Characterizing residual structure and forest recovery following high-severity fire in the western boreal of Canada using Landsat time-series and airborne lidar data

Post-fire regrowth is an important component of carbon dynamics in Canada's boreal forests, yet observations of structural development following fire are lacking across this remote and expansive region. Here, we used Landsat time-series data (1985-2010) to detect high-severity fires in the Boreal Shield West ecozone of Canada, and assessed post-fire structure for > 600 burned patches (> 13,000 ha) using airborne light detection and ranging (lidar) data acquired in 2010. We stratified burned areas into patches of dense (> 50% canopy cover) and open (20-50% canopy cover) forests based on a classification of pre-fire Landsat imagery, and used these patches to establish a 25-year chronosequence of structural development for each class. While structural attributes were similar between dense and open patches during the first ten years since fire (YSF), canopy cover (cover above 2 m) and stand height (75th height percentile) were significantly higher (p < 0.001) for dense patches by the end of the chronosequence (20-25 YSF), suggesting that differences in site productivity were driving patches towards pre-disturbance structure. Our results suggest that growing space remained in stands at the end of the chronosequence, and therefore stem exclusion was not yet reached, as canopy cover was significantly lower (p < 0.001) for patches at 20-25 YSF (mean = 41.9% for dense, 18.6% for open) compared to patches with no recorded burns (mean = 63.3% for dense, 38.6% for open). The lasting impact of high-severity fire on structure was further confirmed by estimates of stand height, which were approximately half as tall for patches 20-25 YSF (4.9 m for dense, 4.2 m for open) compared to patches with no recorded burns (9.8 m for dense, 7.7 m for open). Additionally, we assessed the structural complexity of burned stands using measures of canopy roughness (i.e., rumple) and the distribution shape of lidar returns (i.e., skewness and kurtosis), which provided evidence of young, even-aged structure once a new overstory was formed. As forest inventories are not routinely conducted across Canada's northern boreal, the fusion of Landsat time-series and airborne lidar data provides powerful means for assessing changes in forest structure following disturbance over this large forested area.