Forest resilience to wildfire and harvesting: a comparison of understory plant diversity in island remnants a decade after disturbance

Background/Question/Methods

An important feature of ecosystem-based forest management in the boreal mixedwood of northern Alberta is the emulation of natural disturbances such as wildfire. Post-fire landscapes typically include unburned or partially burned patches of forest (“remnants”). Remnants are thought to provide heterogenous structure, and support understory forest communities within a disturbed landscape. While forest managers leave remnant patches within harvest blocks to emulate the structural effects of fire, it is unknown if they are effective analogues. Our objective was to examine if harvest and fire remnants are effective at maintaining understory vascular plant communities in a post-disturbance landscape, and to determine if they exhibit differences in structure and plant diversity. We further investigated if changes in remnant structure caused by tree mortality were associated with differences in understory microclimates and/or understory vegetation communities. We sampled in three harvested and three burned areas ~a decade after disturbance, and within each area sampled 3-7 island remnants. For each remnant, a “reference” (undisturbed continuous forest) was identified. At each island remnant and reference, the edge of the forest and the interior were sampled for collected microclimate, tree, and understory vegetation data that were analyzed using a combination of taxonomic and trait-based approaches.

Results/Conclusions

We found that fire and harvest remnants support similar plant communities to those of undisturbed forest. Remnants in both fires and harvests had broadly similar richness, cover and diversity of understory species compared to adjacent undisturbed forest. We did however find differences between the edges of fire and harvest remnants: Fire remnant edges had lower percent cover of invasive and early seral species than harvest remnants, but cover was not strongly linked to canopy structure as expected. We found positive associations between canopy cover, tree basal area and late-successional plant species abundance, and also linked canopy cover and tree basal to lower microclimatic temperatures. These findings support the practice of patch retention within clearcuts, finding them to be effective analogues to fire remnants. We confirm that both fire and harvest remnants are effective at providing a forest structure and microclimatic conditions similar to those of undisturbed forest. We believe this research will help empower forest managers to make harvest decisions that enable the forests of the future to retain biodiversity and function effectively.