CLASSIFYING PLANT MORPHOLOGICAL TRAITS AND INVESTIGATING RELATIONSHIPS BETWEEN MODERN LEAF CHARACTERS AND CLIMATES IN EASTERN NORTH AMERICA

For decades, researchers have recognized correspondences between the geographic patterns of plant morphological traits and the distribution of climatic parameters. Leaf area and the absence of margin teeth for example, are positively correlated, respectively, with available moisture and temperature. To investigate these and other relations between climate and leaf characters, as a basis for paleoclimatic reconstructions from fossil leaves, we compared patterns of leaf trait occurrence in modern leaf floras of eastern North America with climatic (seasonal and annual temperature and precipitation) and bioclimatic (mean temperature of the coldest month, growing degree days, and a moisture index) parameters. Leaf characters were evaluated and classified into various leaf trait categories. This task was complicated by the multitude of descriptive terms used by botanists to portray the continuous range of observed variation in the physical characteristics of modern plants. In evaluations of plant morphology, terminology use is highly variable, subjective, and non-quantitative, resulting in inconsistent, non-reproducible descriptions of plant form. To circumvent this problem, we developed quantitative, replicable, methods for assessing and classifying plant morphological traits including leaf, stem, and growth form attributes. Our classification system captures the range of natural plant form variability in a simplified way and facilitates consistent evaluations of the diverse shapes of modern and fossil leaves. The spatial distributions of leaf traits and their correspondences with climatic parameters were determined by: 1) using published accounts to describe the leaf characteristics of individual tree species, 2) digitizing the modern ranges of these species and placing them on a 25-km equal area grid of North America (USGS Prof. Pap. 1650-A), 3) estimating the modern climate at each of the grid points, and 4) using the gridded trait and climatic data to identify the potential climatic controls on the geographic distribution of modern and fossil leaf traits.