Secondary Succession, Gap Dynamics, and Community Structure in a Southern Appalachian Cove Forest

Abstract

Forest succesion was studied in a species—rich cove forest of the Great Smoky Mountains, Tennessee. Long—term compositional and structural changes in secondary stands, 15—63 yr after agricultural abandonment circa 1920, were compared to an adjacent old—growth stand. Later successional trends were projected by computer simulation. The colonizing stand (15 yr) was dominated by Liriodendron tulipifera and Robinia pseudoacacia. Common old—growth species absent at this early stage included Aesculus octandra, Fagus grandifolia, and Tsuga Canadensis. By year 40, Liriodendron dominated the overstory and all old—growth dominants were represented in the understory. After 60 yr, Acer saccharum was clearly the most abundant understory species; gap model projections indicated that after 200 yr it will dominate the forest. The actual old—growth stand was dominated by Acer saccharum. Species richness peaked at mid—succession (°50 yr), while diversity (H') and evenness (J') of species biomass were highest at the old—growth stage. Richness peaked with the coexistence of shade—intolerant colonizers in the overstory and shade—tolerants in the understory. Canopy—gap processes of the secondary and old—growth stands differed sharply. The old—growth canopy was composed of large, spreading, and overlapping crowns, while the second—growth stand was composed of narrow, nonoverlapping crowns. Gaps in the secondary stand tended to be small (<100 m2) but numerous. The regeneration of intolerants was low and individual gap microsites were not markedly different from the understory as a whole. Old—growth gaps were often larger, higher light microsites which contrasted sharply with the shaded forest floor. The wider range of gap sizes in old forests accommodates a variety of species, including intolerants, contributing to the high canopy species diversity of old—growth cove forests.

Disturbance and the Population Dynamics of Liriodendron Tulipifera: Simulations with a Spatial Model of Forest Succession

“…Forests of southern Appalachian coves (small, mid-elevation valleys surrounded by high ridges) fall into this category. Cove forests typically consist of 20-30 mesophytic tree species, often with more than 10 species per hectare in the canopy of old stands (Braun 1950;Whittaker 1956;Golden 1981;Clebsch & Busing 1989). As a whole, shade-intolerant species occur at low levels (c. 3% by density; Barden 1980Barden , 1981 in old-growth canopies, and Liriodendron occurs at low levels or is absent from such stands (Table 1, Fig.…”

“…In young cove forest stands, however, Liriodendron is abundant and often occurs as the sole canopy dominant (Della-Bianca 1983;Clebsch & Busing 1989). Judging from the prolific colonization of artificially disturbed sites by Liriodendron, seed availability is not a limiting factor for this species in cove forests.…”

“…EMPIRICAL DATA Long-term trends in cove forest composition, structure and patch dynamics were inferred from a 60-year study of forest development (Clebsch & Busing 1989) and from old-growth forest plot studies. The importance of Liriodendron in regeneration and overstorey classes at various stages of forest development was determined.…”

“…For example, boreal forests, consisting of narrow-crowned trees, typically have a preponderance of single-tree gaps that are too small for the regeneration of intolerants. In temperate forests, canopy gaps are generally smaller in young stands than in old stands (Clebsch & Busing 1989;Bradshaw & Spies 1992). The difference in gap size between young and old stands can be attributed to the comparatively small crown size of trees comprising the young stands (Clebsch & Busing 1989).…”

“…In temperate forests, canopy gaps are generally smaller in young stands than in old stands (Clebsch & Busing 1989;Bradshaw & Spies 1992). The difference in gap size between young and old stands can be attributed to the comparatively small crown size of trees comprising the young stands (Clebsch & Busing 1989). Like the narrow-crowned boreal forests, young temperate stands also exhibit poor regeneration of intolerants.…”