Terrestrial Palaeoecology and Global Change

Chapter 7. Climate change
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with entire/non-entire leaf margins, providing a major source of palaeoclimatic inference
However, leaf similarities might have only been indirectly related to climate, as in the
case of leaf mimicry owing to plant/insect interactions (III.1.3).
Generally, leaf characters reflect adaptive strategies affected by climate rather than
climate as such. Actually the causal link is not climate – leaves, but climate – vegetation
structure – leaves, with additional complications introduced by fungal and animal symbionts
that make it climate – biotic community – vegetation structure – leaves. These
considerations pertain to climatic inferences based on leaf dimensions and shapes alike.
The size classes of micro-, noto-, meso- and megaphylls, and the leaf indices of
microthemal to megathermal climates based on their ratios have been worked out
(Raunkiaer, 1934; Webb, 1959) for the present-day vegetation types, such as multistratal
canopies of tropical rainforest. Since leaf size is actually related to shade tolerance,
wind velocity and CO 2
concentration on the blade surface, the leaf size/climate
correlations hold for certain types of canopy structures under constant CO 2
levels. An
indiscriminate application of leaf indices can be misleading (Dolph & Dilcher, 1980).
They are of little, if any, meaning for the pre-Pleistocene vegetation types, e.g., the
rainforests of a simpler canopy structure. Correlation between leaf area and precipitation
is likewise affected by the canopy structure, with evolutionary changes over times
(see Wilf et al., 1998).
CO 2
concentration on leaf surface varies with leaf area and wind velocities (Ågren,
1985) and is affected by the leaf roughness and micromorphological structures (the
costal/intercostal differentiation, stomatal pits, trichomes, striation) related to the boundary
layer decoupling leaf surface from the ambient air (III.1.1). Since wind velocities are
greater over open vegetation than under canopy, this factor might have contributed to
morphological divergence of leaf shapes (dimensions) in respect to vegetation structure.
Under similar climatic conditions, wind roughness will be better expressed morphologically
in riparian woodlands than under the canopy of old-growth forests. The alternative
segmentation/fusion tendencies in leaf evolution might have reflected either climate change
or fluctuations of atmospheric CO 2
levels or evolution of vegetation structures. In particular,
marginal fusion of leaflets, parallel in the initially compound leaves of Permian gigantopterids,
Triassic peltasperms or Cretaceous platanoids, resulting in a broad entire
blade, is correlated with species richness and, by inference, with an increasing complexity
of contemporary vegetation structures.
Both leaf venation and marginal characters convey an indirect response to seasonality.
In secondary veins of pinnate venation, the angle of departure increases with compactness
of leaf folding, which is inversely related to unfolding rates that fall under
climatic control (tightly folded leaf primordia take longer to expand, hence correlation
with a long growing season: Kobayashi et al., 1998). Yet under similar climatic conditions
the unfolding rates vary with edaphic conditions, seral status, pollination strategy,
etc. Rapid leafing is advantageous in temperate climates of a short growing season, but
may also be disadvantageous in deciduous plants that flower before leaves.