biostudies-literatureMalcomb JDNational Science Foundation137https://www.ebi.ac.uk/biostudies/studies/S-EPMC12307545biostudies-literatureUnknown207(8)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% OecologiaDivergent water use efficiency trends among eastern North American temperate tree species.PMC123075451562055156147315621271562019Malcomb JDVadeboncoeur MAEpstein HEDruckenbrod DLLanning MScanlon TMWang LAsbjornsen HfalseDivergent water use efficiency trends among eastern North American temperate tree species.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% 2025-01-01T00:00:00Z2025 Jul2026-05-09T17:53:46.487Z2025-08-23T03:07:20.452ZS-EPMC123075454073087210.1007/s00442-025-05753-w