Terrestrial Palaeoecology and Global Change

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Chapter 3. Palaeoecology 39 from water drops. Most morphological characters conventionally related to climate are likewise ambiguous (VII.1.3). Climatic adaptations are those pertaining to temperature and vapour pressure at the leaf/air or root/soil interface. Both root mass and leaf mass (their ratios) are thus affected by climate. But an excessive development of underground organs (both gametophytic and sporophytic) in early land plants and their derived arboreal spore plants (stigmaria of Palaeozoic lepidophytes) might also reflect a less advanced mycorrhizal system of the primitive rhizospheres. Plant growth is limited by the water conductivity of vascular supply, the morphological elaborations of which (bordered pits, perforation plates, etc.) prevent emboly – a plugging of conductive elements by air bubbles and their spread over the system. A risk of emboly increases with water stress, hence the smaller dimensions – higher density of vascular elements, a differentiation of seasonal increments in the seasonally dry/freezing climates, a correlation of early/late xylem ratios with latitudes (Carlquist, 1978). However, since water stress is regulated from above by leaf pumping it eventually depends upon evapotranspiration. Reduction of evapotranspiration in dry climate clashes with the prevention of embolism. Xeromorphism is a morphological expression of this physiological problem. Xeromorphy develops in respect to a deficiency/inaccessibility of soil moisture or exposure to direct sunshine/strong wind pressure, as well as an ineffective vascular system. The latter factor might have had an evolutionary dimension explaining the xeromorphy of early land plants. A restriction of the plant/air interface (dense leaf packing, appressed leaves, thick leaf blades, revolute margins, small leaf area, deciduousness) is the most general solution of the water stress problem. Xeromorphic plants also reduce evapotranspiration by decreasing stomatal conductivity (papillate stomata, wax plugs) or increasing aerodynamic roughness of their leaves to wind pressure (higher in conifers than in broadleaves: Meinzer, 1993). These foliar variables are related to the boundary layer that decouples vapour pressure at leaf surface from that of the ambient air by accumulating the transpired vapour (Fig. 19). Leaf surface micromorphology, in particular the prominent venation network over stomatiferous lower surfaces, scabrate cuticle, striation, cuticular folds, stomatal pits and trichomes pertain to the decoupling efficiency (measured as the decoupling coefficient: Meinzer, 1993) of the boundary layer that tends to increase from conifers to deciduous broadleaves to evergreen broadleaves. In thickly cutinized leaves, the boundary layer is restricted by the areas of densely packed stomata, such as stomatal grooves in cordaites, peltasperms, bennettites, taxaceous and sciadopityaceous conifers, etc. Such leaf microstructures frequently occur in helophytic life forms. Xeromorphy broadly overlaps with scleromorphy, an excessive development of mechanical tissues in relation to distrophy, in such features as an elevated root/shoot ratio, small fibrous leaves, thick cuticles, sunken stomata, etc. (Figs. 20, 21). In palaeoecology, their separation is not always feasible, the more so since in evergreen vegetation dry habitats usually associate with low-nutrient soil types (Hill, 1998).
40 Valentin A. Krassilov. Terrestrial Palaeoecology Fig. 19. The excessively trichomate phyllodes in a protognetalean plant Dinophyton from the Triassic redbeds of Arizona (Krassilov & Ash, 1988). a c b Fig. 20. Scleromorphy in a helophytic bennettite, Otozamites lacustris, from the Early Cretaceous of Mongolia, represented by the shed pinnules of pinnate leaves (Krassiliov, 1982): (a) festooned epidermal cell walls, (b) stomata sunken in the large-celled spongy mesophyll, (c) dense ridged venation.
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Terrestrial Palaeoecology and Global Change

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