Annual dynamics and resilience in post-fire boreal understory vascular plant communities

Abstract

Boreal forests in western North America are considered to be resilient to wildfire disturbance, demonstrated by paleoecological evidence and adaptive regenerative traits possessed by many species. However, little is known about drivers of fine-scale temporal changes in understory communities in boreal forests immediately following fire. Knowledge of these changes, and their relationships with burn severity and pre-fire forest stand conditions, could help us determine recovery of forests as wildlife habitat. Such information is urgently needed in the face of climate warming-induced changes in fire frequency and severity. We used a high-quality, long-term dataset of annual measurements of understory vascular plant communities in sub-arctic boreal forest stands dominated by jack pine (Pinus banksiana), black spruce (Picea mariana), or a mix of the two in the Northwest Territories, Canada. Here, we describe the initial 10years of annual post-fire understory plant community dynamics and assess the important drivers shaping understory composition during this critical period of post-disturbance community assembly. First, we determined the relative importance of burn severity, pre-fire forest type, bare ground, woody debris, and number of years post-fire on understory species richness and composition dynamics following fire. Second, we explored annual dynamics in these communities and determined if compositional change was directional and predictable over time. We found that pre-fire forest type, burn severity, bare ground, woody debris, and number of years post-fire were important predictors of post-fire species richness and composition. Pre-fire forest type explained the greatest variation in understory plant composition, followed by burn severity. Across forest types, most species established within 1–3years following fire and initial species composition determined directional changes in composition. Our results suggest that targeting monitoring efforts in the years immediately post-fire may be sufficient to understand forest successional direction with respect to composition and the important drivers of those changes over the first decade post-fire. However, the recent and ongoing impacts of climate change in boreal regions of western North America leads to uncertainty surrounding the continued ability of these forests to demonstrate resilience under an altered fire regime so these interactions should continue to be considered across a range of forest types and burn severities.