LAUSR

As 21st -century climate and disturbance dynamics depart from historical baselines, ecosystem resilience is uncertain. Multiple drivers are changing simultaneously, and interactions among drivers could amplify ecosystem vulnerability to change. Subalpine forests in Greater Yellowstone (Northern Rocky Mountains, USA) were historically resilient to infrequent (100-300 year), severe fire. We sampled paired short- (< 30-year) and long- (> 125-year) interval post-fire plots most recently burned between 1988 and 2018 to address two questions: (1) How do short-interval fire, climate, topography, and distance to unburned live forest edge interact to affect post-fire forest regeneration? (2) How do forest biomass and fuels vary following short- versus long-interval severe fires? Mean post-fire live tree stem density was an order of magnitude lower following short- versus long-interval fires (3,240 versus 28,741 stems ha-1 , respectively). Differences between paired plots were amplified at longer distances to live forest edge. Surprisingly, warmer-drier climate was associated with higher seedling densities even after short-interval fire, likely relating to regional variation in serotiny of lodgepole pine (Pinus contorta var. latifolia). Unlike conifers, density of aspen (Populus tremuloides), a deciduous resprouter, increased with short- versus long-interval fire (mean 384 versus 62 stems ha-1 , respectively). Live biomass and canopy fuels remained low nearly 30 years after short-interval fire, in contrast to rapid recovery after long-interval fire, suggesting that future burn severity may be reduced for several decades following reburns. Short-interval plots also had half as much dead woody biomass compared to long-interval plots (60 versus 121 Mg ha-1 ), primarily due to the absence of large snags. Our results suggest differences in tree regeneration following short- versus long-interval fire will be especially pronounced where serotiny was high historically. Propagule limitation will also interact with short-interval fire to diminish tree regeneration but lessen subsequent burn severity. Amplifying driver interactions are likely to threaten forest resilience under expected trajectories of future fire.