Overestimated gains in water-use efficiency by global forests

Area of Science:

• Ecology
• Plant Physiology
• Climate Science

Background:

• Terrestrial water-use efficiency (WUE) increases have been observed, potentially altering global carbon and water cycles.
• Estimating long-term WUE gains is challenging due to difficulties in accounting for plant acclimation.
• Mesophyll conductance significantly influences WUE estimations by affecting CO2 diffusion.

Purpose of the Study:

• To re-evaluate 20th-century WUE gains using updated mesophyll conductance data.
• To assess the impact of mesophyll conductance on WUE estimations across diverse biomes and species.
• To understand the long-term constraints on carbon and water cycling in forests.

Main Methods:

• Analysis of 464 tree-ring δ13C chronologies from 143 species globally.
• Incorporation of new knowledge on mesophyll conductance into WUE calculations.
• Comparison of adjusted WUE gains with conventional modeling estimates.

Main Results:

• 20th-century WUE gains were smaller (0.15 ppm/year) when adjusted for mesophyll conductance compared to conventional estimates (0.26 ppm/year).
• Mean sensitivity of WUE to atmospheric CO2 was 0.15 ppm/ppm.
• Internal-to-atmospheric CO2 ratios (ci/ca) remained relatively stable but varied by biome and plant type.

Conclusions:

• Forest WUE may not have increased as much as previously suggested.
• Constrained ratios of chloroplastic-to-atmospheric CO2 (cc/ca) indicate synchronized stomatal and mesophyll conductance responses.
• Future climate forcing projections, particularly CO2 fertilization effects, may require adjustments based on these findings.