Fire interval and post-fire climate effects on serotinous forest resilience

Abstract

Background: Climate change is eroding forest resilience to disturbance directly through warming climate and indirectly through increasing disturbance activity. Forests characterized by stand-replacing fire regimes and dominated by serotinous species are at risk when the inter-fire period is insufficient for canopy seed bank development and climate conditions for recruitment in the post-fire growing season are unsuitable. Although both factors are critical to serotinous forest persistence, their relative importance for post-fire regeneration in serotinous forests remains poorly understood. To assess the relative effects of each factor, we established plots in severely burned knobcone pine (Pinus attenuata Lemmon) forests in Oregon and California, USA, representing a range of past fire intervals (6 to 31+ years). Specifically, we evaluated effects of fire interval and pre-fire canopy seed bank (proxies for seed supply) and post-fire climate on three metrics of post-fire tree regeneration (seedling density, probability of self-replacement, percent population recovery). Results: Seed supply consistently had the strongest effect on post-fire regeneration. Between 6- and 31-year fire intervals, post-fire seedling density increased from 1000 to 100,000 seedlings ha−1, while probability of self-replacement increased from ~ 0 to ~ 100% and percent population recovery increased from 20 to 2000% of the pre-fire population, respectively. Similarly, increasing the canopy seed bank by two orders of magnitude increased seedling density and percent population recovery by two orders and one order of magnitude, respectively, and increased the probability of self-replacement by > 50%. Greater post-fire climatic moisture deficit exacerbated the effect of seed supply; an additional 4–6 years between fires was required under high moisture stress conditions to reach similar regeneration levels as under low moisture stress conditions. Conclusion: The overriding effect of seed supply—strongly driven by pre-fire stand age—on post-fire regeneration suggests that altered fire frequency (an indirect effect of climate change) will have a profound impact on serotinous forests. Although direct effects of hot and dry climate are lower in magnitude, they can alter forest recovery where seed supply nears a threshold. These findings reveal how fire interval and climate combine to determine changes in forest cover in the future, informing management and vulnerability mapping.