The uncontrolled, global increase in atmospheric CO2 concentration (ca 80 ppmv) and decline in δ13Cair (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 δ13Cplant and estimation of isotopic discrimination (Δ) permitted calculation of water-use efficiency (A/g). The δ13Cplant chronologies of C4 Atriplex spp. show some promise as a reliable proxy for δ13Cair because their mean trends approximate the known δ13Cair 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.
Pedicino, Lisa C.; Leavitt, Steven W.; Betancourt, Julio L.; and Van de Water, Peter K.
"Historical variations in δ13Cleaf of herbarium specimens in the southwestern U.S.,"
Western North American Naturalist: Vol. 62
, Article 11.
Available at: http://scholarsarchive.byu.edu/wnan/vol62/iss3/11