Following mountain pine beetle outbreaks, the aerial seedbanks of serotinous lodgepole pine are vulnerable to relocation from the canopy to the ground that may affect postfire seedling establishment. Branch breakage places cones on the ground during fire, which exposes cones to longer flame residence times, ignition, and combustion. We asked the overarching question, how is lodgepole pine regeneration influenced by wildfire burning through gray-phase outbreak conditions? To answer this question, we quantified the effect of four potential contributing factors to variability in seedling recruitment: (1) cone retention in canopies, (2) seedbed availability, (3) beetle outbreak severity, (4) snag fall, and (5) postfire climate. Our field investigation was based on three fires that burned through high mortality, gray-phase lodgepole pine dominated forests in 2012, 2013, and 2014 across central interior British Columbia, Canada. These fires provided a natural experiment to evaluate lodgepole pine regeneration in response to beetle-fire interactions within the epicenter of the outbreak. In 2016, we inventoried 63 plots that spanned gradients of outbreak severity, burning conditions, and fire severity, plus 20 unburned plots. We used generalized linear models to evaluate seedling density against a set of ten candidate variables associated with our four contributing factors.
Results/Conclusions
Our analyses affirm that lodgepole pine seedling density is influenced by variables from three of our five potential contributing factors. Significant relationships were included using α=0.05. Cone retention in canopies demonstrated a direct positive relationship with seedling density, which signifies the importance of the canopy seedbank. Seedlings declined with increasing outbreak severity (i.e., tree mortality from beetles) under severe crown fire, which may be a function of snags torching and/or cone relocation to the ground. There was suggestive evidence (α=0.1) that seedlings declined with increasing snag fall during severe surface fire, which indicates that cones located near the ground may be vulnerable to flame exposure. Seedling density was over four times higher in plots where wildfire burned through beetle outbreak compared to unburned plots with only beetle-killed trees emphasizing that wildfire is a critical mechanism for seed release and lodgepole pine recruitment following beetle outbreak. While outbreak severity reduces postfire seedling establishment under certain burning conditions, it still results in more recruitment than unburned patches with a single disturbance of beetle outbreak. Further research is needed to determine if successional trajectories will progress toward the mature forest composition and structure that preceded the most recent beetle outbreak and large wildfires.