Secondary forest regeneration under fast-growing forest plantations on degraded Imperata cylindrica grasslands

Natural regeneration of native tree species in the understoreys of fast-growing plantation tree species (Acacia mangium, Gmelina arborea and Paraserianthes falcataria) and in adjacent natural riverine forest and non-cultivated grassland areas were studied on Imperata cylindrica grassland sites in Riam Kiwa plantation area, South Kalimantan, Indonesia. Seedling and sapling densities as well as species composition and richness were assessed in a total of 35 ha by a systematic line plot survey.Floristic composition was similar in A. mangium and P. falcataria stands which differed distinctly from G. arborea stands and Imperata grassland area. Riverine forest was characterized by later successional species. In A. mangium and P. falcataria stands, a total of 63 species from 24 families were recorded. Seedling stock consisted merely of pioneer and secondary species, but also several fairly commonly occurring primary forest species. Euphorbiaceae was the most common family representing 42% of sapling and 29% of seedling densities. The most abundant regeneration occurred in A. mangium stands (4,706 seedling/sapling ha^-1), followed by P. falcataria (2,242 ha^-1) and G. arborea (554 ha^-1). Seedling recruitment was significantly lower in plots dominated by grass vegetation than those dominated by no vegetation or shrub vegetation. Breast height diameter and basal area of canopy trees as well as coverage and height of ground vegetation did not explain seedling/sapling density. Results show that plantation establishment with suitable fast-growing tree species facilitates recruitment of a variety of native tree species. This provides potential for rapid restoration of forest ecosystem and for further development of the mixed stands of fast-growing exotics and naturally regenerated native species.