Disturbance facilitates rapid range expansion of aspen into higher elevations of the Rocky Mountains under a warming climate

Aim Trembling aspen (Populus tremuloides Michx.) is absent in the upper foothills region of west-central Alberta because of the cold conditions and short growing season at this high elevation. However, in recent years it appears that aspen has been establishing from seed in this zone and that it has been doing so mainly as a result of forest harvesting. The objectives of this study were to determine the frequency of and types of microsite required for the successful establishment of aspen seedlings at these higher elevations. Location Rocky Mountains Upper Foothills Natural Subregion of west-central Alberta, Canada. Methods The current distribution of mature aspen and the presence and absence of aspen seedlings in harvested areas were determined in an area c. 300 km2 in size, using ground and aerial surveys. In an intensive study, 12 belt transects (180 m long and 5 m wide) were established in areas disturbed by forest harvesting at high elevations where no aspen was present prior to harvesting. Transects were surveyed seven growing seasons after disturbance and the microsites occupied by aspen seedlings were characterized according to their substrate and microtopography. Similarly, the availability of different substrates and microtopographic positions were assessed by systematic point sampling on these sites. Results On level surfaces, aspen seedling regeneration was found up to 200 m higher in elevation than the mature aspen in the original undisturbed forests. Overall, there were 428 seedlings ha-1 established on these transects, and the age distribution indicates that aspen seedlings had established in each of the seven growing seasons since the disturbance. Nearly all of the seedlings (93%) were established on mineral soil microsites and virtually no seedlings were established on undisturbed forest floor layers. Significantly more seedlings were found in concave microtopographic positions. Main conclusions This study indicates that aspen establishment from seed is currently not a stochastic event and demonstrates that aspen is rapidly expanding its range upslope in the Canadian Rocky Mountain region as a result of forest management practices that expose mineral soil substrates in conjunction with a warming climate. The change in canopy composition from conifer to deciduous forests at these higher elevations will have far-reaching implications for ecosystem processes and functions.