DISTRIBUTION PATTERNS AND CARBON ACQUISITION PHYSIOLOGY OF THE C4 SHRUBS ATRIPLEX SPP. IN THE SOUTHWESTERN USA ACROSS THE LAST GLACIAL-INTERGLACIAL CYCLE

A key question about past and future vegetation dynamics involves differences in performance for C₃ vs. C₄ plants with past and ongoing changes in atmospheric CO₂ levels. For the C₄ halophytic shrubs Atriplex canescens and A. confertifolia, in the southwestern USA, we compared past and present distribution patterns with carbon acquisition physiology inferred from δ¹³C of modern and fossil leaves. During the last glacial period when atmospheric CO₂ was 30% lower than during the Holocene, the geographic and elevational ranges of both species were similar to modern except for a slight decrease in the upper elevation limits of A. canescens. The glacial-interglacial range stability of Atriplex spp. contrasts with broad scale changes noted for C₃ plant distributions, attributed in part to greater efficiencies of C₄ over C₃ physiology under lowered CO₂ concentrations of the last glacial period. This does not mean that C₄ plants are unaffected by changing atmospheric CO₂ levels. For A. canescens, physiological parameters reconstructed from δ¹³C of fossil leaf cellulose indicate significant glacial-interglacial differences in isotopic discrimination, C_i/C_a ratio, calculated leakage rates from the bundle sheath cells, and water-use efficiency. These differences are smaller but still in the same direction as those inferred for C₃ pinyon pines (Pinus spp.) and junipers (Juniperus spp.) in comparable data sets. These results indicate that the stability of these past population distributions reflect the relative amount of physiological changes occurring between photosynthesis systems.