MORPHOLOGICAL VARIABILITY ALONG A CLIMATE GRADIENT IN TAXODIUM DISTICHUM (CUPRESSACEAE): IMPLICATIONS FOR PALEOENVIRONMENTAL RECONSTRUCTION

Structure and physiognomy of plants has been shown repeatedly to vary with climate and other environmental factors. Understanding how morphology relates to climate within modern species will improve interpretation of the fossil record, even potentially providing an opportunity to investigate climatic gradients of plant distributions in the geologic record. Gymnosperms have a longer fossil record than angiosperms, and are therefore of interest as environmental proxies that extend further back in time. In order to characterize the relationship between morphology and environment in Taxodium distichum (Cupressaceae), a conifer inhabiting lowland riparian habitats across Southeastern United States, morphological variation along an environmental gradient was investigated. Three branchlets each from three different trees were collected at fifteen sites representing the range of environments that T. distichum inhabits. Leaves were characterized for anatomical features including transverse area, leaf width and thickness, area of vascular bundle, resin canal area, epidermal thickness, mesophyll and palisade layer thickness, and stomatal features. Canonical correspondence analysis of anatomical data and 14 BioClim environmental variables showed first three components explained most of the variation in the data (~98.5%). Most significantly, axis 1 (~71.5% variance) was most strongly correlated with precipitation seasonality, temperature seasonality, mean temperature of coldest quarter, mean annual temperature, temperature annual range, precipitation of wettest quarter, and resin canal area. Axis 2 (~20.5% variance) was most strongly correlated with area of vascular bundle, leaf thickness, and mesophyll and palisade layer thickness. Taxodium has a historically widespread distribution and long fossil record throughout the Northern Hemisphere, extending from the Cretaceous to present. Results from this study provide context for interpreting the fossil record of Taxodium and using this taxon to reconstruct past environments.