Importance of shade trees (Grevillea robusta) in the dispersal of forest tree species in managed tea plantations of southern Western Ghats, India

Abstract:

Abandoned plantations of coffee, tea and other commercial crops offer opportunities for understanding ecological processes in modified forest ecosystems. Unlike tree plantations tea is maintained as a shrub with a continuous dense short canopy that precludes large-frugivore activity thereby limiting dispersal of forest species to such areas. In this study we determine how location and density of Grevillea robustaa shade tree in tea plantations and proximity of plantations to forests influences seed arrival from forests into the plantations. We also estimate the importance of dispersal modes in the colonization processes. We laid 10 × 10-m plots at three distance intervals from the forest edge in three different plantation types with varying shade tree densities. Within the plots we laid four 1× 1-m subplots at the corners of the plot. We estimated species richness, abundance and categorized the seeds into dispersal modes in these plots. Grevillea robustaincreased species richness of seeds by three times and abundance of seeds by 3–30 times compared with plantations without them. Higher density of G. robustaincreased seed input changed species composition and altered species dominance in the plantations. Distance to forests influenced seed arrival in plantations without G. robustatrees and plots 95 m from the forest did not have any seeds in them. No such effect was seen in plantations with G. robustatrees. Seeds dispersed by birds or a combination of birds and mammals contribute 30% of the seeds reaching the plantations with G. robustaand this was not influenced by distance from the forest. In plantations without G. robustabird dispersal is restricted to 25 m from the forest edge. In general density of shade trees has a strong influence on seed arrival which can negate the forest proximity effect and enhance natural forest colonization.