Author Identifier
Karma Jamtsho: https://orcid.org/0000-0003-0846-489X
Date of Award
2025
Document Type
Thesis
Publisher
Edith Cowan University
Degree Name
Doctor of Philosophy
School
School of Science
First Supervisor
Eddie van Etten
Second Supervisor
Mark Lund
Third Supervisor
David Blake
Abstract
The proliferation of impervious cover (IC) - including roads, buildings, and pavements - resulting from urbanisation is rapidly transforming fragile riparian ecosystems in Himalayan cities across South Asia. In these regions, economic growth often takes precedence over environmental protection, and the ecological impacts of expanding IC remain poorly understood. Globally, riparian research has largely overlooked these regions due to financial and logistical constraints, raising concerns about the adequacy of the existing literature and the appropriateness of the urban environment management strategies implemented. This thesis aims to address a critical gap by examining the cumulative impacts of IC expansion on the Himalayas' fragile and narrow riparian ecosystems, using Thimphu City, Bhutan, as a case study. Through the integration of remote sensing, field surveys, metagenomic analysis, and microplastic assessment, this study investigated spatial and temporal land cover changes, plant community composition and structure, soil bacterial and functional diversity, and microplastic pollution in riparian areas of both urban and peri-urban reference sites along the Wang River (also referred to as Raidāk River).
Between 1997 and 2022, the area of IC in Thimphu’s riparian areas increased by 44.4%, while agricultural and low-vegetation land cover declined by 47.7%. Despite this reduction, vegetation cover slightly increased, primarily due to plantations and the spontaneous downstream spread of the exotic tree species Salix babylonica. Urban sites exhibited higher plant species richness and a greater presence of exotic species; however, due to the dominance of a few species, overall diversity was lower. Exotic species were found to be highly resistant to the impacts of IC, thriving even at IC levels as high as 80%, whereas native species richness began to decline at IC levels above 40%. Sites with higher IC were dominated by non-riparian, ruderal, and stress-tolerant species such as Rumex nepalensis and Calamagrostis scabrescens, which exhibit limited dispersal strategies, unlike the more functionally diverse plant communities observed at reference sites.
Given the lower plant species diversity observed in areas with higher IC, it was hypothesised that soil bacterial and functional diversity would exhibit a similar pattern of change. However, the results revealed a contrasting trend: higher bacterial and functional diversity in areas with greater IC. This suggests that plant diversity may not be a reliable indicator of soil bacterial diversity in urban environments subject to multiple human disturbances. The soil carbon-to-nitrogen ratio significantly explained variations in bacterial diversity and community composition. This suggests that IC indirectly influences microbial communities by contributing to nitrogen enrichment, likely through sewage discharges and organic waste deposits.
Further, the abundance of soil microplastics highlighted another indirect effect of IC on riparian ecosystems. A significant positive association was observed between elevated phosphorus levels, potentially originating from wastewater and sewage discharges, and the abundance of microplastics. These pollutants are transported from urban centres into riparian zones via IC structures such as stormwater drains and drainage systems, accumulating microplastics in riparian soils. Urban soils accounted for 93.3% of all detected microplastic particles.
This study provides a comprehensive assessment of the ecological impacts of increasing IC expansion in Thimphu City, Bhutan, offering valuable baseline data for understanding the effects of urbanisation on narrow and fragile riparian ecosystems in the Himalayas. It introduces evidence-based IC thresholds and a tipping point model to guide and check IC expansion in climatically and ecologically sensitive regions. Building on these findings, the study underscores the need for future research to adopt a collaborative, multi-scale approach across urban centres at varying stages of development. This approach should incorporate long-term ecological monitoring to capture seasonal and inter-annual dynamics in IC-plant–microbe interactions and hydrological processes.
DOI
10.25958/63nt-dn56
Access Note
Access to this thesis is embargoed until 12th August 2026
Recommended Citation
Jamtsho, K. (2025). Impacts of impervious cover expansion on riparian areas in a rapidly urbanising Himalayan landscape. Edith Cowan University. https://doi.org/10.25958/63nt-dn56