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Reconstructing Extinct Plant Water Use for Understanding Vegetation–Climate Feedbacks: Methods, Synthesis, and a Case Study Using the Paleozoic-Era Medullosan Seed Ferns

Published online by Cambridge University Press:  21 July 2017

Jonathan P. Wilson*
Department of Biology, Haverford College, 370 Lancaster Ave., Haverford, PA 19041 USA
Joseph D. White
Department of Biology, Baylor University, One Bear Place #97388, Waco, TX 76798–7388 USA
William A. Dimichele
Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, P.O. Box 37012, Washington D.C., 20013-7012 USA
Michael T. Hren
Center for Integrative Geosciences, University of Connecticut, Beach Hall 207, U-1045, 354 Mansfield Road, Storrs, CT, 06269 USA
Christopher J. Poulsen
Department of Earth and Environmental Sciences, University of Michigan, 2534 C.C. Little Building, 1100 North University Ave., Ann Arbor, MI, 48109-1005 USA
Jennifer C. McElwain
Earth Institute, School of Biology and Environmental Science, University College Dublin, Stillorgan Road, Belfield, Dublin 4, Ireland
Isabel P. Montañez*
Department of Earth and Planetary Sciences, University of California, Davis, One Shields Avenue, Davis, CA 95616 USA
Corresponding authors
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Vegetation affects feedbacks in Earth's hydrologic system, but is constrained by physiological adaptations. In extant ecosystems, the mechanisms controlling plant water used can be measured experimentally; for extinct plants in the recent geological past, water use can be inferred from nearest living relatives, assuming minimal evolutionary change. In deep time, where no close living relatives exist, fossil material provides the only information for inferring plant water use. However, mechanistic models for extinct plant water use must be built on first principles and tested on extant plants. Plants serve as a conduit for water movement from the soil to the atmosphere, constrained by tissue-level construction and gross architecture. No single feature, such as stomata or veins, encompasses enough of the complexity underpinning water-use physiology to serve as the basis of a model of functional water use in all (or perhaps any) extinct plants. Rather, a “functional whole plant” model must be used. To understand the interplay between plant and atmosphere, water use in relation to environmental conditions is investigated in an extinct plant, the seed fern Medullosa ((Division Pteridospermatophyta), by reviewing methods for reconstructing physiological variables such as leaf and stem hydraulic capacity, photosynthetic rate, transpiration rate, stomatal conductance, and albedo. Medullosans had the potential for extremely high photosynthetic and assimilation rates, water transport, stomatal conductance, and transpiration—rates comparable to later angiosperms. When these high growth and gas exchange rates of medullosans are combined with the unique atmospheric gas composition of the late Paleozoic atmosphere, complex vegetation-environmental feedbacks are expected despite their basal phylogenetic position relative to post-Paleozoic seed plants.

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Copyright © 2015 by The Paleontological Society 

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