Grassland-woodland boundaries in flux: landcover change and grassland dynamics in the Terai-Duar ecoregion

Bhowal, Swapnil (2026) Grassland-woodland boundaries in flux: landcover change and grassland dynamics in the Terai-Duar ecoregion. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Swapnil Bhowal (19RS100)) 19RS100.pdf - Submitted Version Restricted to Repository staff only Download (7MB)

Abstract

In tropical latitudes, regions with high rainfall usually support forest ecosystems, yet some tropical landscapes with relatively high rainfall remain dominated by grasslands. This long-recognised “Savannah paradox” has intrigued ecologists for decades. The Terai-Duar Savanna and Grasslands ecoregion (hereafter referred to as Terai-Duar), located at the base of the Himalayas, is a prominent example where extensive grasslands persist despite high precipitation. The mechanisms, including climatic, environmental, and disturbance factors, that maintain these unique grasslands in conditions where trees should be favoured are poorly understood. Meanwhile, there are reports of substantial loss of grasslands in this ecoregion due to woody encroachment in recent decades. The conservation of these grasslands is essential because their transition to woodland would significantly undermine the diverse flora and fauna, as well as the varied goods and services provided by these ecosystems. In this thesis, we investigate the intricate dynamics of the Terai-Duar grassland-woodland mosaics through three complementary studies. First, we examined decadal-scale grassland-woodland transitions in one protected area in the Terai-Duar ecoregion, Manas National Park in Assam. Using land-use and land-cover (LULC) classifications derived from Landsat imagery (at 30 m resolution) over a 34-year period (1988-2022), we analysed vegetation transitions over multiple shorter intervals during this period. Overall, we found high dynamism, with balanced interchange between grasslands and woodlands even over brief time intervals. However, in the last 15 years of the study (2008 onwards), the changes have favoured woodland formation, resulting in a 24% decline in grassland area. Regression models reveal that grassland persistence was favoured by fire and anthropogenic influences (using distance from human habitation as a proxy), whereas a greater perimeter of the grass-woodland ecotone and proximity to water bodies reduced persistence. The decline in grassland persistence is correlated with a decrease in the effect of fire and anthropogenic influences on grassland persistence in recent years. We expected topography to affect grassland persistence because of seasonal flooding, but elevation had no consistent influence. Second, to understand how grassland areas are maintained despite losses, we quantified the emergence and expansion of grasslands from non-grassland cover across eight protected areas in India and Nepal between 1988 and 2019. Using satellite-based land cover classifications (Landsat TM/OLI) across this interval, we found that grassland emergence was observed in 9.6% of the non-grassland area, accounting for ~37% of the current grassland extent. We applied a two-tiered Bayesian modelling framework: (1) a landscape-scale mixed-effects binomial model (with protected area as a random effect) to identify environmental drivers that promote grassland emergence, and (2) site-specific spatial regression models with conditional autoregressive priors to capture local patterns of grassland emergence, while accounting for spatial autocorrelation in the data. The models revealed that emergence was promoted in sites with steep slopes, high soil moisture, and frequent fire, and was inhibited at higher elevations. Notably, greater grassland emergence near human settlements suggests that anthropogenic disturbance (such as grazing and fire) can mimic natural processes that often maintain open habitats. Finally, in light of the high variability in grassland dynamics, we characterised short-term changes in plant community composition in response to environmental and spatial heterogeneity in Manas National Park. Through repeat vegetation censuses of 50 plots across five sampling periods (November 2021, May 2022, November 2022, May 2023, and November 2023), we documented a total of 168 species. Species composition and diversity fluctuated seasonally and between years. We quantified community dynamics using beta-diversity metrics and complementary ordination and regression-based approaches. We observed high spatial and temporal beta diversity (0.64-0.69 among sampling periods and 0.53-0.72 within plots across sampling periods), overwhelmingly driven by species turnover (84-94%). Principal Coordinates Analysis (PCoA) revealed clear temporal structure, including a strong seasonal separation between May and November along the third and fourth axes. Generalized dissimilarity models (GDM) and distance-based redundancy analyses (dbRDA), together with dbRDA-based variation partitioning, highlighted geographic distance and habitat gradients – particularly flood hazard, soil nutrients, and moisture availability – as influential drivers of compositional change, with their relative importance varying across sampling periods, while spatial structure also explained a moderate fraction of variation. This investigation offers a comprehensive picture of short-term vegetation dynamics, highlighting the key roles of species turnover and environmental heterogeneity in structuring this unique mesic tropical grassland-savanna ecosystem. Collectively, these findings underscore the ecological significance of disturbance regimes (fire, flood, and human influence) and resource availability (soil nutrients and moisture) in maintaining the dynamic equilibrium of the Terai-Duar ecoregion. In the face of serious anthropogenic influences through climate change, habitat loss, and fragmentation, the insights gained from this study can inform effective habitat management and conservation. Maintaining grassland-woodland mosaics is critical for the conservation of both flora and fauna in this highly dynamic and heterogeneous region. Keywords: Terai-Duar, savanna paradox, woody encroachment, land-cover change, disturbance regimes, grassland persistence, grassland emergence, species turnover.

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