Seasonal Response of Grasslands to Climate Change on the Tibetan Plateau Haiying Yu1,2, Jianchu Xu1,2, Erick Okuto3, Eike Luedeling 3 * 1 Key Laboratory of Biodiversity and Biogeography, Kunming Institute of Botany, Chinese Academy of Sciences, Kunming, China, 2 World Agroforestry Centre, East-Asia Program, Kunming, China, 3 World Agroforestry Centre, Gigiri, Nairobi, Kenya Abstract Background: Monitoring vegetation dynamics and their responses to climate change has been the subject of considerable research. This paper aims to detect change trends in grassland activity on the Tibetan Plateau between 1982 and 2006 and relate these to changes in climate. Methodology/Principal Findings: Grassland activity was analyzed by evaluating remotely sensed Normalized Difference Vegetation Index (NDVI) data collected at 15-day intervals between 1982 and 2006. The timings of vegetation stages (start of green-up, beginning of the growing season, plant maturity, start of senescence and end of the growing season) were assessed using the NDVI ratio method. Mean NDVI values were determined for major vegetation stages (green-up, fast growth, maturity and senescence). All vegetation variables were linked with datasets of monthly temperature and precipitation, and correlations between variables were established using Partial Least Squares regression. Most parts of the Tibetan Plateau showed significantly increasing temperatures, as well as clear advances in late season phenological stages by several weeks. Rainfall trends and significant long-term changes in early season phenology occurred on small parts of the plateau. Vegetation activity increased significantly for all vegetation stages. Most of these changes were related to increasing temperatures during the growing season and in some cases during the previous winter. Precipitation effects appeared less pronounced. Warming thus appears to have shortened the growing season, while increasing vegetation activity. Conclusions/Significance: Shortening of the growing season despite a longer thermally favorable period implies that vegetation on the Tibetan Plateau is unable to exploit additional thermal resources availed by climate change. Ecosystem composition may no longer be well attuned to the local temperature regime, which has changed rapidly over the past three decades. This apparent lag of the vegetation assemblage behind changes in climate should be taken into account when projecting the impacts of climate change on ecosystem processes. Citation: Yu H, Xu J, Okuto E, Luedeling E (2012) Seasonal Response of Grasslands to Climate Change on the Tibetan Plateau. PLoS ONE 7(11): e49230. doi:10.1371/journal.pone.0049230 Editor: Justin Wright, Duke University, United States of America Received February 28, 2012; Accepted October 4, 2012; Published November 16, 2012 Copyright: 2012 Yu et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This research is funded by National Natural Science Foundation of China (31101753). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: E.Luedeling@cgiar.org Introduction Vegetation has an essential function in regulating carbon and energy exchange through phenology [1–4], photosynthetic activity [5,6], autotrophic and heterotrophic respiration [6]and other processes. Several studies have considered changes in vegetation productivity as the primary cause of variation in terrestrial net carbon uptake [6–9]. Recent climate change has exerted significant influences on terrestrial ecosystems and impacts are projected to be even greater in the future [10]. Considerable evidence shows that temperature in the late 20th century was anomalous in relation to the last 1,800 years [11], and the period from 2000 to 2009 was the warmest since instrumental measurements began [6]. Vegetation growth in mid to high latitudes of the Northern Hemisphere is very sensitive to temperature changes [12,13], and a climate-driven greening trend has been well-documented since the 1980s [14– 16]. With its Northern Hemisphere mid-latitude position and high elevation, the Tibetan Plateau is experiencing even faster temperature increases than the global average [17]with net primary productivity (NPP) increasing remarkably, especially in alpine meadows [18]. Assessments of the impact of climate change on plant growth need to examine changes in mean growth rates, seasonality and variability [19]. The instrumental climate record shows high variation in seasonal temperature change: winter months have warmed more rapidly than summer months; night-time temper- atures are more affected than temperatures during the day [19,20]; and the number of frost days (minimum air temperature,0uC) has declined [21]. Overall change in precipitation is harder to generalize, because of substantial temporal and spatial variation [20]. In ecosystems that receive less than 600 mm mean annual precipitation, primary production is largely constrained by water availability, which is, in part, controlled by the nature and timing of rainfall events [22–25]. PLOS ONE | www.plosone.org 1 November 2012 | Volume 7 | Issue 11 | e49230