摘要：The carbon balance of northern terrestrial ecosystems is particularly sensitive to climatic changes in autumn and spring (Goulden et al. 1998, Piao et al. 2008, Luus et al. 2013). During recent decades, a greening trend has been observed in Eurasia (Zhou et al. 2001, Bogaert et al. 2002), characterized by a longer growing season and greater photosynthetic activity, thus enhancing carbon sequestration and extending the period of net carbon uptake. However, the relationship between inter-annual temperature variability and northern vegetation productivity might be weakening (Piao et al. 2014), possibly due to saturating temperature responses of vegetation in summer, and complex feedbacks from expansion of more southerly species. The strength of the relationship varies according to continent and region (Bi et al. 2013). In temperate ecosystems, the weakening of the relationship coincides with an increase in drought. There is evidence that the increasing water stress created by more frequent regional droughts play a significant role also in boreal ecosystems, increasing the tree mortality in boreal forests (Peng et al. 2011). In addition to the definitive but often relatively limited area mortality effect, droughts may temporarily reduce gross primary productivity (GPP) and increase respiration thus educing net carbon storages over a large region. The drought sensitivity of trees differs according to species, forest heterogeneity, soil characteristics and topography (e.g. Lloyd & Fastie 2002, Kljun et al. 2007). Some field studies have shown that deciduous broadleaved species are more sensitive than evergreen coniferous species (Welp et al. 2007) though spruce dominated forests were also found vulnerable (Beck & Goetz 2011). Mixed stands may be more drought-sensitive than pure stands (Grossiord et al. 2014). However, globally many forest species operate with narrow hydraulic safety margins for xylem water transport and show convergence in their vulnerability to drought (Choat et al. 2012).
