Species identity and tree size drive residual tree mortality in island remnants in burned and harvested boreal forests

Ecosystem-based Forest Management (EBM) is a global paradigm with different forms of implementation. In North America, a key aspect of EBM is the emulation of natural disturbances such as wildfire, with retention forestry representing one dominant approach to implementation. Retention is intended to reduce the contrast between ecological legacies present in fire-disturbed and harvested stands by leaving unharvested patches or island remnants. These island remnants provide refugia for forest species, accelerate forest regeneration, and supply coarse woody material over time. However, the objectives of leaving retention patches could be compromised by excessive post-disturbance tree mortality, which may erode canopy cover in patches. Comparisons of residual tree mortality in dispersed retention or partial harvest to reference forests and clearcuts exist, but rarely have comparisons to island remnants created by wildfires been made, despite this being the model for these patches. Here, we sampled island remnants in burned and harvested areas in aspen-dominated mixedwood forests in Alberta, Canada, a decade after disturbance. We sought to: (i) test for differences in post-disturbance tree mortality in island remnants (fire versus harvest) (ii) determine if forest island edges experience higher tree mortality than plots located in the interior, and (iii) examine whether tree species, tree size, and stand density influence tree mortality patterns. Post-disturbance tree mortality was quantified using visual field surveys. Additionally, we dated a sub-sample of trees to quantify temporal patterns of tree mortality. We found that harvest-created island remnants did not exhibit higher residual tree mortality than similar-sized fire island remnants. Edge effects also had no impact on tree mortality, irrespective of disturbance type. Aspen (Populus tremuloides) had higher mortality than spruce (Picea glauca), and small-sized trees had a higher probability of mortality than large-sized trees. While previous studies have reported elevated rates of mortality for retention trees in relation to reference stands, we found similar mortality rates for trees in fire and harvest island remnants. These results support the continued implementation of retention forestry using island remnants within the scope of EBM in Alberta to promote forest recovery and resilience following disturbances.

Automated summary

Ecosystem-based Forest Management (EBM) is a global paradigm that has different implementations. A study in Alberta, Canada found that retention-created island remnants and fire-created island remnants have similar tree mortality rates. Edge effects and disturbance type did not significantly impact tree mortality. Aspen had higher mortality rates compared to spruce, and small-sized trees were more likely to die than large-sized trees.