Evaluating immaturity risk in young stands of the serotinous knobcone pine (Pinus attenuata)

As wildfire becomes increasingly frequent, many plant populations risk
local extirpation if fire recurs too soon, a problem dubbed “immaturity
risk”. We studied the regeneration of a serotinous conifer species,
knobcone pine (Pinus attenuata), as a function of the time between
high-severity fires (6-79 years). We evaluated age, cone production, and
regeneration at two burned sites in northern California, the Ranch Fire
(2018) in Mendocino National Forest, and the Carr Fire (2018) in
Whiskeytown National Recreation Area. We found an average of 30.5 filled
seeds in closed, brown cones (42% viability of filled seeds). The bulk of
the older (gray) cones were partially or fully open, and thus available
seed in them at the time of fire was approximately halved. Adjusting for
survivorship, the estimated minimum age for knobcone pine to produce one
recruit per tree (self-replacement) was 8.2 years. Characterizing the
probability of burning as a negative exponential model and using an
81-year return time, we found the likelihood of reburn before 8.2 years
was 0.09. Our study demonstrated the importance of understanding the size
and time to reproductive maturity in serotinous tree species to adequately
quantify immaturity risk. Based on our results and current estimates of
return time, we suggested that immaturity risk is very low for knobcone
pine. Our approach could be broadly applied to better quantify immaturity
risk in other conifer species.