Terrestrial Palaeoecology and Global Change

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).