Pre-fire deciduous tree species drive forest regeneration after wildfire in interior Alaska

Abstract:

As climate has warmed, wildfires in interior Alaska have been increasing in size, frequency and severity, which may shift forest composition from historically dominant black spruce (Picea mariana) to deciduous tree species (Populus tremuloides and Betula neoalaskana) after fire. While deciduous species are less flammable than black spruce, deciduous forests also burn. However, drivers of post-fire regeneration in these forests are not well understood. The goal of our research was to understand factors that determine post-fire regeneration across a gradient of spruce- to deciduous-dominated forests. We hypothesized that post-fire relative tree density would be dominated by deciduous trees 1) when pre-fire relative deciduous tree density was high because of their high capacity for post-fire resprouting, 2) where residual soil organic layers (SOL) were thinnest because exposure of mineral soil promotes germination, and 3) when post-fire environmental conditions were moist and warm. To test these hypotheses, we assessed pre- and post-fire species composition, density, and basal area of 42 recently burned (6–16 years since fire) forest stands in interior Alaska in the summer of 2019. We combined this with data collected in 2012 from 40 stands that burned in 2004. We used structural equation modeling (SEM) to test our hypotheses.

Almost all stands that had pre-fire relative deciduous tree density >30% regenerated to deciduous dominance (>50%) after fire. Even when pre-fire relative deciduous density was <30%, 77% of stands regenerated to deciduous dominance. Pre-fire relative deciduous density was also the most important direct factor influencing regeneration in the SEM. We found no evidence that post-fire relative deciduous dominance was affected by SOL thickness. On the other hand, we found that climate moisture index in August three years after fire had a positive correlation with post-fire relative deciduous density. We conclude that the most important driver of post-fire regeneration across this species compositional gradient is pre-fire relative deciduous dominance. Even a small fraction of deciduous trees in the pre-fire stand can drive a state change in forest composition after fire.