It takes a few to tango: Changing climate and fire regimes can cause regeneration failure of two subalpine conifers

Environmental change is accelerating in the 21st century, but how
multiple drivers may interact to alter forest resilience remains
uncertain. In forests affected by large high-severity disturbances,
tree regeneration is a resilience linchpin that shapes successional
trajectories for decades. We modeled stands of two widespread western
U.S. conifers, Douglas-fir (Pseudotsuga menziesii var. glauca), and
lodgepole pine (Pinus contorta var. latifolia), in Yellowstone
National Park (Wyoming, USA) to ask (1) What combinations of distance
to seed source, fire return interval, and warming-drying conditions
cause postfire tree-regeneration failure? (2) If postfire tree
regeneration was successful, how does early tree density differ under
future climate relative to historical climate? We conducted a
stand-level (1 ha) factorial simulation experiment using the
individual-based forest process model iLand to identify combinations
of fire return interval (11–100 yr), distance to seed source (50–1,000
m), and climate (historical, mid-21st century, late-21st century)
where trees failed to regenerate by 30-yr postfire. If regeneration
was successful, we compared stand densities between climate periods.
Simulated postfire regeneration were surprisingly resilient to
changing climate and fire drivers. Douglas-fir regeneration failed
more frequently (55%) than lodgepole pine (28% and 16% for
nonserotinous and serotinous stands, respectively). Distance to seed
source was an important driver of regeneration failure for Douglas-fir
and non-serotinous lodgepole pine; regeneration never failed when
stands were 50 m from a seed source and nearly always failed when
stands were 1 km away. Regeneration of serotinous lodgepole pine only
failed when fire return intervals were ≤20 yr and stands were far (1
km) from a seed source. Warming climate increased regeneration success
for Douglas-fir but did not affect lodgepole pine. If regeneration was
successful, postfire density varied with climate. Douglasfir and
serotinous lodgepole pine regeneration density both increased under
21st-century climate but in response to different climate variables
(growing season length vs. cold limitation). Results suggest that,
given a warmer future with larger and more frequent fires, a greater
number of stands that fail to regenerate after fires combined with
increasing density in stands where regeneration is successful could
produce a more coarse-grained forest landscape.