| Fragmentation and non-native species are two of the most serious threats to biodiversity today. The extent of edge effects and non-native invasion were quantified in ten forest fragments (beech-maple and oak-hickory types) in southern Michigan. Aspect significantly influenced the extent of edge effects based on vegetation structure, but not vegetation composition. Edge effects penetrated up to 90 m into forest interiors and were greater in beech-maple (x = 12.7 m) than in oak-hickory forests (x = 5.8 m). Herb layer structure and understory composition were most sensitive to edge effects. Non-native species made up 24% of sampled species richness. In beech-maple forests, most non-natives were limited to within 20 m of the edge. Non-native richness (43 vs. 26 species), cover (0.08% vs. 0.01%), and stem density (1.2 vs. 0.1 stems/m 2) were highest in oak-hickory forests. Soil seed bank density at five sites was higher at the edge (3.85 vs. 3.35 seeds/m2). Non-native species made up 30% of the seed bank. Most abundant non-native species in the vegetation were absent from the sampled seed bank, suggesting that the seed bank does not contribute to their invasiveness. While basal area, stem density, and species richness of Tourney Woodlot, a small, old-growth beech-maple fragment, are similar to larger old-growth forests, it is dominated by Acer saccharum rather than Fagus grandifolia . This difference could be caused by the woodlot's small size and the influence of edge effects on the competitive balance between the dominants. |
