Abstract
Abstract. The climate regime shift during the 1980s had a substantial impact on the terrestrial ecosystems and vegetation at different scales. However, the mechanisms driving vegetation changes, before and after the shift, remain unclear. In this study, we used a biophysical dynamic vegetation model to estimate large-scale trends in terms of carbon fixation, vegetation growth, and expansion during the period 1958–2007, and to attribute these changes to environmental drivers including elevated atmospheric CO2 concentration (hereafter eCO2), global warming, and climate variability (hereafter CV). Simulated leaf area index (LAI) and gross primary production (GPP) were evaluated against observation-based data. Significant spatial correlations are found (correlations > 0.87), along with regionally varying temporal correlations of 0.34–0.80 for LAI and 0.45–0.83 for GPP. More than 40 % of the global land area shows significant positive (increase) or negative (decrease) trends in LAI and GPP during 1958–2007. Regions over the globe show different characteristics in terms of ecosystem trends before and after the 1980s. While 11.7 % and 19.3 % of land have had consistently positive LAI and GPP trends, respectively, since 1958, 17.1 % and 20.1 % of land saw LAI and GPP trends, respectively, reverse during the 1980s. Vegetation fraction cover (FRAC) trends, representing vegetation expansion and/or shrinking, are found at the edges of semi-arid areas and polar areas. Environmental drivers affect the change in ecosystem trend over different regions. Overall, eCO2 consistently contributes to positive LAI and GPP trends in the tropics. Global warming mostly affects LAI, with positive effects in high latitudes and negative effects in subtropical semi-arid areas. CV is found to dominate the variability of FRAC, LAI, and GPP in the semi-humid and semi-arid areas. The eCO2 and global warming effects increased after the 1980s, while the CV effect reversed during the 1980s. In addition, plant competition is shown to have played an important role in determining which driver dominated the regional trends. This paper presents new insight into ecosystem variability and changes in the varying climate since the 1950s.
Highlights
Climate variability and change, including global warming, and elevated atmospheric CO2 concentrations, have profound impacts on the terrestrial biosphere at global and regional scales (Garcia et al, 2014), while the terrestrial biosphere, in turn, affects the global climate by altering the exchanges of carbon, water, and energy between the atmosphere and land surface (Cox et al, 2000; Xue et al, 2004, 2010; Friedlingstein et al, 2006; Ma et al, 2013)
We investigate the effect of elevated atmospheric CO2 concentration (eCO2) and climate drivers including global warming and climate variability on the trends of fraction cover (FRAC), leaf area index (LAI), and gross primary production (GPP) during the period 1958–2007 by using the SSiB4/TRIFFID (Simplified Simple Biosphere model version 4/Top-down Representation of Interactive Foliage and Flora Including Dynamics) Dynamic global vegetation models (DGVMs) (Xue et al, 1991; Cox, 2001; Zhan et al, 2003; Zhang et al, 2015; Harper et al, 2016) at both grid and plant functional type (PFT) levels, and using satellite products whenever they are available
0.69 1.31 −3.26 −1.14 0.93 sign reversal from positive to negative is found in western North America, South American savanna, and east Africa, which accounted for 2.9 % (LAI) and 2.7 % (GPP) of the land surface
Summary
Climate variability and change, including global warming, and elevated atmospheric CO2 concentrations (referred to as eCO2 in this paper), have profound impacts on the terrestrial biosphere at global and regional scales (Garcia et al, 2014), while the terrestrial biosphere, in turn, affects the global climate by altering the exchanges of carbon, water, and energy between the atmosphere and land surface (Cox et al, 2000; Xue et al, 2004, 2010; Friedlingstein et al, 2006; Ma et al, 2013). Important trends in terrestrial ecosystem carbon fixation, growth, and expansion in the past 60 years have been detected (Myneni et al, 1997; Piao et al, 2011, 2015; Ichii et al, 2013; Los 2013; Zhu et al, 2016). Liu et al.: Global vegetation variability and its response to elevated CO2, global warming, and climate variability neni et al, 1997; Piao et al, 2011; Ichii et al, 2013; Los, 2013) and leaf area index (LAI, defined as the one-side leaf area per ground area) products (Piao et al, 2011, 2015; Zhu et al, 2016). Vegetation fractional coverage (hereafter FRAC) has been changing, including some large-scale increases in total vegetation cover (Piao et al, 2005; Donohue et al, 2009; McDowell et al, 2015), and shifts in the spatial distributions of plants species, such as woody plants encroachment in the savanna area (Stevens et al, 2017) and shrubification in the tundra biome (Epstein et al, 2012; Mod and Luoto, 2016)
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