Alpine ecosystem vulnerability to climate change on the Tibetan Plateau: Global implications for carbon balance, regional consequences for local pastoralists

The Tibetan Plateau is predicted to undergo climate warming much greater than the global average, as well as shifts in its currently monsoon-dominated precipitation regime. These changes will likely affect the vegetation composition, carbon balance, and nutrient cycling of this alpine, social-ecological system. In 2009 we established a fully factorial experiment to test ecosystem responses to predicted climate changes on the Tibetan Plateau. Our experiment site (4870 m) is located in the foothills of the Nyanchenthanglha Mountains, where local pastoralists graze their livestock. The site is representative of central Tibet’s alpine meadow ecosystems, with the turf-forming sedge, Kobresia pygmaea, as both the dominant species and preferred forage of yaks. Our climate treatments are spring snow addition, which is added at 1-m depth to simulate severe snowstorms, and warming with open top chambers, which elevate air temperatures by an average of 1.2 degrees Celsius. The climate treatments are fully crossed with controlled grazing by yaks, which represents the primary livelihood practice of herders at our study site and throughout Tibet’s grasslands. To better understand the ecosystem shifts that may occur under climate change in this alpine system and to elucidate the drivers of these shifts, we collected data from a suite of measurements in each of our plots. Using a LiCOR 6400 infrared gas analyzer, we measured CO2 fluxes at 4 periods throughout the growing season to obtain values for net ecosystem productivity (NEP), ecosystem respiration, and gross primary productivity. We also measured available nitrogen (N) across three distinct moisture regimes (snowmelt, dry-down, and monsoon). Finally, we quantified changes in vegetation composition and recorded air and soil temperature and soil moisture throughout the growing season. After two years of applying treatments, our findings suggest that Tibet’s alpine grasslands are particularly vulnerable to climate change. We will present data indicating that soil moisture drying seems to be the proximate cause of shifts in vegetation composition, NEP, and N availability, with climate warming as the ultimate cause. Our results also indicate that some of the negative effects of warming may be mitigated by spring snowstorms that provide a pulse of moisture at the start of the growing season. However, in contrast to temperatures that are expected to rise steadily under climate change, snow events on the Tibetan Plateau are much less predictable across both time and space. This mismatch in increased rates of warming and added precipitation to the system will likely affect the landscape patchily. Furthermore, it will leave local herders with the conundrum of how to cope most successfully with the impacts of multiple climate changes, thus illustrating the complexity associated with integrating a fine-scale understanding of ecosystem responses to climate change with coherent strategies for adaptation.