Terrestrial Palaeoecology and Global Change

90 Valentin A. Krassilov. Terrestrial Palaeoecology
The conifer rainforests of circum-Pacific mountain ranges still retain a relict taxodiaceous
element indicating their origin from the widespread Late Cretaceous to Palaeogene
lowland forests with Parataxodium, Parasequoia, Geinitzia, Sequoia, Metasequoia
and Glyptostrobus as dominant trees. With the circum-Pacific orogenic belt rising
since the mid-Cretaceous, the Mesozoic taxodiaceous forests might have become
segregated into the montane and lowland redwood types, perhaps in parallel with segregation
of their allopolyploid hybrid complexes into the extant species.
In the montane forests, taxodiaceous conifers were gradually replaced by pinaceous
conifers. Over the Sikhote Alin ranges, this process is documented by the entries of
piceoid and laricoid conifers in the terminal Cretaceous plant assemblages of intermontane
depressions (Krassilov, 1989b) and their subsequent increase through the Palaeogene.
Fossil wood remains indicate the development of a modern-type montane taiga in
the Late Eocene to Early Oligocene, just prior to a widespread temperization of the
northern mid-latitudes. The Siziman “fossil forest” assemblage of that age (Blokhina,
1985) includes Metasequoia in association with Laricioxylon xylotypes related to extant
larch species of the same mountainous area.
Montane taiga is precursive of the lowland boreal forests. Until the Miocene, the
northern high latitudes were covered with a mixed Arcto-Tertiary forest. A much
more recent segregation of the boreal conifer forests was related to a spread of cold
front generated by the Plio-Plistocene continental glaciation. The present-day boreal
forest zone occurs between the winter and summer positions of the Arctic atmospheric
front dividing cold dry Arctic air from the warmer and moister Pacific air. Solar
angle is one of the variables controlling the extent of this zone (Bonan & Shugart,
1989). In the boreal conifers, both leaf surface and leaf mass are larger than in the
cool-temperate hardwood forests (Sprugel, 1989). In contrast to the leaf litter of deciduous
hardwoods, the needle litter does not inhibit moss growth. In boreal conifer
forests, moss cover plays a crucial role in sustaining water balance. Their annual moss
production is nearly equal to their arboreal production (Bonan & Shugart, 1989). Since
there is evidence of a diverse bryophytic ground cover in coniferous Jurassic forests
(Krassilov, 1973c), this aspect of boreal ecology might have already appeared in the
Mesozoic wetland forerunners.
IV.2.4. Xeromorphic vegetation
Xeromorphic vegetation overlaps with sclerophyllous vegetation (III.1.3) over a broad
zone of seasonally dry climates. The present-day xeromorphic plants are mostly angiosperms,
though xeromorphy also occurs in lichens, lycopsids, ferns, cycads, conifers
and gnetophytes. Cladonia is prominent in the present-day lichen tundra while Selaginella
is widespread both in savannas and tundras, as well as over the Pleistocene savannoid
(“tundra-steppe”) vegetation.