Int J Biometeorol DOI 10.1007/s00484-014-0817-5

ORIGINAL PAPER

Assessing phenological change and climatic control of alpine grasslands in the Tibetan Plateau with MODIS time series Cuizhen Wang & Huadong Guo & Li Zhang & Shuangyu Liu & Yubao Qiu & Zhongchang Sun

Received: 19 April 2013 / Revised: 27 February 2014 / Accepted: 3 March 2014 # ISB 2014

Abstract The Tibetan Plateau, a unique cold and dry region recognized as the Earth’s third pole, is primarily composed of alpine grasslands (>60 %). While a warming climate in the plateau is being recorded, phenology of alpine grasslands and its climatic dependencies are less investigated. This study tests the feasibility of the frequently observed Moderate Resolution Imaging Spectroradiometer (MODIS) time series (500 m, 8 days) in examining alpine phenology in the plateau. A set of phenological metrics are extracted from the MODIS Normalized Difference Vegetation Index (NDVI) series in each year, 2000–2010. A nonparametric Mann-Kendall trend analysis is performed to find the trends of these phenological metrics, which are then linked to monthly climatic records in the growing season. Opposite trends of phenological change are observed between the east and west of the plateau, with delayed start of season, peak date, and end of season in the west and advanced phenophases in the east. The correlation analysis indicates that precipitation, with a decreasing trend in C. Wang (*) Department of Geography, University of South Carolina, Columbia, SC, USA e-mail: [email protected] H. Guo : L. Zhang : S. Liu : Y. Qiu : Z. Sun Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing, China H. Guo e-mail: [email protected] L. Zhang e-mail: [email protected] S. Liu e-mail: [email protected] Y. Qiu e-mail: [email protected] Z. Sun e-mail: [email protected]

the west and increasing trend in the east, may serve as the primary driver of the onset and peak dates of greenness. Temperature increases all over the plateau. While the delay of the end of season in the west could be related to higher lateseason temperature, its advance in the east needs further investigation in this unique cold region. This study demonstrates that frequent satellite observations are able to extract phenological features of alpine grasslands and to provide spatiotemporally detailed base information for long-term monitoring on the plateau under rapid climate change. Keywords Tibetan Plateau . Alpine grassland . MODIS time series . Climate change

Introduction Vegetation in terrestrial ecosystems often serves as an indicator of global climate change because it interacts with atmosphere, soil, and water via its photosynthetic processes. Vegetation growth is physiologically limited by temperature, water availability, and radiation that are geographically different (Nemani et al. 2003). While global climate change is being recognized, biotic response on terrestrial land has been frequently observed in aspects of phenological variations. de Beurs and Henebry (2004a) define land surface phenology to measure the timing of periodic vegetation growth from frequent satellite observations. The spatially and temporally extensive remote sensing data, together with ground observation networks in different regions, provide rich information for global phenology monitoring (Betancourt et al. 2005). The Tibetan Plateau in central Asia is one of the predominant geological features on Earth and is undergoing rapid warming (Lin and Zhao 1996; Liu and Chen 2000; Liu et al. 2006; Harris 2010). Climatic records show that it increased

