Global environmental change is rapidly altering the dynamics of terrestrial vegetation, with consequences for the functioning of the Earth system and provision of ecosystem services 1,2 . Yet how global vegetation is responding to the changing environment is not well established. Here we use three long-term satellite leaf area index (LAI) records and ten global ecosystem models to investigate four key drivers of LAI trends during 1982-2009. We show a persistent and widespread increase of growing season integrated LAI (greening) over 25% to 50% of the global vegetated area, whereas less than 4% of the globe shows decreasing LAI (browning). Factorial simulations with multiple global ecosystem models suggest that CO 2 fertilization e ects explain 70% of the observed greening trend, followed by nitrogen deposition (9%), climate change (8%) and land cover change (LCC) (4%). CO 2 fertilization e ects explain most of the greening trends in the tropics, whereas climate change resulted in greening of the high latitudes and the Tibetan Plateau. LCC contributed most to the regional greening observed in southeast China and the eastern United States. The regional e ects of unexplained factors suggest that the next generation of ecosystem models will need to explore the impacts of forest demography, di erences in regional management intensities for cropland and pastures, and other emerging productivity constraints such as phosphorus availability.Changes in vegetation greenness have been reported at regional and continental scales on the basis of forest inventory and satellite measurements 3-8 . Long-term changes in vegetation greenness are driven by multiple interacting biogeochemical drivers and land-use effects 9 . Biogeochemical drivers include the fertilization effects of elevated atmospheric CO 2 concentration (eCO 2 ), regional climate change (temperature, precipitation and radiation), and varying rates of nitrogen deposition. Land-use-related drivers involve changes in land cover and in land management intensity, including fertilization, irrigation, forestry and grazing 10 . None of these driving factors can be considered in isolation, given their strong interactions with one another. Previously, a few studies had investigated the drivers of global greenness trends 6,7,11 , with a limited number of models and satellite observations, which prevented an appropriate quantification of uncertainties 12 .Here, we investigate trends of leaf area index (LAI) and their drivers for the period 1982 to 2009 using three remotely sensed data sets (GIMMS3g, GLASS and GLOMAP) and outputs from ten ecosystem models run at global extent (see Supplementary Information). We use the growing season integrated leaf area index (hereafter, LAI; Methods) as the variable of our study. We first analyse global and regional LAI trends for the study period and differences between the three data sets. Using modelling results, we then quantify the contributions of CO 2 fertilization, climatic factors, nitrogen deposition and LCC to the observed trends...

No abstract

18The phenology of spring leaf unfolding influences regional and hemispheric-scale carbon 19 balances 2 , the long-term distribution of tree species 9 , and plant-animal interactions 10 . Changes in 20 the phenology of spring leaf unfolding can also exert biophysical feedbacks on climate by 21 modifying the surface albedo and energy budget 11,12 . Recent studies have reported significant 22 advances in spring phenology as a result of warming in most northern hemisphere regions 1,3,4 . 23Climate warming is projected to further increase 13 , but the future evolution of the phenology of 24 spring leaf unfolding remains uncertain -in view of the imperfect understanding of how the 25 underlying mechanisms respond to environmental stimuli 12,14 . In addition, the relative 26 contributions of each environmental stimulus, which together define the apparent temperature 27 sensitivity of the phenology of spring leaf unfolding (advances in days per degree Celsius 28 warming, S T ), may also change over time 6,8,15 . An improved characterization of the variation in 29 3 phenological responses to spring temperature is thus valuable, provided that it addresses temporal 1 and spatial scales relevant for regional projections. 2Numerous studies have reported advanced spring leaf unfolding which matches warming trends 3 over recent decades 1,3,4 . However, there is still debate regarding the linearity of leaf unfolding 4 response to the climate warming 6,7 . Recent experimental studies of warming using saplings have 5 shown that S T weakens as warming increases 7 . Experimental manipulation of temperature for 6 saplings or twigs, however, might elicit phenological responses that do not accurately reflect the 7 response of mature trees 16,17 . We therefore investigated the temporal changes in S T in adult trees 8 monitored in situ and exposed to real-world changes in temperature and other climate variables. 9These long-term data series were obtained across Central Europe from the Pan European regression for the entire period and for two 15-year periods, namely 1980-1994 and 1999-2013, 25 that had slight difference in preseason lengths (Extended Data Fig. 3a). The leaf unfolding dates (Fig. 1a). But the surprising result is that S T 3 significantly decreased by 40.0% from 4.0 ± 1.8 days °C -1 during 1980-1994 to 2.3 ± 1.6 days °C -4 1 during 1999-2013 (t=-37.3, df=5473, P<0.001) (Fig. 1b). All species show similar significant 5 decreases in S T (Fig. 1a), although the extent of reduction was species-specific. For example, 6Aesculus hippocastanum (see caption to Fig. 1 for English common names) had the largest 7 decrease in S T (-2.0 days °C -1 ), while S T decreased only slightly (but still significantly) in Fagus 8 sylvatica (-0.9 days °C -1 ) (Fig. 1a). Similar results were also obtained using a fixed preseason 9 length determined either in the time period 1980-1994 or in 1999-2013 10 and 3c). The declining S T could, however, also have been an artifact resulting from the 11 ‗encroachment' of leaf unfolding dates...

