Interactions with a dwarf shrub structure herbaceous plant diversity in the Trans-Himalayan rangelands
The Trans-Himalayan rangeland ecosystems support a unique assemblage of animals and sustains agro-pastoralist tribes. Dwarf shrubs are a dominant growth form in many alpine rangelands, where they co-occur with herbaceous plants. Theoretical expectations of their effects on herbaceous diversity are unclear, with a possibly complex balance of competition and facilitation occurring between them.
We studied the plant community associated with Caragana versicolor, a dominant dwarf shrub in the Spiti Valley, India, to understand how interactions between plants shapes herbaceous diversity. We surveyed herbaceous plants in paired plots within and outside the Caragana canopy, in 4 watersheds differing in dominant grazer (livestock and bharal) and altitude (4200-5000m). All plants growing in 30 paired plots per watershed were counted, and we calculated the effects of Caragana on plot level diversity using log response ratios. To compare the effects of the shrub on dominant and rare species at the landscape scale, we created Renyi diversity profiles for each watershed. Additionally, we calculated the relative interaction index for every species to see how plant growth form mediates interactions. Finally, in an experiment, we removed herbaceous vegetation from shrub canopies throughout a growing season, and compared the flowering of these shrubs to paired, un-manipulated controls.
The plant community within Caragana had a higher richness (by ~25%) at the plot level, and soil within the canopy had 28% more organic matter. The canopy periphery harboured a more diverse community than the larger core, possibly due to reduced shading and greater space. When classified based on life form, graminoids and erect forbs showed positive association with the shrub, while prostrate forbs showed a negative association. Renyi diversity profiles for the community at the scale the watershed showed that while Caragana did not increase gamma diversity, it did increase diversity of rare species at the high grazing and high altitude sites.
Removal of the herbaceous plant community from the Caragana canopy increased flowering of the shrubs relative to controls, suggesting that presence of herbaceous plants imposes a fitness cost on the shrub.
In conclusion, Caragana versicolor does modify the plant community of these rangelands, increasing local diversity by harbouring more species of grasses and forbs, but shading out plants with prostrate growth forms. These patterns are consistent across a wide variation in grazing intensity and altitude. Association of the plants with Caragana suggests facilitation of some plants by the shrub, along with reciprocal costs to the shrub.