Canopy gaps facilitate establishment of invasive plants in a subtropical broadleaved forest of central Nepal

Abstract

Plant invasions are major threats to the biodiversity and ecosystem function worldwide. In particular, forests are one of the highly affected ecosystems by invasive alien plant species (IAPS). In spite of several previous studies on plant invasion issues in forests, the role of tree canopy gaps in the establishment of IAPS is poorly understood, particularly in Nepal Himalaya. In this study, we examined the effect of tree canopy gaps on IAPS diversity and abundance in Shorea robusta dominated subtropical mixed broadleaved forest in central Nepal. Fifty pairs of plots (10 m × 10 m) were sampled, with a plot of a pair sampled in a canopy gap and its counterpart 20 m away into the forest interior within tree canopy. In every plot, the cover of the individual IAPS was estimated visually, and other environmental variables such as tree canopy, light intensity, slope, litter depth, tree basal area were measured. Higher number of IAPS (9 species) and their cover (31%) were measured in the canopy gaps than in the plots with tree canopy (6 species; 5% cover), suggesting that tree canopy in these forests provide moderate level of resistance to plant invasions. Moreover, the IAPS cover increased with increasing size of canopy gap. Similarly, IAPS richness increased with the age of gap but it was independent of the gap size. Non-metric multidimensional scaling revealed that the IAPS community within forest canopy was a subset of species found in forest gaps, suggesting that the canopy opening has facilitated establishment of IAPS in the studied forests. Therefore, monitoring forest gaps can be helpful in detecting IAPS in their early stage of invasion so that the effective control measures can be implemented before the IAPS are widespread and problematic at the landscape level.