China has the largest afforested area in the world (∼62 million hectares in 2008), and these forests are carbon sinks. The climatic effect of these new forests depends on how radiant and turbulent energy fluxes over these plantations modify surface temperature. For instance, a lower albedo may cause warming, which negates the climatic benefits of carbon sequestration. Here, we used satellite measurements of land surface temperature (LST) from planted forests and adjacent grasslands or croplands in China to understand how afforestation affects LST. Afforestation is found to decrease daytime LST by about 1.1 ± 0.5°C (mean ± 1 SD) and to increase nighttime LST by about 0.2 ± 0.5°C, on average. The observed daytime cooling is a result of increased evapotranspiration. The nighttime warming is found to increase with latitude and decrease with average rainfall. Afforestation in dry regions therefore leads to net warming, as daytime cooling is offset by nighttime warming. Thus, it is necessary to carefully consider where to plant trees to realize potential climatic benefits in future afforestation projects. T he area of planted forest (PF) in China has increased by ∼1.7 million hectares per year (about 41% of the global afforestation rate) during the last 2 decades (1, 2). China had the largest PF area in the world in 2008, at ∼62 million hectares ( Fig. 1), or ∼23% of global plantation area (264 million hectares) (1, 2). The Chinese government launched several projects to convert croplands (CR) and marginal lands into forests, to reduce soil and water quality degradation, in the 1980s and 1990s (2). This afforestation contributed to increased carbon storage (3, 4) but also altered local energy budgets, which has the potential to offer feedback on local and regional climates (5-10).Forests generally have a lower albedo than grasslands (GR) and CR. Thus, afforestation increases the amount of absorbed solar radiation at the surface (9, 10). Surface cooling will result if this extra energy is dissipated as evapotranspiration (ET) (11) or heat convection (7); otherwise, afforestation will result in surface warming. The biophysical effects of afforestation on local climate can be much larger than the small global cooling effect resulting from uptake of CO 2 by growing forests (8,12,13). However, these biophysical effects are also complex and depend on "background" climate (14). Afforestation generally cools the surface in tropical areas but warms it in boreal lands (6,(8)(9)(10). The effects of afforestation in temperate regions are not clear. The large area under afforestation in China, the diversity of projects (over former CR, GR, or marginal lands), and the broad range of background climates (most plantations are in temperate regions with varying degrees of annual average rainfall) provide an interesting test bed to assess how afforestation affects local temperature.In this article, we investigate how plantations affect land surface temperature (LST) across China, using satellite-derived LST data sets from Earth Observin...

Satellite-derived Normalized Difference Vegetation Index (NDVI), a proxy of vegetation productivity, is known to be correlated with temperature in northern ecosystems. This relationship, however, may change over time following alternations in other environmental factors. Here we show that above 30°N, the strength of the relationship between the interannual variability of growing season NDVI and temperature (partial correlation coefficient R NDVI-GT ) declined substantially between 1982 and 2011. This decrease in R NDVI-GT is mainly observed in temperate and arctic ecosystems, and is also partly reproduced by process-based ecosystem model results. In the temperate ecosystem, the decrease in R NDVI-GT coincides with an increase in drought. In the arctic ecosystem, it may be related to a nonlinear response of photosynthesis to temperature, increase of hot extreme days and shrub expansion over grass-dominated tundra. Our results caution the use of results from interannual time scales to constrain the decadal response of plants to ongoing warming.