Species diversity in forests established before and after 1950 in a southern Appalachian watershed.

Forest cover in many southern Appalachian watersheds has increased since the early 1900s due to changing patterns of land use. Changing economics led to the abandonment of pastures and croplands on sites with steep slopes and less-fertile soils. Since 1950 land cover on many sites has changed from nonforest to forest, resulting in greater coverage and less fragmentation of forest habitats. We hypothesized that sites with younger, aggrading forests may not yet support many of the plant species present in older forests. Therefore, we compared the diversity and abundance of vascular plants at two types of presently forested sites: (a) sites with forest established pre-1950 and (b) sites with forest established after 1950. We found that species richness for both woody and herbaceous plants was lower at sites with post-1950 forests. Species with limited dispersal capabilities or specialized habitat needs were absent from these sites. Generalist species were frequently present in these young forests but in lower densities. Many exotic, weedy species persisted in post-1950 forests. Differences between the two types of forests were mitigated by topographic factors, proximity to older forests, and land use history. Paired study plots were selected using maps of forest cover in 1950 and 1990 provided by Paul Bolstad. Each pair of plots had one member in a pre-1950 forest stand and one member in a post 1950 forest stand. Plots were located as to minimize topographic and edaphic differences between the two members of the pair. Twenty-four pairs of plots were sampled in June 1997. Study plots were 0.10 ha in area. The diversity and abundance of forest herb and shrub species were recorded in nine 0.5 m2 quadrats. The percent coverage (of the ground) was visually estimated for each species in each quadrat. All trees occurring in a 20 x 20 m area within the plot were recorded. Diameter at breast height (dbh) was recorded for individuals >10 cm dbh