Urbanisation and specifically impervious cover alter riparian plant communities in a rapidly urbanising landscape in the Himalayas

Impervious covers, such as roads, pavements, buildings, and parking lots, prevent water infiltration, thereby increasing surface runoff. The expansion of impervious cover along riverbanks in urban areas poses a significant threat to riparian ecosystems by altering species diversity and composition through hydrological changes. As dynamic ecosystems at the interface between aquatic and terrestrial environments, riparian areas play a crucial role in aquatic ecology, particularly in terms of biodiversity, bank stability, nutrient dynamics, and hydrological processes. Employing plot-based floristic sampling, this study investigated the effects of urbanisation, particularly impervious cover (quantified as PTIA, Percentage of Total Impervious Area), on riparian plant communities in the rapidly urbanising landscapes of Thimphu City, Bhutan. Additionally, other environmental variables and soil chemical properties were examined to determine whether they confounded the impacts of PTIA. The results showed that PTIA had a significant impact on the diversity and composition of riparian plant communities. Overall, higher levels of PTIA supported a greater number of plant species, including exotic species, but exhibited the lowest α-diversity and reduced tree abundance compared to reference sites (p < 0.001). These areas favoured Rumex nepalensis–Equisetum spp. community, while low PTIA supported the Calamagrostis scabrescens–Elaeagnus umbellata community. Significant changes in species diversity and community structure were observed at a PTIA threshold of approximately 60 % (p < 0.05), indicating substantial ecological alterations beyond this point. Except for soil pH, which influenced plant communities independently rather than confounding PTIA’s effects, other environmental variables and soil chemical properties did not have any significant effects on riparian plant communities. Our study revealed that PTIA was the primary predictor for changes in plant communities in rapidly developing urban riparian landscapes. The relationship between PTIA and plant communities highlights the importance of considering PTIA in urban biodiversity management, as it can predict the extent of vegetated land required to sustain healthy riparian ecosystems. These findings suggest that rapidly urbanising cities can realign their urban development and riparian biodiversity management strategies by assessing existing impervious cover and establishing appropriate PTIA thresholds to balance urban growth with the conservation of riparian environments.