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The environmental factors that influence tree–grass abundances in tropical savanna and grasslands are not well understood, particularly for woodland–grassland mosaics in humid zones. We studied the effects of abiotic and spatial variables on woody and herbaceous species distributions in a Terai ecosystem of northeastern India. We evaluated the importance of climatic and non-climatic factors that maintain variable tree–grass ratios across the landscape, and also accounted for spatial connectivity and dispersal. We measured species abundances of woody and herbaceous plant species in 134 plots with each 30 m × 30 m in a 519 km2 protected Terai habitat, and derived several climatic and non-climatic environmental factors. We constructed variables based on different models of spatial connectivity among sites, to test their influence on species abundances. We then used redundancy analyses and variation partitioning to quantify the importance of environmental variables and spatial structure on variation in tree–grass abundances. We found that environmental variables including rainfall, fire, water stress, topography and soil nutrients had statistically significant effects on species abundance and tree–grass ratios. Spatial structure was significant, and the best spatial model was an inverse distance-weighted model with linkage extending to 23.5 km, indicating weak dispersal limitation. About 21% of the variation in species abundance was explained by the selected environmental and spatial factors. The results indicate that dynamic plant communities in which spatial–temporal variation in environmental factors may drive stochasticity in species distribution and abundance, thus dominantly influencing on the vegetation mosaic. [ABSTRACT FROM AUTHOR]

[Display omitted] • Terai vegetation is dynamic, and grasslands have declined in the last thirty years. • Fire occurrence has the strongest influence on grassland to woodland transformation. • Grassland decline was linked with increased area under croplands in some sites. The Terai-Duar Savanna and Grassland ecoregion (hereafter Terai), located at the base of the Himalayas in the Indian subcontinent, support diverse ecosystems with pure grasslands, savanna, and forests in varying proportions. There are indications that grasslands and savanna are being lost to woody encroachment across the ecoregion. A significant fraction of plant and animal taxa are endemic to grasslands, so the decline of grasslands is likely to impact biodiversity and ecosystem function. We assessed large-scale vegetation changes to Terai habitats over three decades (1989–2019) and quantified the role of environmental factors in driving the observed changes. Selecting eight large protected wildlife habitats (four each from India and Nepal), we performed supervised vegetation classification for three-time points. We then quantified grassland-woodland transitions and used Bayesian Conditional Autoregressive spatial models to test the influence of climatic, environmental, and anthropogenic factors on observed changes. Including the protected sites with substantial grassland areas, we found that the total extent of grasslands was 1417 km2 (28 % of total area) in 1988–1989, which declined by 34.4 % to 923 km2 in three decades, while woodland area increased by 8,7% from 3235 km2 to 3516 km2. Grasslands were also converted to cropland or inundated by flooding, but to a lesser extent. Dry season grass fire had the strongest influence on grassland persistence, followed by anthropogenic impacts. However, the eight sites differed with respect to the nature and extent of the changes in vegetation transitions, perhaps reflecting site differences in climatic conditions and anthropogenic influence. [ABSTRACT FROM AUTHOR]

• Terai ecosystems in India are threatened by habitat loss and dry-season fires. • Fires are used to control woody species encroachment into grassland areas. • The ratio of grassland to woodland appears not determined by climate. • Using long-term satellite data, we found that non-climatic drivers influence fires. • Current fire management disproportionately affects roadside habitats. The Terai ecoregion of the Himalayan foothills is among the most fire-affected ecosystems in the Indian sub-continent. Although most of the Terai has already been lost to agriculture and urbanization, the few remaining native habitats are strictly protected due to their high biodiversity and ecological importance. The use of fires to maintain vegetation and wildlife habitat in these protected areas is an integral part of forest management. Although fires are initiated by forest managers or local people, their eventual spread and behaviour are not controlled. We hypothesize that distributions of fires are determined by several direct and indirect drivers like fuel load, fuel moisture content, presence of natural or artificial fire breaks, and climatic attributes of precipitation and temperature. Using the moderate-resolution (375m) satellite sensor-based data we studied the environmental influence on the spatial-temporal patterns of fire events over 18 years (2000-2018) in a 519 km2 protected area in northeastern India. The park has a mosaic of vegetation formations – including dry and swampy alluvial grassland, early successional woodland, and moist tropical forest. Despite high rainfall, there is an intense dry season that renders the herbaceous vegetation susceptible to fires. Using spatial and spatial-temporal Poisson regression models in a rigorous conditional autoregressive Bayesian framework, we found that net primary productivity (a proxy of vegetation type and fuel load), distance to roads (a measure of human influence), elevation (through its influence on floods and vegetation type), and river area extent (by determining the area under vegetation) had a significant influence on the spatial distributions of fires. The climatic signal on interannual variation in fires was weak, but dry season rainfall reduced fire incidence. The disproportionate distribution of fires adjacent to roads compared to the interior, and the repeated burning of some grassland patches need to be addressed in fire management. [ABSTRACT FROM AUTHOR]

Summary: One of the multiple threats to protected areas worldwide, invasive plant species have the potential to decrease biodiversity and ecosystem function. We studied changes in infestation by two widespread invasive plant species – Chromolaena odorata and Mikania micrantha – in India's Manas National Park, a critical conservation site for threatened flora and fauna. Based on field surveys in 2011 and 2019, we found that C. odorata and M. micrantha were present in most of the sampled plots and had newly invaded over 20% of the plots. However, the abundance of M. micrantha decreased in 45% of the plots while C. odorata increased in >50% of the plots. We used a decision tree-based regression with environmental variables as predictors to generate the distribution, abundance and invasion risk maps of the two species. Among environmental variables, road proximity and fire frequency had the strongest influences, respectively, on C. odorata and M. micrantha. Invaded quadrats exhibited lower native-plant diversity than non-invaded quadrats, and C. odorata specifically had a strong negative association with native-plant community structure. These invasive species have increased their range and abundance, and our predicted invasion risk maps indicate the areas where management intervention is urgently needed. [ABSTRACT FROM AUTHOR]