Terrestrial Palaeoecology and Global Change

248 Valentin A. Krassilov. Terrestrial Palaeoecology
Trochodendroidion, Palaeocene – 43°N
Parataxodion, Mid-Cretaceous – 50°N
Phoenicopsion, Mid-Jurassic – 35°N
Phoenicopsion, Late Triassic – 42°N
Ruflorion, Late Permian – 38°N
Archaeopteridion, Late Devonian -54°N
The resulting curve (Fig. 100) fluctuates between the latitudes of 35°-54° north. The
corresponding meridional amplitude amounts to about 2220 km (111 km per 1° of meridian
over the mid-latitudes).
Since vegetation zones are jointly controlled by temperature and precipitation gradients,
their shifts indicate a general direction of climate change, such as temperization or
tropicalization, rather than individual climatic variables. Deciduous vegetation spreads
with a temperization of the mid-latitudes caused by the surges of polar air generated by
the polar ice caps (IV.2.2). Hence the deciduous zone is fairly prominent over the Late
Palaeozoic, mid-Jurassic and Late Cenozoic, but shrinks in the essentially non-glacial
periods. These are also periods of relatively mild climatic contrasts, or equability, inducing
tropicalization of the mid-latitudes (Axelrod, 1992). A penetration of thermophilic
elements into the temperate realm is due not only to a lower temperature gradient, but
also to a decrease in water uptake with the rise of atmospheric CO 2
level rendering plant
species less dependent on precipitation (VII.2.4).
An interpretation in terms of temperature changes is no more than a crude approximation,
warranted in the case of the nemoral zone by a correlation of deciduousness
with the cold month component of the mean annual temperatures, which increase by
about 1°C per 3° of meridian. A recalculation of latitudes into temperatures gives a
cooling effect of about 6°C from the mid-Devonian to present. A prominent Cretaceous
Fig. 100. Climatic curve based on the latitudinal positions of the southern limit for temperate deciduous
biomes in Central Asia (Fig. 99). Open peaks correspond to episodes of obliterate zonation supposedly
reflecting a greenhouse climate (Krassilov, 1997c).