Print PageAspen (Populus tremuloides) forests in the southern Rocky Mountains promote extremely rich and productive understory plant communities. While recent concerns about aspen forests in the region have focused on Sudden Aspen Decline (SAD), evidence from across the western US suggests that many otherwise healthy forests may be experiencing accelerating rates of tree mortality. Such changes in forest structure are likely to affect forest understory plant communities through increased light levels. We resampled forest structure, canopy cover and understory composition in 19 healthy aspen stands (not showing characteristics of SAD) originally sampled in 1964 and 1994 near Crested Butte, Colorado to assess recent patterns of canopy and understory change.
Results/Conclusions
Annual rate of forest decline has more than doubled from -0.96% to -2.38% over the two sampling intervals (1964-1994, 1994-2010). Over the last 46 years, stand density has decreased from 3151 ha-1 to 1605 ha-1, a 49% loss. Basal area, which did not change 1964-1994, also dropped significantly and rapidly from 1994-2010. While understory plant species composition has remained relatively consistent, we found significant increases in cover by graminoids and N-fixing species over the last 16 years, which appear closely linked to increased light levels. Even in these apparently healthy aspen forests, accelerating canopy dieback is leading to changes in community structure and composition, with possible consequences for biological diversity, ecosystem services, and aesthetic and economic values.
