of burn severity levels. Compared to a site without previous burn, canopy height and AGBD experienced lower magnitudes of change after subsequent wildfires, explained by a negative feedback between high frequency wildfires and biomass loss ($\overline {\Delta \mathbf{Canopy}\ \mathbf{Height}} $_{single wildfire} = 3.03 m; $\overline {\Delta \mathbf{Canopy}\ \mathbf{Height}} $_{successive wildfire} = 2.47 m; $\overline {\Delta \mathbf{AGBD}} $_{single wildfire} = 8.40 Mg/ha; $\overline {\Delta \mathbf{AGBD}} $_{successive wildfire} = 6.69 Mg/ha). This study provides new insights into forest recovery dynamics following fire disturbance, which is particularly relevant given increased fire frequency and intensity in boreal ecosystems resulting from climate change.