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

The response of forests to climate change depends in part on whether the photosynthetic benefit from increased atmospheric CO₂ (ΔC_a = future minus historic CO₂) compensates for increased physiological stresses from higher temperature (ΔT). 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 ΔC_a and ΔT, where acclimated forests adjusted leaf area, photosynthetic capacity, and stand density to maximize productivity while avoiding stress. Without acclimation, the ΔC_a-driven boost in net primary productivity (NPP) was compromised by ΔT-driven stress and mortality associated with vascular failure. With acclimation, the ΔC_a-driven boost in NPP and stand biomass (C storage) was accentuated for cooler futures but negated for warmer futures by a ΔT-driven reduction in NPP and biomass. Thus, hotter futures reduced forest biomass through either mortality or acclimation. Forest outcomes depended on whether projected climatic ΔC_a/ΔT ratios were above or below physiological thresholds that neutralized the negative impacts of warming. Critically, if forests do not acclimate, the ΔC_a/ΔT 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 ΔC_a/ΔT must rise above ca. 67 ppm·°C⁻¹ for NPP and biomass to increase, a lower threshold met by 71% of projections.