ArticleDeciphering impacts of climate extremes on Tibetan grasslands in the last fifteen years
Introduction
There is high confidence that anthropogenic activities have not only led to a gradual change in mean climate state, but have also altered the variability of temperature and precipitation, resulting in an observed increased intensity and frequency of climate extremes (e.g. drought, extreme wet, extreme cold and extreme hot) in the past several decades [1], [2], [3], [4], [5], [6]. Furthermore, these climate extremes are projected to become more intense and more frequent with stronger increases at higher levels of global warming [7]. Evidence gleaned from remote sensing, tree rings and global-change manipulative experiments have clearly shown that the occurrence of climate extreme events could trigger profound impacts on ecosystem carbon cycling [8], [9], [10], which would, in turn, accelerate or limit climate change through climate-carbon cycle feedbacks [11], [12]. Climate extremes are therefore becoming an increasingly important driver of changes in terrestrial ecosystem carbon cycling, and their impacts on terrestrial ecosystems are reported to be one of the largest uncertainties in carbon cycling [8], [10], [12].
Nearly two-thirds of the Tibetan Plateau, also known as the “Third Pole”, are covered by alpine grasslands, which provides forage for grazing livestock and is the major vegetation type covering the headstream regions of the major Asian rivers. The stability of Tibetan grassland production is crucial to local herders and for downstream water resources. Observations from Tibetan meteorological stations indicate that the occurrence of climate extreme events generally increased over the last several decades [13], [14], [15]. But our current understanding of Tibetan grassland responses to climate extremes is mainly derived from ecosystem-level manipulative experiments which artificially impose extreme conditions (e.g. precipitation exclusion and heating) [16]. These manipulation experiments can improve our mechanistic understanding of grassland growth responses, but they do not adequately represent grasslands located in different climate regimes, nor do they consider realistic extreme conditions in an historical sense [17]. We therefore still lack a full description of the occurrence of climate extremes and an estimate of the relative magnitude of growth deviations induced by climate extremes over Tibetan grasslands.
Various lines of evidence indicate that reduced precipitation due to a weakened Indian summer monsoon [18], [19] has been a crucial driver of grassland browning in southern Tibet since 2000 [20], suggesting that water availability is the main limiting factor for plant growth over the Tibetan Plateau. It has been presumed that droughts, with severe departures from normal precipitation, have strong negative impacts on grassland growth in the region [20], [21]. Besides water availability, temperature is also recognized as an important constraint on plant growth in relatively cold environments [22]. In the last few decades, the warming rate over the Tibetan Plateau has been twice as fast as the global average [23], and the occurrence of temperature extremes, especially extreme hot spells, have become much more frequent [13], [24]. These temperature extremes are generally expected to exert negative impacts on the carbon cycle, since the optimum temperature for plant photosynthesis could be exceeded during periods of extreme heat, and, during extreme cold, frosts could cause damage to plants [8], [25], [26], [27]. However, grasslands that have adapted to the cold, alpine environment over the Tibetan Plateau might differ from other ecosystems with respect to carbon-cycle responses to temperature extremes. Quantifying the impacts and mechanisms of climate extremes over alpine grasslands could provide a comparison to studies of grasslands in other regions, culminating in a better understanding of the role of climate extremes in grassland carbon cycling.
Nearly real-time satellite remote sensing, with a short return interval to the same locations, offers the research community unprecedented information to detect terrestrial biosphere responses resulting from climate extremes in a consistent way [8]. Here, we used climate data and remote sensing proxies of vegetation growth to characterize spatio-temporal patterns of climate extremes (drought, extreme wet, extreme cold and extreme hot) and their impacts on grassland growth over the Tibetan Plateau since 2001. The knowledge gained from such satellite-based studies can provide invaluable information for improving the model representation of climate-extreme induced perturbations in ecosystem carbon cycling. With this in mind, we used the satellite-derived results to assess how state-of-the-art process-oriented ecosystem models simulate grassland responses to naturally occurring climate extreme events over the Tibetan Plateau.
Section snippets
Study area
The Tibetan Plateau is one of the largest and highest plateau in the world, with an average altitude of over 4,000 m. The climate of the Tibetan Plateau is controlled by the Indian summer monsoon in the summer and westerlies in the winter [28], [29], [30]. Precipitation in the region mainly occurs in the summer with the total amount of annual precipitation varying from over 1,000 mm in the southeast to less than 50 mm in the northwest [30]. The temperature over the Tibetan Plateau is generally
Frequency of climate extremes
The identified drought frequency at the pixel level is around 1.5 months decade−1, ranging from 0.5 months decade−1 in the southwestern plateau to 3.5 months decade−1 in the central plateau (Fig. 1a). Higher drought frequency occurs in the central plateau, and lower frequency in the northeastern and southwestern plateau areas. The distribution of drought frequency shows a clear altitude dependence, with an increase with altitude until 4,500 m and then a decrease at higher altitudes (Fig. 1b).
Conclusions
We have performed an overall assessment of the impacts of four types of climate extremes on alpine grassland growth over the Tibetan Plateau. Drought was recognized as the most severe extreme event that leads to a widespread decline in grassland growth in the region. The recent increase in frequency of extreme wet can potentially negate the negative impacts caused by drought. Extreme hot events, which mainly occur in May, generally stimulate grassland growth in this relatively cold environment.
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgments
This work was supported by the Strategic Priority Research Program (A) of the Chinese Academy of Sciences (XDA20050101), the National Natural Science Foundation of China (41530528, 41871104), the Chinese Postdoctoral Science Foundation Project (Y7Gc011012), the Key Research and Development Programs for Global Change and Adaptation (2017YFA0603604), the Second Tibetan Plateau Scientific Expedition and Research (STEP) Project and the Thousand Youth Talents Plan Project in China. Mr. Tao Yang
Dan Liu is currently a postdoc at the Key Laboratory of Alpine Ecology, Institute of Tibetan Plateau Research, Chinese Academy of Sciences. She received her Ph.D. at Beijing Normal University in 2016. Her research focuses on the combined use of satellite data and models to understand carbon cycle in cold regions.
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Dan Liu is currently a postdoc at the Key Laboratory of Alpine Ecology, Institute of Tibetan Plateau Research, Chinese Academy of Sciences. She received her Ph.D. at Beijing Normal University in 2016. Her research focuses on the combined use of satellite data and models to understand carbon cycle in cold regions.
Tao Wang is a professor at the Key Laboratory of Alpine Ecology, Institute of Tibetan Plateau Research, Chinese Academy of Sciences (ITPCAS). Before joining in ITPCAS, he worked at Laboratory of Glaciology and Geophysics of the Environment and Laboratory of Climate Science and Environment in France. His research interest focuses on the responses and feedback of cold ecosystems to global change based on field experiments, satellite observations and modeling.
SPECIAL TOPIC: The Second Tibetan Plateau Scientific Expedition and Research (I).