Can Austroeupatorium inulifolium Invasion Favour Tree Species Establishment on Highly-Degraded Man-made Grasslands

Abstract

It is well known that invasive species alter vegetation structure and composition, but most ofthe evidence to support this claim come from natural ecosystems. In many instances, thesestudies reported negative consequences on these natural habitats. Within last few years, thehighly degraded grasslands at the Knuckles Conservation Area (KCA) have rapidly invadedby Austroeupatorium inulifolium, a notorious invasive species. The present study was aimedat investigating the impacts of Austroeupatorium on grassland communities, and also to seeany positive role in forest expansion process towards these invasive grasslands by borderingforest remnants.

A complete vegetation sampling was carried out in 3 grassland communities having differentlevels of invasion; grasslands densely-invaded with Austroeupatorium (HIG), and lessdenselyinvaded (LIG) and the forest-grassland edge (FGE). For each grassland community,nine randomly placed 4 m2 quadrates were used to enumerate the vegetation. Anothersampling was carried out using two forest-grassland edge (FGE) communities; forest remnantbordered by LIG (FGE- LIG) and forest remnant bordered by HIG (FGE- HIG). At each site,5 linear quadrates (3 m x 12 m) were placed parallel to the FGE at different distances; 0(edge), 5 and 10 m away from the edge towards the grassland and towards the forest interior.Six such transects were sampled for each FGE category.

A total of 30 species have been identified belonging to 20 families in HIG while 21 species(belonged to 14 families) and 14 species (belonged to 9 families) were identified from LIGand FGE respectively. Tree saplings (4 species) were recorded only in HIG. The two edgehabitats showed somewhat different vegetation composition. Of the 182 and 430 individualsof tree saplings encountered at FGE-LIG and FGE-HIG respectively, majority of them werefound in quadrates laid inside the forest (76 and 88% respectively). Abundance and richnessof tree saplings were significantly higher at 10 m towards the forest interior bordered by HIGthan that of LIG.

Present results suggested that Austroeupatorium invasion has a facilitative effect on treeseedling establishment probably through enhanced shading effect and improved edaphicconditions. The results also suggest that Austroeupatorium invasion has the potential tochange the community structure in forest-grassland edges which eventually influence thevegetation community in the forest interior, at least up to 10 m. The facilitative effect ofAustroeupatorium invasion on the forest tree establishment needs further studies to verify itslong-term effect on these grasslands.