Simulated postfire tree regeneration suggests reorganization of Greater Yellowstone forests during the 21st century

Abstract Tree regeneration underpins forest resilience, but how pathways of postfire tree regeneration will unfold with future climate and fire regimes is difficult to anticipate. We conducted a simulation study in the Greater Yellowstone Ecosystem (GYE; United States) using a process‐based model, iLand, to ask how rates, composition, and spatial patterns of postfire tree regeneration vary with 21st‐century climate. Subalpine forest and fire dynamics were simulated through 2100 under four climate scenarios, 2 × 2 factorial of aridity (wet and dry) and temperature (warm and hot), in five GYE landscapes. We tallied postfire tree seedling density by species in simulated fires (>400 ha) at five years postfire. To assess changes in regeneration rates (total and by species) to 2100 in each landscape × climate scenario combination, we fit generalized linear models of regeneration versus time and estimated slope coefficients. To analyze spatial patterns of recovery, we compared regeneration to prefire forest state. Postfire regeneration rates were maintained through 2100 in wet scenarios but declined in the hot‐dry scenario. Seedling composition was generally consistent throughout the simulations across wet scenarios, except for declines of Engelmann spruce ( Picea engelmannii ). Regeneration of fire‐sensitive species declined in the hot‐dry scenario, with Engelmann spruce experiencing the steepest declines (−20% to −71%) and failing by late century. Lodgepole pine ( Pinus contorta var. latifolia ) and subalpine fir ( Abies lasiocarpa ) declined in the hot‐dry scenario, but regeneration never failed. Regeneration of fire‐tolerant Douglas‐fir ( Pseudotsuga menziesii ) and aspen ( Populus tremuloides ) was sustained or increased in dry scenarios (+4% to +6%). The proportion of burned area where regeneration failed increased in all dry scenarios but never exceeded 20%. Declining tree regeneration and shifts in dominant tree species revealed that changes in forest structure and composition—and not a conversion to non‐forest—are the dominant response to future climate across broad swaths of the simulation landscapes. Our results suggest that postfire reorganization may be widespread during the 21st century and enable forests to persist in a warming climate with more fire.