Topography and overstory mortality interact to control tree regeneration in spruce-fir forests of the southern Rocky Mountains

Climate change is driving rapid and dramatic changes in forests across the globe, as highlighted by recent widespread drought-induced tree mortality events. Long-term changes in these forest ecosystems will be dependent upon the ability of trees to regenerate following overstory mortality under a warmer and drier climate. The goal of this study was to assess how tree mortality, and environmental gradients imposed by topographic and edaphic conditions, interact to influence tree regeneration in subalpine spruce-fir forests in a region experiencing rapid climate warming. We evaluated regeneration of two widely distributed subalpine forest species of the Rocky Mountains, subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea engelmannii), across gradients of elevation, aspect, soil type, and overstory subalpine fir mortality. In both species regeneration was strongly associated with local topography and appeared to not be adversely affected by subalpine fir overstory mortality. We found that subalpine fir seedling (height <1.4 m) density was strongly and positively related to subalpine fir basal area prior to mortality. We also found that in areas with overstory mortality, subalpine fir regeneration was negatively associated with high morning heat load (i.e. south and east facing aspects), likely due to water stress associated with less canopy shade and greater morning insolation. In contrast, Engelmann spruce juvenile density was strongly negatively associated with elevation and did not appear vulnerable to projected warming temperatures. Taken together, our results suggest that regeneration contributes to resilience of these forests by compensating for some overstory mortality, but continued warming may adversely affect subalpine fir regeneration, particularly on south and east facing aspects.