Tsechoe Dorji

Plant functional traits mediate reproductive phenology and success in response to experimental warming and snow addition in Tibet

Global climate change is predicted to have large impacts on the phenology and reproduction of alpine plants, which will have important implications for plant demography and community interactions, trophic dynamics, ecosystem energy balance, and human livelihoods. In this article we report results of a 3-year, fully factorial experimental study exploring how warming, snow addition, and their combination affect reproductive phenology, effort, and success of four alpine plant species belonging to three different life forms in a semiarid, alpine meadow ecosystem on the central Tibetan Plateau. Our results indicate that warming and snow addition change reproductive phenology and success, but responses are not uniform across species. Moreover, traits associated with resource acquisition, such as rooting depth and life history (early vs. late flowering), mediate plant phenology, and reproductive responses to changing climatic conditions. Specifically, we found that warming delayed the reproductive phenology and decreased number of inflorescences of Kobresia pygmaea C. B. Clarke, a shallow-rooted, early-flowering plant, which may be mainly constrained by upper-soil moisture availability. Because K. pygmaea is the dominant species in the alpine meadow ecosystem, these results may have important implications for ecosystem dynamics and for pastoralists and wildlife in the region.

Plant Species Richness, Evenness, and Composition along Environmental Gradients in an Alpine Meadow Grazing Ecosystem in Central Tibet, China

Abstract Plant community properties such as species richness, evenness, and composition vary along environmental gradients. Arid and semi-arid ecosystems, such as the central Tibetan Plateau, are thought to be sensitive to changes in temperature and water availability, and also influenced by a long history of herbivore grazing. We used linear mixed effect models and Canonical Correspondence Analysis to explore how plant community properties varied along gradients of elevation, soil moisture, grazing intensity, solar radiation, ground surface roughness (ground concavity), and pika abundance in an alpine meadow ecosystem in central Tibet. We found that species richness increased with elevation. Species evenness increased with soil moisture at lower elevation, but decreased with soil moisture at higher elevation. Species composition was significantly associated with all environmental variables except solar radiation. The abundance of the dominant plant species, K. pygmaea, which is driven primarily by soil moisture, was also an important variable. We conclude that open patches (habitat), associated with elevation, number of pika burrows and surface roughness, and soil moisture and its effects on K. pygmaea were the most important environmental variables creating variation in plant community properties across this landscape.

Are Droppings, Distance From Pastoralist Camps, and Pika Burrows Good Proxies for Local Grazing Pressure?

Abstract Grazing is regarded as one of the most important factors influencing rangeland ecosystems. Many previous studies have used indirect measures (proxies) of grazing intensity, such as the distance from congregation areas and the number of herbivore excreta, to estimate effects of grazing on vegetation parameters in rangeland ecosystems. However, little is known about how such proxies reflect the true grazing intensity, and if their suitability is consistent across landscape conditions. We measured grazing intensity based on the amount of plant ramets being grazed in 75 plots positioned across a rangeland landscape in central Tibet. Several grazing intensity proxies (distance from pastoral camp sites, number of yak dung patches (old and fresh), sheep and goat dropping piles, and pika (Ochotona curzoniae) burrows (active and abandoned)) were also measured in each plot. Regression models were used to examine the suitability of these proxies in predicting our measure of real grazing intensity. None of the examined proxies satisfactorily predicted real grazing intensity in our study system. The distance from camp sites was the best proxy for grazing intensity (P < 0.001), but it explained only 30% of its variation. Fresh yak dung and active pika burrows explained only 10% and 3% of the variation in grazing intensity, respectively. The suitability of using proxies for grazing intensity did not significantly differ along landscape parameters (elevation, aspect directions, and slope angle) (P > 0.05), except that the number of animal excreta and pika burrows functioned better as proxies at low, compared to high, elevation (P < 0.05). Our results should be of concern since many studies use proxies to estimate effects of grazing on vegetation dynamics and ecosystem properties. We recommend awareness of potential limitations and suitability of proxies, when real grazing intensity is not measured.