Tropical rainforest restoration on Imperata grasslands under fast-growing plantations in South-Kalimantan, Indonesia

High rates of deforestation and forest degradation have led to urgent need of forest restoration, especially in tropical domain. In Southeast Asia, deforested areas are often occupied by Imperata grasses, which decelerates successional processes and prevent natural forest regeneration. Establishing plantations on Imperata grasslands based on fast-growing tree species can improve site conditions and foster natural regrowth of a forest. In the late 1980s in an experimental area in South Kalimantan, Indonesia, fast-growing plantations were established to study reforestation methods on Imperata grasslands. As a result, indigenous species began to regenerate naturally under the plantations and developed secondary forest patches and individual trees in the site. This study investigated how well the experimental site has been restored in 30 years, estimating the diversity of woody species and aboveground biomass (AGB). The results were compared with a reference forest, an old-growth forest nearby. In addition, the impact of plantation species selection was investigated. Diversity analyses were conducted with Shannon and Simpson’s indices, species accumulation curve and non-metric multidimensional scaling (NMDS). The AGB was estimated with mixed-species tree biomass allometric model, based on diameter at breast height (DBH) and species-specific wood density. In the restored site, the mean AGB was 165 Mg ha-1, while in the old-growth forest the mean AGB was 188.2 Mg ha-1, respectively. The mean species richness and Shannon index in the restored site were 14 and 3.6, while in the old-growth forest the mean species richness and Shannon index were 20 and 4.19. The results show that the AGB of the restored site had almost reached the AGB of the old-growth forest (87.7%), but the woody species diversity was still significantly lower in the restored site than in the old-growth forest. NMDS ordination plot described that the research sites differed in their species composition. In addition, the species composition between the sample plots were more similar in the oldgrowth forest than in the restored site. The AGB did not differ significantly in plantations stands of different nurse species but the mean species richness and Shannon index were significantly lower in G. arborea (12 and 1.9) than under A. mangium (15 and 2.4) or P. falcataria (20 and 2.6) plantation stands. These results indicate that with appropriate methods, Imperata grasslands can be restored in 30 years close to the pre-disturbance state in terms of structural characteristics, but the recovery of species diversity and composition takes longer. Nurse species selection affects the restoration outcome, when restoring tropical rainforests with fast-growing plantations