The impact of rising CO2 and acclimation on the response of US forests to global warming

John S. Sperry, Martin D. Venturas, Henry N. Todd, Anna T. Trugman, William R. L. Anderegg, Yujie Wang, and Xiaonan Tai

PNAS December 17, 2019 116 (51) 25734-25744

Significance

The benefit of climate change for forests is that higher atmospheric CO₂ allows trees to use less water and photosynthesize more. The problem of climate change is that warmer temperatures make trees use more water and photosynthesize less. We predicted the outcome of these opposing influences using a physiologically realistic model which accounted for the potential adjustment in forest leaf area and related traits to future conditions. If forests fail to adjust, only 55% of climate projections predict a CO₂ increase large enough to prevent warming from causing significant drought and mortality. If forests can adjust, the percentage of favorable outcomes rises to 71%. However, uncertainty remains in whether trees can adjust rapidly and in the scatter among climate projections.

Abstract

The response of forests to climate change depends in part on whether the photosynthetic benefit from increased atmospheric CO₂ (∆C_a = future minus historic CO₂) compensates for increased physiological stresses from higher temperature (∆T). We predicted the outcome of these competing responses by using optimization theory and a mechanistic model of tree water transport and photosynthesis. We simulated current and future productivity, stress, and mortality in mature monospecific stands with soil, species, and climate sampled from 20 continental US locations. We modeled stands with and without acclimation to ∆C_a and ∆T, where acclimated forests adjusted leaf area, photosynthetic capacity, and stand density to maximize productivity while avoiding stress. Without acclimation, the ∆C_a-driven boost in net primary productivity (NPP) was compromised by ∆T-driven stress and mortality associated with vascular failure. With acclimation, the ∆C_a-driven boost in NPP and stand biomass (C storage) was accentuated for cooler futures but negated for warmer futures by a ∆T-driven reduction in NPP and biomass. Thus, hotter futures reduced forest biomass through either mortality or acclimation. Forest outcomes depended on whether projected climatic ∆C_a/∆T ratios were above or below physiological thresholds that neutralized the negative impacts of warming. Critically, if forests do not acclimate, the ∆C_a/∆T must be above ca. 89 ppm⋅°C⁻¹ to avoid chronic stress, a threshold met by 55% of climate projections. If forests do acclimate, the ∆C_a/∆T must rise above ca. 67 ppm⋅°C⁻¹ for NPP and biomass to increase, a lower threshold met by 71% of projections.

See https://www.pnas.org/content/116/51/25734

Fig. 1.

Basis for modeling tree-level acclimation of LA and photosynthetic capacity (maximum carboxylation capacity, V_max25, coupled to electron transport capacity, J_max25). (A) Trees acclimate LA and V_max25 to achieve a homeostatic ratio between leaf internal (C_i) and atmospheric (C_a) CO₂ concentrations (C_i/C_a = 0.7) under favorable site “reference” conditions. The C_i is determined by the balance between V_max25 (V_max25 arrows) and the stomatal conductance to CO₂ (absolute value of slope of dashed gray line). Stomatal conductance depends on LA through its effect on the tree hydraulic conductance per LA (LA arrows). (B) For each set of environmental reference conditions there are infinite combinations of LA and V_max25 that satisfy C_i/C_a = 0.7 (solid black contour for historic conditions and gray dashed contour for future conditions), but only one LA–V_max25 combination is optimal (historic, black circle; future, gray triangle). (C) The optimal LA–V_max25 maximizes the ROI (Max ROI arrow), calculated as the difference between the net whole canopy assimilation (solid A_net curve) and leaf construction cost amortized over the GS (gray cost line).