Rapid viable seedbank fosters resilience via pyrosilviculture in serotinous bishop pine

Abstract

Background In ecosystems where fire has been excluded, pyrosilviculture can restore fire regimes while mitigating risk where fire is inevitable. Pyrosilviculture in crown fire-adapted forests is, however, limited by insight into the temporal window of fire return matching canopy seedbank development. Here, we characterized demographic responses to fire and non-native pine pitch canker infection in a chronosequence of serotinous bishop pine stands burned at high-severity to quantify: (1) temporal patterns of seedbank development given seed viability and density, cone production, and tree density, and (2) pine pitch canker incidence across tree ages and sizes to assess how infection might influence pyrosilviculture. Stands were burned to promote resilience while mitigating WUI wildfire, and we use to our findings to elucidate pyrosilvicuture as a means of meeting these objectives in crown-fire adapted forests. Results Bishop pine produces an abundant, viable seedbank within six years, with high production and viability persisting across developmental stages and age classes. Seed abundance and viability are exceptionally high at even the earliest age (median > 600,000 seeds ha− 1 & 97% viability at 6 years) and remain high, with the oldest stands (36 years) maintaining median densities of > 500,000 seeds ha− 1 and viability of 95%. We additionally learned that pine pitch canker infection is most severe during the sapling stage (8–10 years post-fire), potentially shifting the timing and development of fuel structure to earlier successional stages as well as limiting recruitment into the canopy. Conclusions In bishop pine and equally-fecund serotinous species, burning at short fire return intervals is feasible given early development of a robust, viable seedbank, addressing two common management challenges in fire-adapted forests - restoring fire to promote resilience while mitigating wildfire risk to the WUI. Moreover, frequent fire in pine pitch canker infected stands may also provide disease mitigation. Although the long-term effects of pine pitch canker infection remain unknown, bishop pines’ viable and persistent seedbank suggests that managers can ignite prescribed fire across a broad return interval – as short as six to eight years and as long as several decades – to promote ongoing bishop pine resilience, mitigate disease infection rates, and reduce wildfire risk in WUI-adjacent ecosystems.