Change within and among forest communities: the influence of historic disturbance, environmental gradients, and community attributes.

Understanding how ecological communities change over time is critical for biodiversity conservation, but few long-term studies directly address decadal-scale changes in both the within- and among-community components of diversity. In this study, we use a network of permanent forest vegetation plots, established in Great Smoky Mountains National Park (USA) in 1978, to examine the factors that influence change in community composition within and among communities. In 2007, we resampled 15 plots that were logged in the late 1920s and 15 plots that had no documented history of intensive human disturbance. We found that understory species richness decreased by an average of 4.3 species over the 30-yr study period in the logged plots, but remained relatively unchanged in the unlogged plots. In addition, tree density decreased by an average of 145 stems ha ―1 in the logged plots, but was relatively stable in the unlogged plots. However, we found that historic logging had no effect on within-community understory or tree compositional turnover during this time period. Instead, sites at lower elevations and sites with lower understory biomass in 1978 had higher understory compositional turnover than did sites at higher elevations and sites with higher understory biomass. In addition, sites with lower soil cation exchange capacity (CEC) and with lower tree basal area in 1978 had higher tree compositional turnover than did sites with higher soil CEC and higher tree basal area. Among-community similarity was unchanged from 1978 to 2007 for both the logged and unlogged plots. Overall, our results indicate that human disturbance can affect plant communities for decades, but the extent of temporal change in community composition may nevertheless depend more on environmental gradients and community attributes.