Regeneration of montane forests 24 years after the 1988 Yellowstone fires: A fire‐catalyzed shift in lower treelines?

Abstract

AbstractForests near the lower limit of montane tree cover are expected to be particularly vulnerable to warming climate, potentially converting to non‐forest for prolonged periods if affected by canopy‐removing disturbances. Such disturbance‐catalyzed shifts are by nature stochastic, offering few opportunities to test these predictions. We capitalized on a landmark event exemplary of recent large disturbances—the 1988 wildfires in Yellowstone National Park (USA)—to investigate long‐term (24‐yr) regeneration dynamics in Douglas‐fir (Pseudotsuga menziesiivar.glauca) forests, which compose the lower treeline yet have received little study. We tested the hypothesis that, under current conditions (warmest decades in last 6000 yr), dry lower‐margin stands are less apt to attain self‐replacement than adjacent, but more mesic, stands farther from the ecotone. Mesic sites characterized by dense forests prefire regenerated robustly regardless of burn severity, even in large high‐severity patches, whereas dry sites did so only if nearby seed sources survived (i.e., small patch size or moderate burn severity). Regeneration continued over two decades, peaking between ~5 and 15 yr, with mesic stands continuing regenerating beyond 15 yr to a greater degree than dry stands. Self‐replacement of stands by 24 yr postfire was nearly universal in mesic stands, variable but common in dry stands not adjacent to the lower ecotone, and uncommon in dry stands near the ecotone—particularly if burned at high severity. Whether the lack of regeneration in ecotonal stands is categorically directional with warming climate or part of a long‐term cycle is unresolved, but we estimate that the 1988 fires converted > 4000 ha of lower‐montane forest to grass/sage steppe for at least several decades, possibly indefinitely given current trends in climate. These data support the notion that climate‐driven shifts in vegetation cover are likely to occur in punctuated fashion with disturbances, with relatively abrupt implications for montane forest cover, biogeography, and ecosystem function.