Climate change and grazing drive cascading responses in plant communities and carbon cycling on the Tibetan Plateau

Background/Question/Methods

Changing climate and land use practices will have interactive and cascading effects on ecological communities, processes, and the ecosystem services on which humans depend.  Short-term responses in ecosystem functioning, such as carbon cycling, may either be exacerbated or mitigated by longer-term shifts in plant community composition, which may not be captured within the timeframe of many field experiments.  Like other alpine ecosystems, the Tibetan Plateau is already experiencing above-average climate warming, which is occurring simultaneously with restrictions placed on traditional livestock grazing practices.  To understand how changing climate and land use will affect this alpine meadow ecosystem, we set up a climate change and grazing experiment in central Tibet.  We measured plant species composition, soil nutrients, and ecosystem carbon dioxide (CO₂) fluxes in our experiment plots as well as in different plant communities across the landscape.  We link results from the first four years of our experiment with landscape-level observations and a time series of satellite images to develop a longer-term view of how the alpine meadow system will likely respond to climate and land use change, as well as a causal understanding of the factors driving shifts in plant community composition and carbon sequestration on the Tibetan Plateau.

Results/Conclusions

After four years of experimental climate and grazing manipulations, plant species composition changed significantly, with corresponding changes in ecosystem CO₂ fluxes.  Yak grazing maintained the alpine meadow community structure by promoting the growth of the dominant sedge species and suppressing shrub growth, but warming caused a significant die-back of the dominant sedge and a four-fold increase in shrub production.  Warming-induced shifts in plant community composition reduced ecosystem CO₂ uptake, suggesting that over time, less carbon will be sequestered by the system as the climate continues to warm.  Analyses of satellite images and measurements in different plant communities across the landscape also suggest that shrubland production has been increasing, while production of the meadow community has been decreasing in this region over the past two decades, a time period during which ambient temperatures have been rising and livestock grazing has been increasingly restricted.  Furthermore, plant communities dominated by shrubs had lower soil carbon than the healthy meadow communities.  These results suggest that warming-induced, longer-term shifts toward more woody plant growth will be unlikely to mitigate losses of ecosystem function, both in terms of soil carbon storage and production of forage for livestock and wildlife.