Terrestrial Palaeoecology and Global Change

Chapter 7. Climate change
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calculated atmospheric pCO 2
levels with the Late Paleozoic and Neogene glaciations is
taken as a verification of the model. However, the outgassing rates are derived from the
supposed sea floor spreading rates in turn inferred from the observed sea-level fluctuations.
In effect, Berner’s curve actually reflects sea-level fluctuations that are climatically
significant with or without mediation of CO 2
(VII.2.3). Instead of being relegated to
mere evidence of alleged sea-floor spreading rates, sea-level fluctuations has to be considered
as a major force under greenhouse events due to their effect on oceanic circulation
as well as through the land area – terrestrial biomass – pedogenic weathering correlations.
The factual evidence of CO 2
fluctuations over geological times is controversial. The
carbon isotope ratios defined for marine and pedogenic carbonates (that reflect photosynthetic
carbon isotope fractionation related to the atmospheric CO 2
concentrations)
indicate a high CO 2
level in the Ordovician, actually an icehouse period (Cerling, 1991;
Freeman & Hayes, 1992). The Late Palaeozoic to Mesozoic estimates (e.g., Ghosh et
al., 2001) fall within a very broad uncertainty margin (except for the Jurassic), whereas
the Cenozoic ones are totally incongruous.
The stomatal method is based on the response of stomatal frequencies to CO 2
concentrations
on the leaf surface that depends on the atmospheric pCO 2
(which is no
longer a limiting factor at 200% of the present-day level), leaf area and wind velocity
(Ågren, 1985). The method is verified by comparison of stomata per cell ratios (stomatal
index) in pre-industrial herbarium specimens and living plants of the same species. Objectives
and limitations of the method are discussed in Kürschner (1996) and Roger
(2001).
Inverse response of stomatal frequency to the ambient CO 2
levels has been found in
36% of hitherto studied species. CO 2
fluctuations of about 30% over the last glacial/
interglacial transition correspond to about 17% drop of stomatal index in Pinus flexilis
and Salix cenerea (Van der Water et al., 1994). The estimates for Quercus petreaea,
a long-ranging Mediterranean species, seem congruous with other indicators of the Neogene
climate change (Van der Burgh et al., 1993). Other limitations aside, the stomatal
method requires long stratigraphic records that are rare in fossil plant species. The longliving
genera, such as Ginkgo, seem promising (Chen et al., 2001), although interspecies
comparisons are scarcely justified by the method. Moreover, the Mesozoic species of
Ginkgo (Ginkgoites) differ from extant G. biloba in the densely trichomate foliage
leaves, similar to juvenile leaves of the living species, suggesting a different ecology.
It must be made clear that the stomatal index method is based on species-specific
response to CO 2
concentration in the ambient air determined by the genome. When the
comparisons transgress species boundaries, we are no longer dealing with the genomespecific
responses, but rather rely on the convergent ones. Hence, it makes little diffrence
whether the selsected species are phyllogenetically related or not.
Mean stomatal length is another epidermal variable correlated with atmospheric pCO 2
(Kürschner, 1996) that is more likely to show a convergent response in a group of con-