Historical variations in δ¹³C_leaf of herbarium specimens in the southwestern U.S.

The uncontrolled, global increase in atmospheric CO₂ concentration (ca 80 ppmv) and decline in δ¹³C_air (ca 1.5%) since industrialization provide experimental boundary conditions by which to assess physiological response of vegetation. To examine consequences of these global atmospheric changes in the southwestern U.S., 350 specimens of Atriplex confertifolia, A. canescens, Ephedra viridis, Pinus edulis, P. flexilis, Juniperus scopulorum, and Quercus turbinella of precisely known age spanning the last 150 years were acquired from 9 herbaria. Cellulose analysis of δ¹³C_plant and estimation of isotopic discrimination (Δ) permitted calculation of water-use efficiency (A/g). The δ¹³C_plant chronologies of C₄ Atriplex spp. show some promise as a reliable proxy for δ¹³C_air because their mean trends approximate the known δ¹³C_air chronology. However, the high variability would necessitate multiple samples at any time period to accurately represent the mean. The generally increasing A/g trends of the 5 C₃ species are particularly pronounced for P. edulis and, after the 1950s, for J. scopulorum, but less evident for P. flexilis, E. viridis, and Q. turbinella, evidencing possible differences in species response to rising CO₂ concentrations. The trends are statistically noisy, however, possibly due to complex microclimates, extreme seasonality, and great interannual variability typical of the southwestern U.S. Herbarium specimens, at least in the Southwest, may be less useful for precise detection of direct CO₂ effects on plant physiology than tree rings, where the variability can be constrained to a single individual over time.