Habitat loss and fragmentation are recognized as major threats to biodiversity worldwide. Though most biomes on Earth have continued to suffer losses in area and increases in the degree of fragmentation since the mid-20th century, the temperate deciduous forest biome in North America has experienced an increase in area, due to abandonment of agricultural land and subsequent reforestation. This study investigated the degree to which reforestation and changes in the degree of fragmentation have influenced current patterns of biodiversity in these ecosystems. We used a sequence of aerial photos from 1941 through 2010 to analyze changes in area and configuration of three oak-hickory forest fragments in southwestern Illinois. Vegetation data were collected from 130 circular plots randomly located throughout the fragments, with the herbaceous ground layer community sampled both in spring and summer. Biodiversity indicators, including richness (S), Simpson diversity (D2), and meanC (a measure of conservation value or quality) were computed for each plot for the tree, shrub/sapling and ground layer communities. Predictive regression models incorporating measures of successional age (Age) and mean distance from edge (DE) derived from the historic photo analysis, as well as measures of exotic species abundance, were developed for each biodiversity indicator.
Results/Conclusions
One fragment showed little change over the past 70 years, while the other two have increased significantly in area and experienced marked decreases in sinuosity and edge-to-interior ratio. No predictive models were found for S and D2 of the tree and shrub layers but tree meanC increased with Age and shrub meanC increased with both Age and DE. For the summer herbaceous community, S and D2 decreased with exotic shrub density, while meanC increased with Age and decreased with exotic shrub density. Both S and meanC of the spring ephemeral community were positively related to Age and DE and negatively related to exotic shrub density and exotic ground layer cover. D2 of the spring ephemeral community increased with Age and decreased with exotic shrub density. Overall the results suggest that reforestation has had a positive effect on biodiversity, especially richness of the spring ephemeral community and meanC of all vegetation strata. Seventy years is probably too little time for increases in richness and diversity of woody species to be observed. We recommend that management of deciduous forest fragments to maximize biodiversity should concentrate on increasing the area and decreasing the sinuosity of fragments and reducing the abundance of exotic species.