Author's personal copy Short communication Assessing the recent grassland greening trend in a long-term context based on tree-ring analysis: A case study in North China Eryuan Liang a, *, Dieter Eckstein b , Hongyan Liu c a Institute of Tibetan Plateau Research, Chinese Academy of Science, P.O. Box 2871, Beijing 100085, China b University of Hamburg, Department of Wood Science, Hamburg 21031, Germany c College of Urban and Environmental Sciences, Peking University, Beijing 100871, China 1. Introduction Remote sensing data and climatic models point to an increasing greening since 1982 in the middle and high latitudes of the northern hemisphere (e.g., Braswell et al., 1997; Myneni et al., 1997; Tucker et al., 2001; Zhou et al., 2001; Hicke et al., 2002; Lucht et al., 2002; Gong and Shi, 2003; Nemani et al., 2003; Slayback et al., 2003). Such a trend is also reported for China (e.g., Fang et al., 2003, 2005; Xiao and Moody, 2004; Piao et al., 2006). Most of these large-scale studies have attributed this greening to the recent warming. However, some investigations in the middle latitudes including China, Europe and North America also related it to a raised availability of precipitation (Osborne and Woodward, 2001; Fang et al., 2005; Cao et al., 2004). The typical steppe in North China is highly sensitive to climate change (Xiao et al., 1995; Fang et al., 2005). However, the records of the remote sensing Normalized Difference Vegetation Index (NDVI) and of the grassland productivity are short, not allowing to reliably capture the natural variation of the grassland dynamics. A long-term retrospective assessment of the grassland dynamics has therefore to rely on proxy data. The NDVI of forest ecosystems and the tree-ring width and carbon isotope variation in North America and Eurasia share a common signal reflecting funda- mental aspects of tree physiology (D’Arrigo et al., 2000; Kaufmann et al., 2004; Wang et al., 2004; Chiesi et al., 2005; Lopatin et al., 2006; Leavitt et al., 2008). Recently, Liang et al. (2005) and He and Shao (2006) also revealed significant relationships between the average growing-season NDVI of grassland and tree-ring width series in east Inner Mongolia and on the northeast Tibetan Plateau. However, no attempt has been made to look on the recent grassland greening trend in a long-term context. Our case study is, therefore, aimed at comparing the steppe NDVI record in east Inner Mongolia, North China, with tree-ring chronologies of the same area and assessing the recent greening trend in a long-term retrospection. 2. NDVI and tree-ring data NDVI is often taken as an indicator for the photosynthetic activity and for the Net Primary Productivity (NPP) of the terrestrial vegetation (Tucker and Sellers, 1986; Myneni et al., 1995). In the steppe in east Inner Mongolia the growing-season NDVI of grassland is also considered to be a promising indicator of above-ground grassland biomass or of NPP (Fang et al., 2005; Liang Ecological Indicators 9 (2009) 1280–1283 ARTICLE INFO Article history: Received 9 October 2008 Received in revised form 17 February 2009 Accepted 27 February 2009 Keywords: Tree ring NDVI Greening trend Natural variability Typical steppe North China ABSTRACT Records of the Normalized Difference Vegetation Index (NDVI) and of the productivity of grassland are not long enough to reliably capture natural variation of the grassland dynamics. Herein, we compared a 20-year long growing-season NDVI time series of a typical steppe in North China, and the first principal component (PC1) of four tree-ring width chronologies (based on 69 trees from 3 Pinus tabulaeformis sites and 1 Picea meyeri site) with climate data. Both variables are significantly correlated to each other and to April–July rainfall as well as relative air humidity. This is why PC1 was taken as an indicator of greenness of vegetation in a typical steppe since 1850. The recent greening trend may have happened several times in the past as part of the moisture-driven natural variability. Periods of low growing-season NDVI values derived from tree rings were validated by historical documents. In spite of the ecological complexity, tree-ring analysis provides a simple and efficient method to mirror the key characteristics of the underlying ecological process that was primarily controlled by moisture availability. Thus, tree-ring width time series, clearly documenting the signals of environmental influences, conveys invaluable information about the steppe greenness dynamics and enables us to put the recent greening trend in a long-term context. ß 2009 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +86 10 62849380; fax: +86 10 62849886. E-mail addresses: liangeryuan@yahoo.com, liangey@itpcas.ac.cn (E. Liang). Contents lists available at ScienceDirect Ecological Indicators journal homepage: www.elsevier.com/locate/ecolind 1470-160X/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecolind.2009.02.007