Aspen (Populus tremuloides) stand dynamics and understory plant community changes over 46 years near Crested Butte, Colorado, USA

Aspen stands in the Rocky Mountains form especially productive and diverse plant understory communities. However, little is known about canopy–understory relationships or understory dynamics, especially in light of widespread aspen decline. The purpose of our research was to assess recent aspen plant community dynamics by resampling 19 sites sampled in 1964 and 1994 near Crested Butte, Colorado. In 2010, we replicated previous sampling methods to measure canopy structure and understory composition at each site. In contrast with the conclusions of previous work in the study area, sampled aspen stands did not exhibit long-term stability, nor were they succeeding to conifers. Over the 46-year sample period, live aspen density and basal area both decreased significantly (3151–1605 stems ha−1; 39.2–33.7 m2 ha−1), with more pronounced decreases over the most recent sampling interval, 1994–2010. These changes appear linked to natural patterns of stand maturation, but may also have been enhanced by recent, drought-triggered mortality, as well as constraints on aspen regeneration imposed by elk and livestock. A striking decrease in the cover and frequency of fireweed (Chamerion angustifolium) was consistent with expectations of aging stands initiated by historic fire, and the expansion of Kentucky bluegrass (Poa pratensis) indicated strong effects of ungulate herbivory. Recent overstory losses had little effect on understory communities, which were dominated by a suite of large perennial herbs that showed little change over the sampling interval, including Fendler’s meadowrue (Thalictrum fendleri), oshá (Ligusticum porteri), elk and dryspike sedge (Carex geyeri and C. siccata), and blue wildrye (Elymus glaucus). Understory community composition was not closely related to current stand conditions or light levels, but was linked with regional elevational and geographic gradients and historic (1994) stand structure