Sessa, Emily , Givnish, Thomas .
Leaf form and photosynthetic physiology of eastern North American Dryopteris (Dryopteridace.
Ferns occupy a critical position in the evolutionary history of land plants. They are sister to the seed plants, and are physiologically and ecologically intermediate between the seed plants and the non-vascular bryophytes. They have long been recognized as having unique physiological features, including low rates of photosynthesis, leaf hydraulic conductance, and vein density. Despite the ubiquity of ferns and recognition by botanists that their physiology is unique among land plants, we know very little about the physiological traits that may be driving fern diversification, or to what extent physiological diversification may play a role in the persistence of hybrid taxa. Most studies on fern physiology to date have focused only on a few locally distributed, usually distantly related species. No previous study has attempted to examine physiological adaptations in a group of widespread taxa who are closely related and whose relationships are well understood. This study is the first to report leaf form and physiological measures for such a group, the eleven eastern North American species of Dryopteris (Dryopteridaceae), and to examine differences in these parameters for evidence of adaptation to light availability. Field studies allowed us to begin characterizing the range of native light environments occupied by members of this group, and to examine variation across species in photosynthetic light response and several morphological traits for evidence of adaption to light availability. Common-garden experiments allowed "soft" and "hard" tests of adaptation, by comparing reaction norms with the qualitative expectations of economic theory, and by testing the quantitative prediction that species should show higher rates of photosynthesis per unit leaf mass than other species when grown in light regimes similar to those they occupy in the field. We present a novel means for incorporating phylogeny in tests of correlated evolution in a reticulate lineage. Trends in physiological and morphological traits generally agreed with qualitative predictions; phylogenetically unstructured analyses showed correlations among several traits traditionally viewed as adapted to light supply, and light levels in the common-garden study had a significant effect on the expression of these traits. However, there was no quantitative support at the species level for light availability being the most important variable driving morphological and physiological adaptation in this group. We propose instead that hydraulic factors related to water balance may have played a larger role in determining these species' morphological and physiological variation.
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1 - University of Arizona, Department of Ecology & Evolutionary Biology, P.O. Box 210088, Tucson, AZ, 85721, USA
2 - University Of Wisconsin, Department Of Botany, Birge Hall, 430 Lincoln Drive, Madison, WI, 53706, USA
Presentation Type: Oral Paper:Papers for Sections
Location: Union E/Hyatt
Date: Monday, July 9th, 2012
Time: 8:15 AM