Serotinous forest resilience in a warmer and more fire-prone world

Abstract

Warming climate and increased fire activity are expected to lead to decreased capacity of woody plant-dominated ecosystems to recover following fire (i.e., the erosion of resilience) in ecosystems globally. Systems characterized by stand-replacing fire regimes and dominated by serotinous plants (plants that release seeds from cones when heated by fire) may be particularly at risk. Two key conditions for resilience to fire in systems dominated by serotinous species are the sufficient production of cones following one fire and prior to the occurrence of subsequent stand-replacing fire (reburn) and climate conditions suitable for recruitment in the first post-fire year. Expected increases in fire frequency, hot and dry conditions during the inter-fire period, and occurrence of post-fire drought may lead to significant decreases in serotinous plant population persistence (interval squeeze). While the interval squeeze has been demonstrated for some serotinous species (e.g., Banksia spp. in Western Australia), key knowledge gaps exist regarding resilience mechanisms of serotinous species in North American forests and how populations may respond in a warmer future with more fire. My dissertation used closed-cone pine (Pinus attenuata and P. muricata) forests of California, USA as a study system to empirically test for evidence of the interval squeeze. Specifically, I assessed the developmental trajectories of fuels and cones over a chronosequence of three decades since stand-replacing fire to understand how reburn and population self-replacement potential develop over time. Building on these trajectories, I assessed the effects of fire interval and post-fire climate on in situ post-fire recruitment within the first two years post-fire, using a plot network of stands that burned in 2018, on intervals from 6 - 31 years. I found that 1) fuel development occurs rapidly in these forests, with fuels available to support a high severity reburn by ~10 years post-fire, 2) canopy seed bank development to support stand self-replacement occurs by ~15 years post-fire, 3) in situ post-fire recruitment was present following all fire intervals, and reaches stand self-replacement for fire intervals ≥15 years on average, but under harsh post-fire climate conditions, fire intervals must be ≥20 years to reach stand self-replacement. Collectively, these results suggest that closed-cone pine forests are resilient to relatively frequent stand-replacing fires and warming that has occurred thus far. However, if fire recurs on an interval of <15 years, or post-fire growing seasons co-occur with severe drought conditions, resilience may be eroded. Combined with a review of the literature on the environmental conditions that have given rise to serotiny as a trait and variability in trait expression, this work provides a framework for understanding how coniferous forests dominated by serotinous species may respond to future climate and changing fire activity in ecosystems across North America, with significant implications for forest and fire management in rapidly changing environmental conditions.