Ecologists investigate what makes a fire healthy in Diablio Canyon

Prescribed burning is increasingly used in forests of the western USA to restore natural fire regimes and mitigate wildfire hazards. However, uncertainty around canopy seedbank resilience can make prescribed fire plans challenging, especially in forests adapted to high-severity crown fire. A new study in Bishop Pine (Pinus muricata) forests of coastal California reveals some promising news for managers looking to expand the use of prescribed fire. Published in Fire Ecology, the research by Sarah Bisbing and colleagues found that bishop pines rapidly produce a viable aerial seedbank following stand-replacing fire.

An aerial seedbank is the sum of seeds stored in the canopy of trees, typically in cones that only open and release seeds when exposed to high heat. This adaptation allows certain fire-dependent tree species, like Bishop Pine, to regenerate after fire rips through a stand. The aerial seedbank is a resilience mechanism that ensures there will be a flush of new seedlings post-fire to re-establish the forest.

Quantifying the timing of aerial seedbank development as well as its density and viability, is crucial for managing fire-adapted forests. This helps determine acceptable windows for prescribed burning that avoid regeneration failures. Robust canopy seedbanks allow for more flexible burn intervals, while species with slower seedbank development require more cautious use of fire.

Bisbing and colleagues looked at past fires that had occurred over recent decades in bishop pine forests surrounding the Diablo Canyon Nuclear Power Plant in coastal California to understand how quickly bishop pines can regenerate after a wildfire. They visited 50 sites within areas burned from 1982 to 2012 by both wildfires and prescribed fires. At each location, they mapped out plots and counted all the pine trees still living as well as dead tree remains. This survey let them see how dense the young pine forests were after different fires. They also counted the cones on trees to estimate seed production. By collecting and testing seeds from the cones, they could check how many were still viable and able to sprout new seedlings.

Lastly, the researchers inspected living pines for signs of pine pitch canker, a disease affecting these trees. Tracking seed production and health across sites burned over the last 30 years gave the researchers a timeline of how quickly the pines can bounce back after major fires and how long their canopy seed banks can supply new growth.

The study revealed that Bishop Pines can rapidly rebuild a resilient supply of seeds after being burned in severe wildfires. Within only eight years after a blaze, the trees had produced an abundant canopy seedbank filled with viable seeds capable of sprouting new seedlings. This aerial seedbank persisted for at least 36 years in stands burned decades prior. The viability of seeds stayed remarkably high over time, remaining above 95% even three decades after a fire.

The research also showed that pine pitch canker, a disease impacting these trees, peaks in sapling stands around 8-10 years post-fire. The disease was likely making it harder for some young pines to survive and mature. However, the findings suggest using prescribed fire may help control pine pitch canker infestations.

Overall, the rapid recovery of bishop pine’s aerial seedbank after high-severity fire gives managers flexibility in using prescribed burning to restore these forests. Since so many viable seeds are available after just eight years, prescribed fires could be timed anywhere from eight years to several decades after a previous fire and still allow bishop pines to regenerate successfully. Such flexible windows for repeat burning will be vital for reintroducing fire in these ecosystems. Managers can tailor the timing based on factors like crew availability, weather patterns, and proximity to homes and communities. Burning may also mitigate disease impacts if it helps control pine pitch canker in regenerating stands.

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Bisbing, S.M., Urza, A.K., York, R.A., Hankin, L.E. and Putz, T.R. (2023) “Persistent, viable seedbank buffers serotinous bishop pine over a broad fire return interval,” Fire Ecology, 19(1). Available at: https://doi.org/10.1186/s42408-023-00194-3.