Successional status, seed dispersal mode and overstorey species influence tree regeneration in tropical rain-forest fragments in Western Ghats, India

Abstract

International audienceThe effects of fragmentation and overstorey tree diversity on tree regeneration were assessed in tropicalrain forests of the Western Ghats, India. Ninety plots were sampled for saplings (1–5 cm diameter at breast height(dbh); 5×5-m plots) and overstorey trees (>9.55 cm dbh; 20×20-m plots) within two fragments (32 ha and18 ha) and two continuous forests. We tested the hypotheses that fragmentation and expected seed-dispersal declines(1) reduce sapling densities and species richness of all species and old-growth species, and increase recruitment ofearly-successional species, (2) reduce the prevalence of dispersed recruits and (3) increase influence of local overstoreyon sapling densities and richness. Continuous forests and fragments had similar sapling densities and species richnessoverall, but density and richness of old-growth species declined by62%and 48%, respectively, in fragments. Fragmentshad 39% lower densities and 24% lower richness of immigrant saplings (presumed dispersed into sites as conspecificadults were absent nearby), and immigrant densities of old-growth bird-dispersed species declined by 79%. Saplingspecies richness (overall and old-growth) increased with overstorey species richness in fragments, but was unrelatedto overstorey richness in continuous forests. Our results show that while forest fragments retain significant saplingdiversity, losses of immigrant recruits and increased overstorey influence strengthen barriers to natural regenerationof old-growth tropical rain forests