Old-field succession on a Minnesota sand plain.

Soil N concentration increased significantly with field age. Vegetation cover, total aboveground plant biomass, and litter cover increased significantly with soil N. Light penetration to the soil surface was negatively correlated with total plant biomass. Species richness per field and within field heterogeneity in species composition increased with field age. Local species richness decreased with increasing soil N. Cover of annuals and introduced species decreased with field age and N, but annuals contributed an important part of total vegetative cover even in 25yr old fields. Cover of perennials and woody species increased with soil N and field age. Although the fields were bordered by woods, woody species contributed <15% cover even in the oldest fields. While soil N is an important determinant of local species composition and abundance, dispersal and colonization, which are dependent on field age, nevertheless determine which species are present in a field. -from Authors