A dominant dwarf shrub increases diversity of herbaceous plant communities in a Trans-Himalayan rangeland

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. But, 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 center 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 vegetation of these high altitude rangelands, we suggest that the shrub microhabitat be explicitly considered in any analyses of ecosystem health in such rangelands

Sensitivity of grasslands to rainfall variation: the role of resource colimitation

University of Minnesota Ph.D. dissertation. August 2020. Major: Ecology, Evolution and Behavior. Advisors: Elizabeth Borer, Eric Seabloom. 1 computer file (PDF); ix, 94 pages.Identifying the factors shaping ecosystem sensitivity to global change is an important unresolved question in ecology. Grasslands and savannas account for one-third of Earth's terrestrial primary production, and variation in that production is strongly driven by rainfall. The sensitivity of ecosystems to rainfall should depend on the availability of other co-limiting resources, such as nutrients. However, it is unclear how the strength of that co-limitation varies among sites - across gradients of aridity, soil fertility and community composition. I leverage long term experiments within the Nutrient Network research cooperative to analyse how fertilization affects the sensitivity of grassland plant communities to rainfall. I expected that sensitivity to rainfall would decline from arid to mesic ecosystems, while the effects of fertilization on sensitivity would increase along that same gradient. I found the sensitivity of grasslands to extreme drought in 13 sites did not change with aridity, but it was increased by fertilization. For 37 grassland sites around the world, fertilization generally increased the sensitivity of biomass to rainfall variation irrespective of aridity. This showed that grasslands are co-limited by nutrients and water all across the aridity gradient present among our sites. Finally, I found that community shifts due to chronic nutrient addition at more than half the sites studied will still retain species that perform well in both high and low rainfall years. Thus composition change is unlikely to amplify the effects of nutrient addition on biomass sensitivity to precipitation