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Premise: Previous experimental studies have shown that poison ivy (Toxicodendron radicans; Anacardicaceae) responds to elevated CO 2 with increased leaf production, water-use efficiency, and toxicity (allergenic urushiol). However, long-term field data suggest no increase in poison ivy abundance over time. Using herbarium specimens, we examined whether poison ivy and other species shifted leaf traits under natural conditions with increasing atmospheric CO 2 (pCO 2 ) over the past century., Methods: We measured stomatal density, leaf area, leaf N, leaf C:N, leaf carbon isotope discrimination (Δ leaf ), and intrinsic water-use efficiency (iWUE) from 327 specimens collected from 1838 to 2020 across Pennsylvania. We compared poison ivy's responses to two evolutionarily related tree species, Toxicodendron vernix and Rhus typhina (Anacardiacae) and one ecological analog, Parthenocissus quinquefolia (Vitaceae), a common co-occurring liana., Results: Stomatal density significantly decreased (P < 0.05) in poison ivy and the ecologically similar liana P. quinquefolia over the past century, but did not change in the related trees T. vernix and R. typhina. None of these species showed significant trends in changes in leaf N or C:N. Surprisingly, in poison ivy, but not the other species, Δ leaf increased with increased pCO 2 , corresponding to significant declines in iWUE over time., Conclusions: In contrast to the results of short-term experimental studies, iWUE decreased in poison ivy over the last century. Trait responses to pCO 2 varied by species. Herbarium specimens suggest that realized long-term plant physiological responses to increased CO 2 may not be reflected in short-term experimental growth studies, highlighting the value of collections., (© 2023 Botanical Society of America.)