Increased temperatures due to climate warming are expected to drive an upward shift in elevation of species ranges, as they track changes in climate. Lags in tree species migrations have been documented, however, and may be tied to inadequate dispersal or lack of appropriate sites for establishment due to competition, resource limitations, and/or other factors. Disturbances such as wildfire may facilitate a more rapid shift in species ranges by reducing competition, increasing resource availability and producing conditions and microsites suitable for establishment. Recent evidence demonstrates that trembling aspen establishment from seed, rather than suckering, is more common than previously thought, especially in disturbed areas. Consequently, disturbance may be an essential prerequisite for aspen range expansion. To determine whether recent wildfires promoted upward range expansion of aspen in the Colorado Rocky Mountains, we established elevational transects starting from aspen’s upper limit in areas recently burned at high severity and adjacent unburned areas. Seedling surveys were conducted every 40-meter increase in elevation on each transect. Conditions associated with seedling establishment were assessed, including microsite factors and site characteristics.
Results/Conclusions
Aspen seedling establishment occurred only in burned areas from 3408 to 3678 meters in elevation, with a median of 3542 meters. This represents local upward migrations of up to 395 meters in elevation and regional migrations up to 172 meters in elevation. Burned sites had significantly less canopy cover (3 vs. 15%), less duff (5 vs 34%) and less understory vegetation (48 vs. 65%), but were also farther from live adult aspen than unburned areas (2.2 vs. 0.95 km), indicating that aspen seedling establishment is promoted more by reduced competition and the abundant light and mineral soil conditions created after fire than by the presence of nearby seed sources. Associations between seedlings and nurse objects increased on sites with lower heat loads and with elevation, suggesting that logs and large rocks may help shelter seedlings from extreme cold above their upper elevational limit. Our findings indicate that severe wildfire can catalyze the upward migration of aspen, and thus facilitate this species’ persistence in the face of accelerated climate warming.