# Divergent water use efficiency trends among eastern North American temperate tree species

## TL;DR

This study shows that tree water use efficiency in eastern North American forests has increased with CO2, but responses vary by species and pollution levels.

## Contribution

The study reveals divergent iWUE trends among tree species and links these to atmospheric pollution and functional traits.

## Key findings

- Tree iWUE increased by 22.3% between 1950 and 2011, matching a 25.2% rise in atmospheric CO2.
- Needleleaf evergreens showed declining iWUE after 2002, while broadleaf deciduous species continued to increase.
- Higher atmospheric pollution was associated with smaller increases in iWUE.

## Abstract

Both theory and observations suggest that tree intrinsic water use efficiency (iWUE)—the ratio of photosynthetic carbon assimilation to stomatal conductance to water—increases with atmospheric CO2. However, the strength of this relationship varies across sites and species, prompting questions about additional physiological constraints and environmental controls on iWUE. In this study, we analyzed tree core carbon isotope ratios to examine trends in, and drivers of, iWUE in 12 tree species common to the temperate forests of eastern North America, where forests have experienced changes in CO2, climate, and atmospheric pollution in recent decades. Across all site-species combinations, we found that tree iWUE increased 22.3% between 1950 and 2011, coinciding with a 25.2% increase in atmospheric CO2. iWUE trajectories varied markedly among tree functional groups and within species across sites. Needleleaf evergreen iWUE increased until circa 2002 before declining in recent years, while iWUE of broadleaf deciduous species continued to increase. The analysis of environmental controls on iWUE trends revealed smaller increases in iWUE in trees subjected to higher atmospheric pollution loads. Our results suggest that tree functional characteristics and atmospheric pollution history influence tree response to atmospheric CO2, with implications for forest carbon and water balance in temperate regions.

The online version contains supplementary material available at 10.1007/s00442-025-05753-w.