Abstract
Vegetation has different adjustable properties for adaptation to its environment. Examples include stomatal conductance at short time scale (minutes), leaf area index and fine root distributions at longer time scales (days-months) and species composition and dominant growth forms at very long time scales (years-decades-centuries). As a result, the overall response of evapotranspiration to changes in environmental forcing may also change at different time scales. The vegetation optimality model simulates optimal adaptation to environmental conditions, based on the assumption that different vegetation properties are optimized to maximize the long-term net carbon profit, allowing for separation of different scales of adaptation, without the need for parametrization with observed responses. This paper discusses model simulations of vegetation responses to today's elevated atmospheric CO2 concentrations (eCO2) at different temporal scales and puts them in context with experimental evidence from free-air CO2 enrichment (FACE) experiments. Without any model tuning or calibration, the model reproduced general trends deduced from FACE experiments, but, contrary to the widespread expectation that eCO2 would generally decrease water use due to its leaf-scale effect on stomatal conductance, our results suggest that eCO2 may lead to unchanged or even increased vegetation water use in water-limited climates, accompanied by an increase in perennial vegetation cover.
Highlights
Elevated atmospheric CO2 concentrations are generally expected to lead to reductions in stomatal conductance and leaf-scale water use (Wong et al 1979; Drake et al 1997)
Without any model tuning or calibration, the model reproduced general trends deduced from free-air CO2 enrichment (FACE) experiments, but, contrary to the widespread expectation that elevated atmospheric CO2 concentrations (eCO2) would generally decrease water use due to its leaf-scale effect on stomatal conductance, our results suggest that eCO2 may lead to unchanged or even increased vegetation water use in water-limited climates, accompanied by an increase in perennial vegetation cover
The present analysis of the effects of eCO2 on the economics of vegetation water use and carbon gain suggests that the assumption of optimal vegetation leads to results that are similar to observed patterns
Summary
Elevated atmospheric CO2 concentrations (eCO2) are generally expected to lead to reductions in stomatal conductance and leaf-scale water use (Wong et al 1979; Drake et al 1997). There is only limited empirical evidence for the full range of vegetation responses to eCO2, but both theoretical considerations and remote sensing data have led some authors to link the observed global increase in perennial vegetation cover (‘woody thickening’) to increasing atmospheric CO2 concentrations (Ca) (Bond and Midgley 2000, 2012; Berry and Roderick 2002; Eamus and Palmer 2008; Donohue et al 2013), suggesting that stomatal closure is only the first step in a long cascade of potential effects of eCO2 These may include alterations in species compositions, perennial vegetation cover and rooting depths, which come about as the amount of transpiration required to fix a given amount of CO2 declines with increasing atmospheric CO2 concentrations. Such alterations are likely to only become obvious after several generations of plants, which, for perennial plants, can take decades to centuries or beyond
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