Changes in vegetation structure and diversity after grass-to-forest succession in a Southern Appalachian watershed
To document how species richness and diversity (H!) recover from severe large-scale disturbance, we report temporal patterns of species composition and diversity following grass-to-forest succession from a long-term experiment in the Coweeta Basin, western North Carolina. The original experiment -- clear-cutting, 5 yr of grass cover followed by a herbicide treatment, and abandonment in a Southern Appalachian mixed deciduous forest -- represents the most severe human disturbance in the Coweeta Basin. For several years after cessation of management, Robinia pseudoacacia quickly sprouted from roots and exceeded the growth rates of other species. Liriodendron tulipifera increased in density and basal area because of its prolific seedling establishment and rapid growth rate. Regeneration of large seeded species was mixed -- sparse for Quercus rubra and Q. coccinea and nonexistent for Q. prinus and Q. velutina. In the overstory, density-based H! increased from 1958, before grass conversion, to 15 yr and 28 yr following disturbance. In contrast, basal area-based H! had significantly declined at 15 yr, then increased at 28 yr. The initial decline in basal area based H! was attributed to a decline in evenness of species distribution (J!) rather than to a change in species richness. The severe disturbance increased the abundance of early successional woody species and of herbaceous genera that tolerate open habitats, such as Erichtites, Phytolacca, and Erigeron. Shade-tolerant understory ferns and herbs such as Polystichum acrostichoides, Dennstaedtia punctilobula, Galium latifolium, and Viola cucullata gradually became more abundant. The 28-yr-old forest of WS6 had much lower species richness than the adjacent reference watersheds, but more than threefold higher density.