Looking beyond the mean: Drivers of variability in postfire stand development of conifers in Greater Yellowstone

High-severity, infrequent fires in forests shape landscape mosaics of
stand age and structure for decades to centuries, and forest structure
can vary substantially even among same-aged stands. This variability
among stand structures can affect landscape-scale carbon and nitrogen
cycling, wildlife habitat availability, and vulnerability to
subsequent disturbances. We used an individual-based forest process
model (iLand) to ask: Over 300 years of postfire stand development,
how does variation in early regeneration densities versus abiotic
conditions influence among-stand structural variability for four
conifer species widespread in western North America? We parameterized
iLand for lodgepole pine (Pinus contorta var. latifolia), Douglas-fir
(Pseudotsuga menziesii var. glauca), Engelmann spruce (Picea
engelmannii), and subalpine fir (Abies lasiocarpa) in Greater
Yellowstone (USA). Simulations were initialized with field data on
regeneration following stand-replacing fires, and stand development
was simulated under historical climatic conditions without further
disturbance. Stand structure was characterized by stand density and
basal area. Stands became more similar in structure as time since fire
increased. Basal area converged more rapidly among stands than tree
density for Douglas-fir and lodgepole pine, but not for subalpine fir
and Engelmann spruce. For all species, regeneration-driven variation
in stand density persisted for at least 99 years postfire, and for
lodgepole pine, early regeneration densities dictated among-stand
variation for 217 years. Over time, stands shifted from
competition-driven convergence to environment-driven divergence, in
which variability among stands was maintained or increased. The
relative importance of drivers of stand structural variability
differed between density and basal area and among species due to
differential species traits, growth rates, and sensitivity to
intraspecific competition versus abiotic conditions. Understanding
dynamics of postfire stand development is increasingly important for
anticipating future landscape patterns as fire activity increases.

This dataset was used in the publication Braziunas et al. (2018),
Forest Ecology and Management
https://doi.org/10.1016/j.foreco.2018.08.034