Effect of removal of an invasive species, Acacia mearnsii, on plant species diversity in a shola-grassland ecosystem

Abstract

Species invasions have become a global threat to native biodiversity in the past couple of centuries. Getting rid of invasive species from an ecosystem is a difficult task, owing to their aggressive nature; but there are studies being carried out throughout the world for the restoration of invaded spaces. The most common method used to restore native ecosystems is the manual removal of invasive species. Acacia mearnsii, or black wattle, is one such invasive tree species which has invaded the high-altitude grasslands of the shola-grassland ecosystem in the Nilgiris, and the Tamil Nadu Forest Department has been taking efforts to restore these grasslands by clearing wattle trees (cutting them down one site at a time, revisiting it every 2-3 years). This study attempted to quantify changes in plant species richness and cover in grasslands over time, as well as changes in soil carbon and nitrogen content, following the clearing of wattle. This was done by sampling (a) species richness, (b) vegetation cover, and (c) soil carbon and (d) nitrogen of sites that have been cleared of wattle over the past five years. It is seen that the vegetation cover of grassland species in wattle cleared areas is higher than that in intact wattle sites. However, there is not much change in the species richness across different sites, wattle cleared, or intact. This suggests that the wattle patches support most grassland species, but do not give it a conducive environment to spread. Once wattle is removed, grassland species spread and occupy the cleared areas. The richness and cover of grassland species in all wattle cleared sites is much higher than richness and cover of shola or invasive species, indicating that the regeneration of flora after wattle removal is dominated by native grassland species. The soil carbon and nitrogen has not change across years since wattle removal. However, there seems to be an unexpected change in species richness, cover as well as carbon and nitrogen across the two elevations which were sampled. This could be due to factors that were unaccounted for, and needs to be investigated further to get a better understanding of the system.