California subalpine forest post-fire conifer regeneration data

Wildfire may facilitate climate tracking of forest species moving upslope
or north in latitude. For subalpine tree species, for which higher
elevation habitat is limited, accelerated replacement by lower elevation
montane tree species following fire may hasten their extinction risk. We
used a dataset of post-fire tree regeneration spanning a broad geographic
range to ask whether fire facilitated upslope movement of montane tree
species at the montane-to-subalpine ecotone. We sampled tree seedling
occurrence in 248 plots across a fire severity gradient (unburned to
>90% basal area mortality) and spanning ~500 km of latitude in
Mediterranean-type subalpine forest in California, USA. We used logistic
regression to quantify differences in postfire regeneration between
resident subalpine species and the seedling-only range (interpreted as
climate-induced range extension) of montane species. We tested our
assumption of increasing climatic suitability for montane species in
subalpine forest using the predicted difference in habitat suitability at
study plots between 1990 and 2030. We found that postfire regeneration of
resident subalpine species was uncorrelated or weakly positively
correlated with fire severity. Regeneration of montane species, however,
was roughly four times greater in unburned relative to burned subalpine
forest. Although our overall results contrast with theoretical predictions
of disturbance-facilitated range shifts, we found opposing postfire
regeneration responses for montane species with distinct regeneration
niches. Recruitment of shade-tolerant red fir declined with fire severity
and recruitment of shade-intolerant Jeffrey pine increased with fire
severity. Predicted climatic suitability increased 5% for red fir and 34%
for Jeffrey pine. Differing postfire responses in newly climatically
available habitat indicate that wildfire disturbance may only facilitate
range extensions for species whose preferred regeneration conditions align
with increased light and/or other postfire landscape characteristics.