Using an optimality model to understand medium and long-term responses of vegetation water use to elevated atmospheric CO2 concentrations

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.

Keywords: Adaptation; ecohydrology; evapotranspiration; global change; optimality; vegetation.

Figure 1.

Simulated and satellite-derived FPC at the different sites. Simulation results taken from long-term adaptation runs at 317 (solid lines), 350 (dashed lines) and 380 ppm atmospheric CO₂ concentrations (dotted lines), satellite-derived (AVHRR) estimates of fractional foliage cover (grey shaded) derived from Donohue et al. (2008). Note that gaps in the satellite-derived FPC in year 2000 are due to missing data, not catastrophic events.

Figure 2.

Simulated mean annual evapotranspiration rates for different atmospheric CO₂ concentrations (C_a). ‘Medium-term’ refers to simulations where constant vegetation properties (see Table 1) were optimized for C_a = 317 ppm, while dynamic vegetation properties were optimized for the respective C_a. ‘Long-term’ refers to simulations where all vegetation properties were optimized for the respective C_a. The horizontal black dashed lines are a visual guide to see the change relative to the E_T rates at 317 ppm C_a.

Figure 3.

Relative changes in evaporative fluxes and RAI vs. relative changes in (A) surface soil moisture for medium-term and (B) FPC in long-term adaptation. E_t, transpiration; E_s, soil evaporation; Θ₁, relative saturation in the top 0.5 m of soil; RAI, root area index. Subscripts p and s refer to perennial and seasonal vegetation, respectively. Dashed lines link points belonging to a given site (codes following Table 2) and atmospheric CO₂ concentration (subscripts to side codes).

Figure 4.

Sensitivity of transpiration rate (E_t, per unit leaf area) to λ = ∂E_t/∂A_g for different atmospheric CO₂ concentrations (see keys) at high λ (A) and low λ (B). Simulation conditions: 1000 μmol m⁻² s⁻¹ PPFD, 0.02 mol H₂O mol⁻¹ air vapour deficit (equivalent to 2 kPa VPD), 40 ppm Γ_*. Ranges of λ and J_max in (A) and (B) represent simulated values at the wettest site (CT, J_max = 485 μmol m⁻² s⁻¹) and the driest site (VIR, J_max = 250 μmol m⁻² s⁻¹), respectively (see Table 5).

Figure 5.

Summary of effects of eCO₂ on vegetation and water resources for constant climate. Effects specific to either water-limited or energy-limited catchments are in the respective coloured boxes. Note that decrease in transpiration per unit leaf area has an initial effect on increasing soil moisture in all catchments, whereas initially increased soil moisture and enhanced assimilation results in increasing leaf area and increased transpiration per ground area at the water-limited sites, reversing the initial effect on soil moisture.

Figure 6.

Relative response of transpiration, CO₂ assimilation and their ratio to a 20 % increase in atmospheric CO₂ concentrations at constant climate, assuming long-term adaptation.