Impact of CO₂ fertilization on maximum foliage cover across the globe's warm, arid environments

Abstract

[1] Satellite observations reveal a greening of the globe over recent decades. The role in this greening of the "CO 2 fertilization" effect-the enhancement of photosynthesis due to rising CO 2 levels-is yet to be established. The direct CO 2 effect on vegetation should be most clearly expressed in warm, arid environments where water is the dominant limit to vegetation growth. Using gas exchange theory, we predict that the 14% increase in atmospheric CO 2 led to a 5 to 10% increase in green foliage cover in warm, arid environments. Satellite observations, analyzed to remove the effect of variations in precipitation, show that cover across these environments has increased by 11%. Our results confirm that the anticipated CO 2 fertilization effect is occurring alongside ongoing anthropogenic perturbations to the carbon cycle and that the fertilization effect is now a significant land surface process. Citation: Donohue, R. J., M. L. Roderick, T. R. McVicar, and G. D. Farquhar (2013), Impact of CO 2 fertilization on maximum foliage cover across the globe's warm, arid environments, Geophys. Res. Lett., 40,[3031][3032][3033][3034][3035]

Greening of the Earth and its drivers

“…The spatial pattern is consistent with previous analyses 15 that posited large absolute LAI increases due to eCO 2 in the tropics, in the absence of temperature, water and nitrogen limitations 16 , and large relative LAI increases due to eCO 2 in arid regions, where eCO 2 is expected to increase the water use efficiency of plants ( Supplementary Fig. 12) 17 . A simple theoretical model 17,18 was used to diagnose the response of leaf level carbon assimilation to the observed 46 ppm increase of CO 2 over the study period, including the effect of vapour pressure deficit trends and stomatal closure.…”

“…12) 17 . A simple theoretical model 17,18 was used to diagnose the response of leaf level carbon assimilation to the observed 46 ppm increase of CO 2 over the study period, including the effect of vapour pressure deficit trends and stomatal closure. This model gave a similar relative response of carbon assimilation to eCO 2 as the ecosystem models did for LAI (Supplementary Section 12).…”

Temperature and Snow-Mediated Moisture Controls of Summer Photosynthetic Activity in Northern Terrestrial Ecosystems between 1982 and 2011

“…A number of possible factors could be at work but quantifying their relative contribution to these greening trends would require further analysis and is beyond the scope of this study. In part, these greening trends could indicate the effect of CO 2 fertilization on northern vegetation, as seen in modeling studies [85], but such a strong effect is unlikely in cool boreal regions [88]. Changes in permafrost seem to be a factor stimulating greening trends in some subarctic regions.…”

Vegetation plays an important role in mediating future water resources

“…Studies of historical and future runoff (Gedney et al 2006, Davie et al 2013 often point to increased runoff due to CO 2 -induced increases in water use efficiency (in particular, stomatal closure allowing water to be conserved), which can be observed both at the leaf (Field et al 1995) and ecosystem (Keenan et al 2013) scales. However, a recent analysis of historical observations (Ukkola et al 2016) showed these water savings do not necessarily lead to increased runoff in drier climates due to CO 2 -induced vegetation greening (Donohue et al 2013), which acts to increase vegetation water use at the ecosystem scale. Changes in vegetation water use due to CO 2 as well as climatic factors may play a large, yet to date poorly constrained, role in mediating future water resources.…”