COS 26-2
Assessing 40 years of change in aspen forests of the Rocky Mountain National Park (CO)
Forests dominated by aspen provide a variety of ecosystem services, including soil enrichment, watershed protection, wildlife habitat, and recreational opportunities. In the Southern Rocky Mountain region, species richness, nutrient cycling and herbaceous biomass are generally higher under aspen than in conifer-dominated habitats, justifying the interest in protecting these communities. It is unclear whether aspen are increasing, decreasing, or persistent in Colorado, and the recent mountain pine beetle epidemic has raised more questions regarding aspen’s future. Long-term studies based on resampling of previously sampled plots provide unique opportunities to directly and accurately quantify changes in forest vegetation and help develop optimal management strategies. We relocated and resampled a subset of 89 plots containing aspen from among 305 vegetation plots first sampled by Robert Peet during 1972-1973. We resampled these plots during 2011-2013 using the Carolina Vegetation Protocol, which is compatible with and effectively an expansion on the Whittaker protocol followed by Peet. We hypothesized that aspen have decreased in density and basal area over the past 40 years due to reduced fire frequency, increased herbivory and increasingly drier weather, and that the extent of aspen decline decreases with elevation due to higher browsing pressure in the Park’s lower-elevation elk winter ranges.
Results/Conclusions
Our data only partially support our hypothesis of aspen decline. Although there has been an extensive and conspicuous decrease in aspen density on the landscape scale, most of this decline occurred in 11 plots that were part of Peet’s aspen-dominated Populus tremuloides series, accounting for 55.09%, 94.91%, and 94.01% of total aspen decline in herb, shrub, and tree stratum, respectively. Moreover, basal area of aspen in this vegetation type declined by 57%. The considerable declines in aspen stem density in all strata of this vegetation type, coupled with increases in Picea engelmannii, Pinus contorta, and Pseudotsuga mensiezii, indicate that aspen is being replaced by other species. However, our results revealed no elevational trend in the decrease in aspen density. Although our data show decreasing aspen dominance in forest communities where aspen were dominant, they also suggest that aspen in mixed stands have been relatively resistant to conifer encroachment and may be beneficiaries of the mountain pine beetle outbreak.