| During the last deglaciation, 15,000 to 12,000 calendar years ago, global warming and wholesale shifts in regional precipitation patterns produced dramatic changes in vegetation worldwide. Paleobotanical records, namely pollen and macrofossils, have been used not only to reconstruct shifts in plant distributions and abundances, but also to quantify changes in temperature and precipitation amounts or seasonality. In addition to climatic change, during the full glacial period atmospheric CO2 values had dropped 30% to 200 ppmv compared to the Holocene, preindustrial value of 280 ppmv. Hypothetically, variations in atmospheric CO2 affect plant water-use efficiency (carbon gained to plant-water transpired) and thus may have modulated vegetation response as climates change. The studies incorporated in this dissertation focused upon carbon isotope and morphological changes in leaves of key functional groups. The studies concentrated on plant species that are abundant in the fossil record and comprise major floral components of past and present vegetation. Key findings include: (1) that shifts in delta13C in modern populations along steep environmental gradients seldom exceeds inter-plant variability at a given site, (2) inter-plant and intra-site variability in modern and historic herbarium collections of the C4 halophytes Altriplex canescens and A. confertilfolia and packrat midden macrofossils of A. canescens excludes their use as a reliable proxy for atmospheric delta13C, (3) calcium-oxalate crystals are common component in plant tissue and can have a significantly different delta13C value that increases inter-plant variability, especially in C4 plants such as Atriplex canescens and A. confertifolia, (4) carbon isotope and stomatal density/index measurements of macrofossils from packrat middens show species specific adaptation in ecophysiological processes as atmospheric CO2 rose from the full glacial, and (5) the greatest adaptation to low atmospheric CO2 during the last ice age was in the C3 species and that C4 and CAM plants showed few changes in their discrimination against 13C or in the number of stomata on their leaf surfaces. |
