A multiple-scale assessment of long-term aspen persistence and elevational range shifts in the Colorado Front Range

Aspen forests and woodlands are some of the most species-rich forest communities in the northern hemisphere. Changing climate, altered disturbance regimes, land use, and increased herbivore pressure threaten these forests both in Eurasia and North America. In addition, rapid mortality dubbed Sudden Aspen Decline is a concern for aspen's long-term presence in the western United States, especially Colorado and Utah. Yet it is still unclear whether aspen is persistent or declining at the landscape scale. We assessed aspen persistence at different spatial scales in the Colorado Front Range by resampling 89 plots containing aspen from among 305 vegetation plots sampled by Robert Peet during 1972-1973. We hypothesized that aspen density and basal area had decreased at the landscape scale, with notable variability in change depending on the forest community type, and that this overall decrease has been more pronounced at lower elevations. We also assessed elevational range shifts of the major species in these forests. Aspen were no longer present in 22 of the 89 plots and aspen density for stems less than 2.5 cm diameter at breast height (DBH) had declined significantly overall, although density of medium (2.5-10 cm DBH) and large (>10 cm DBH) trees, as well as basal area, had not changed significantly. A comparison between montane (<2700 m elevation) and subalpine (2700-3500 m elevation) plots revealed that the decrease was more pronounced at higher elevations and was mostly the result of substantial decreases of stems in the eleven plots that were part of Peet's aspen-dominated Populus tremuloides series. In these plots, aspen basal area decreased significantly whereas basal area of Abies bifolia, Picea engelmannii, Pinus contorta, and Pseudotsuga menziesii increased substantially. Upslope shifts were observed for most species, especially on northeast facing slopes, suggesting climate-related responses. In summary, aspen have been resilient in mixed forests and may be beneficiaries of recent bark beetle epidemics, but have decreased and been subject to successional transitions in previously aspen-dominated stands. Our results confirm the importance of region-specific, multiple-scale assessments of species persistence to make best management recommendations.