## 4 Discussion  
Although greening in China and India has been highlighted recently and mostly attributed to the land-use management such as reforestation and afforestation efforts and multiple cropping albeit in the process of urbanization [6; 41], it is largely unknown how climate alone has contributed to the greening. Our approach in this study allowed us to obtain a 'cleaner' and unbiased relationship between vegetation greenness and climate change by insulating from the impact of land-cover changes. The greenness of temperate grasslands is primarily precipitation-controlled compared to that of temperate deciduous forests co-controlled by temperature and precipitation [42; 43]. Precipitation, especially in growing season, dominates the greenness dynamics in the temperate grasslands in Northern China (Figure 5). Compared to trees in the forests, grass has shallow-root system which limits its uptake of water/nutrients from deep soil layers. Therefore, green biomass production of temperate grasslands fluctuates with rainfall, especially the growing-season rainfall. The analyses on the gross primary productivity (GPP) over China indicated that the GPP in the temperate semi-arid grasslands is dominated by precipitation [44; 45]. In the US Great Basin, the interannual variation of vegetation greenness was found to strongly correlated with total precipitation, but not surface air temperature [46]. The diurnal and seasonal dynamics of temperature interacts with wet-year precipitation especially that in nongrowing seasons to coregulate greenness of temperate deciduous-broadleaved forests (Figure 5). In addition to the well-developed root systems that hold the moisture from both shallow and deep soil horizons, trees have much more functional biomass than grasses to maintain and higher longevity than grasses which makes trees sensitive to intra- and inter-annual climate variability and phenology changes [7; 14]. Our results showed that wet-year especially nongrowing-season precipitation contributes to the greenness of deciduous-broadleaved forests (Figure 5), whereas grassland greenness is strongly correlated to growing-season precipitation. More importantly, daytime (maximum) and nighttime (minimum) temperatures have asymmetric effects on the production and leaf growth of trees in deciduous-broadleaved forests, and the asymmetric effects are highly spatially heterogeneous (Figure 5). Nighttime warming as reflected in increased minimum temperature in general elevates respiration cost, thus tends to reduce leaf growth. On the other hand, nighttime warming tends to reduce the chances of frost damage so that to extend the growing season in early spring and autumn, hence favors the seasonal peak and average greenness [19; 20]. In the temperature-limited northern part of the deciduous-broadleaved forests, the net effect of nighttime warming tends to enhance the greenness (Figure 5b) due to the benefit from lengthening growing season [14]. However, in the warmer southern part, the higher maintenance cost contributes to a negative net effect of nighttime warming (Figure 5b) which reduces leaf growth and greenness. Daytime temperature directly regulates photosynthetic enzyme activities during growing season, but also causes water loss through evapotranspiration which can exert drought stress on photosynthesis. The contrasting pattern of the greenness responses to daytime temperature (\(T_{\text{max}}\), Figure 5c) between the northern and the southern deciduous-broadleaved forests could be attributed to species composition with birch and poplar dominated in the north while oak dominated in the south. Oak is more drought-tolerant than birch or poplar [12], and thus is less sensitive to drought condition during the growing season. Therefore, the oak-dominated southern part exhibited positive net effect of \(T_{\text{max}}\) on greenness due to enhanced photosynthetic activities.