DELTA¹³C AND STOMATAL DENSITY VARIABILITY

IN MODERN AND FOSSIL LEAVES OF KEY PLANTS

IN THE WESTERN UNITED STATES

Department of Geosciences
The University of Arizona

1999

Peter Kent Van de Water

ABSTRACT

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  δ¹³C 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 Atriplex canescens and A. confertifolia and packrat midden macrofossils of A. canescens excludes their use as a reliable proxy for atmospheric δ¹³C, 3) calcium-oxalate crystals are common component in plant tissue and can have a significantly different δ¹³C 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.