A dominant dwarf shrub increases diversity of herbaceous plant communities in a Trans-Himalayan rangeland
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Plant communities are structured by both competition and facilitation. The interplay between the two interactions can vary depending on environmental factors, nature of stress, and plant traits. However, whether positive or negative interactions dominate in regions of high biotic and abiotic stress remains unclear. We studied herbaceous plant communities associated with a dwarf shrub Caragana versicolor in semi-arid, high altitude Trans-Himalayan rangelands of Spiti, India. We surveyed 120 pairs of plots (within and outside shrub canopies) across four watersheds differing in altitude, aspect, and dominant herbivores. Herbaceous communities within shrub canopies had 25% higher species richness, but similar abundance when compared to communities outside the canopy, with the shrub edge having higher diversity than the centre of the canopy. Grasses and erect forbs showed positive associations with the shrub, while prostrate plants occurred at much lower abundance within the canopy. Rare species showed stronger positive associations with Caragana than abundant species. Experimental removal of herbaceous vegetation from within shrub canopies led to 42% increase in flowering in Caragana, indicating a cost to the host shrubs. Our study indicates a robust pattern of a dwarf shrub facilitating local community diversity across this alpine landscape, increasing diversity at the plot level, facilitating rare species, and yet incurring a cost to hosts from the presence of herbaceous plants. Given these large influences of this shrub on the vegetation of these high altitude rangelands, we suggest that the shrub microhabitat be explicitly considered in any analyses of ecosystem health in such rangelands.
KeywordsFacilitation Alpine Dwarf shrub Altitude Community diversity Grassland Nurse plant
We thank Himachal Pradesh Forest Department for permits and their support. Fieldwork was carried out by SBI and Tenzin Sharaf, with assistance from Tandup Chhering, Rinchen Tobge, and many others in Kibber. We thank Dr. Jayashree Ratnam for inputs in the analysis and planning. We are grateful for helpful critiques from the editor and two anonymous reviewers. SBI was supported through the Kishore Vaigyanik Protsahan Yojana fellowship from the Department of Science and Technology, Government of India at IISER, Pune and NCBS Bangalore. SB acknowledges support from DST-SERB, DBT-IISc, and MoEFCC. MS acknowledges support from NCBS.
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