Functional trait profiles and diversity of trees regenerating in disturbed tropical forests and agroforests in Indonesia

A main question in restoration of degraded forests and forest landscapes recovering from logging and fire is what to expect from natural regeneration through surviving propagules in the soil or seed sources and associated dispersal agents from the surrounding landscape mosaic, as alternative to tree planting. Tree diversity in secondary forests may be high, but based on newcomer species of low wood density and long-distance, abiotic dispersal modes. We compiled and analyzed three pairs of case studies (totaling 815 plots and 11.8 ​ha) of secondary forests recovering from logging, fire and conversion to agroforest in Sumatra and Kalimantan (Indonesia) on mineral soils. Data on tree species diversity, wood density frequency distribution (indicative of successional status) and dispersal modes were compared with those of less disturbed comparator forests in the same landscapes. Relatively undisturbed lowland dipterocarp forest in Kalimantan had close to 200 species of trees (>10 ​cm diameter) at a 1-ha sample scale (and 450 ​at a 10-ha scale). After repeated fires a sample area of 2 ​ha was needed to reach the same species richness. Regulation-based logging had little impact on tree species richness. In rubber agroforest with low-intensity management beyond rubber planting, 50 tree species were found at a 1-ha scale and close to 100 species in 3 ​ha. The Kalimantan forest after repeated fires had a markedly higher fraction of low-wood-density trees (40%), but otherwise, all forests sampled were similar in overall wood density profiles. Selectively logged forest managed by a local community (village forest) and rubber agroforest in Sumatra contained larger fractions of heavy-wood-density trees. The majority of trees (50%–70%) had birds, bats and primates as dispersal agents in all sites. Selectively logged forests had higher fractions of autochorous species (15%) compared to other sites. Anemochorous (wind-dispersed) species, especially Macaranga lowii, were most common (20%) in lowland dipterocarp forest. Comparison between secondary forests and agroforests showed the influence of farmer selection regarding what is allowed to grow beyond the pole stage. Wood density and seed dispersal profiles can be used as degradation indicators of species assemblages across various disturbance levels and types, as they reflect the habitat quality of the surrounding landscape mosaics