Int J Biometeorol

1.8 °C in the past 50 years (Wang et al. 2008), while the global increase was 0.128 °C per decade in the same period and 0.177 °C per decade in the past 25 years (IPCC 2007). Alpine grasslands dominate the plateau and are one of the most fragile ecosystems in the world due to their high elevation and dry, cold climate. It is of great importance to fully understand the responses of alpine grasslands to the changing climate on this geographically, ecologically, and hydrologically unique environment. Due to physical difficulties in accessing the land, however, historical records and in situ observations are limited, especially in the vast, uninhabited interior plateau. More recently, intensive investigation of the plateau is conducted after coarse resolution satellite observations become available. Among these studies, the 15-day, 8-km Global Inventory Modeling and Mapping Studies (GIMMS) AVHRR Normalized Difference Vegetation Index (NDVI) products are most popularly applied. Despite their long-term observations since 1982, these products have led to controversial findings on the Tibetan Plateau. For example, some studies report that the plateau has experienced greening and advanced green-up dates in recent decades (Yang and Piao 2006; Xu et al. 2008a; Zhong et al. 2010; Zhang et al. 2013), while other studies show that the increase is not significant (Zhang et al. 2007). Some studies even find continuous degradation of greenness and delayed spring growth in alpine grasslands (Liang et al. 2007; Mao et al. 2008; Yu et al. 2010). Due to the harsh climate conditions, alpine grasslands, especially alpine steppes and alpine deserts in the western plateau, are sparse with low biomass and short growing season (Ding et al. 2007). At reduced spatial and temporal resolutions, the GIMMS products cannot reveal detailed vegetation growth cycles and interannual trends, and grassland distributions extracted from these data are spatially fragmented (Piao et al. 2003; Ding et al. 2007; Zhang et al. 2007; Xu et al. 2008a). A recent review paper concludes that the current status of alpine grasslands in the plateau and their responses to climate change are poorly documented (Harris 2010). From 2000, the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard satellites Terra and Aqua has provide a rich set of land surface products at different spatial resolutions (250, 500 and 1,000 m) and temporal intervals (8 days, 16 days, and monthly) (https://lpdaac.usgs. gov/products). Although only available since the past decade, these MODIS products have higher spatial and temporal resolutions than the AVHRR GIMMS products, which could extract more detailed information about land surface phenology and, thus, improve our understanding of alpine ecosystems on the Tibetan Plateau. Assisted with the MODIS 8-day, 500-m land surface reflectance products (MOD09A1) and monthly climatic factors such as precipitation and temperature, this study aims to examine phenological change of alpine grasslands as a response to the changing climate in the past decade. With

improved spatial and temporal details, the findings in this study could serve as base information for long-term climatic and ecological monitoring of the Tibetan Plateau.

Materials and methods Study area and data sets Study area The Tibetan Plateau comprises the Tibetan Autonomous Region, Qinghai Province and partial territories of Sichuan Province and Xinjiang Autonomous Region in southwestern China (as outlined in the inset of Fig. 1). Covering an area of 2.5 million km2 with an average elevation higher than 4,000 m, it is recognized as the “Roof of the World” and “Earth’s third pole” (Zhang et al. 2002). Climate on the plateau is characterized as cold and dry with annual mean temperature of 3–5 °C (Yu and Xu 2009). Precipitation is strongly affected by East Asian and Indian monsoons, which bring precipitation in JuneSeptember with a gradient from over 1,000 mm in the southeast to less than 100 mm in the northwest (Zhao 1994). While rapid warming is obvious in past decades, the plateau-level precipitation records do not show a clear trend (Yu and Xu 2009) although some studies report varying local trends such as increased precipitation in the eastern plateau (Lin and Zhao 1996; Xu et al. 2008b; Liao et al. 2013). Alpine grasslands dominate the plateau and are recognized as a fragile ecosystem because of the harsh climate and high elevation. The plateau is divided into a number of ecoregions and climate divisions (Zheng et al. 1979; Bao et al. 2010), mostly in arid/semi-arid and cold conditions. In the recently published Vegetation Atlas of China (ACAS 2001), and an improved alpine grassland map in the plateau (Wang et al. 2013) alpine grasslands cover more than 60 % of the plateau and are categorized into alpine meadow, alpine steppe, and alpine desert grass along the east-west gradient (Fig. 1). The vegetation atlas is a long-term product based on field surveys in the 1960s and 1970s, the Landsat Multi-Spectral Scanner (MSS) images in the 1970s, and the Thematic Mapper (TM) images since the 1980s. Therefore, it has been used as an optimal land cover product of the plateau (Dr. Yili Zhang, Institute of Geography Science and Natural Resources Research, Chinese Academy of Sciences, personal communication, February 2012). The southeastern plateau has a subtropical warm, moist/semi-moist climate and is covered with forests and scrublands. Cultivated lands are rare (

Assessing phenological change and climatic control of alpine grasslands in the Tibetan Plateau with MODIS time series.

The Tibetan Plateau, a unique cold and dry region recognized as the Earth's third pole, is primarily composed of alpine grasslands (>60 %). While a wa...
3MB Sizes 0 Downloads 3 Views