Hoecker, Tyler J. 2021. Anticipating subalpine landscapes of the future: Responses to climate and fire-regime change in the northern US Rocky Mountains. PhD Dissertation, University of Wisconsin-Madison.

Wildfires shaped subalpine forests in the northern US Rocky Mountains for millennia. However, recent climate-driven increases in fire frequency and severity are constraining postfire tree regeneration in forests dominated by fire-sensitive obligate-seeder conifers via reduced propagule pressure and in xeric areas via elevated aridity. Changes in tree regeneration are projected to drive large-scale shifts in the extent and character of future forests, which will cascade to effect critical ecosystem services. This dissertation explored controls on tree regeneration in diverse subalpine forest settings and disturbance contexts and implications for future ecosystem function. I investigated (i) effects of interactions between short-interval fires and topographic position on postfire regeneration using a seed-planting experiment in the Greater Yellowstone Ecosystem (GYE); (ii) consequences of future landscape change for three wildlife species in the GYE using the simulation model iLand and the Maxent algorithm; and (ii) shifts in forest composition and structure after recent fires in Glacier National Park using a field study. Postfire conifer regeneration was low on south-facing aspects affected by high-severity fires at short intervals (<30 yrs). Soils on south-facing slopes were >2°C warmer and >20% drier than north-facing slopes after short-interval burns, and residual fire-killed trees after long-interval fires (>150 yrs) provided microclimate buffering of a similar magnitude. Distribution modeling from 2017-2100 revealed extensive loss of old forest in climate scenarios with substantial warming (RCP 8.5) and drying (HadGEM2-ES). Habitat for three vertebrates (Picoides arcticus, Martes caurina, Tamiasciurus hudsonicus) depended on spatial overlap of persistent forest with suitable climate, which were frequently mismatched. Mesic forests in Glacier remain resilient to fire, but short-interval fire reduced stand density and simplified composition. Postfire regeneration after long-interval fires is dominated by fire-adapted conifers; a second fire erodes the ability of fire-sensitive species to establish. If these shifts persist, the range of mesic conifers is likely to be reduced. Findings indicate climate- and fire-catalyzed changes in forest ecosystems are already emerging in the northern Rockies. Managing forests to preserve historical structure and composition will be increasingly difficult, requiring approaches that direct change toward desired outcomes where possible and accept alternatives where change is unavoidable.