Terrestrial Palaeoecology and Global Change

Chapter 8. Ecosystem evolution
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pioneer riparian trees, such as poplars. It was fairly common in the Mesozoic gymnosperm
forests while the bark-droppers prevailed in the Carboniferous lepidophyte forests. In the
ecologically marginal environments (near the tree-line), large plants tend to degrade to a
lower habit (Stevens & Fox, 1991) with biomass reallocated to underground parts. In grasslands,
the underground biomass amounts to about 25-58% of the total (Titlyanova et al.,
1996). It might have been even larger in the rhizomatic early land plant communities.
VIII.2.1. Biomass build-up and fluctuations
The evolution of ecosystems, and of the biosphere as a whole, is directed toward the
goal of efficiency in producing living matter from non-living matter with a minimal waste
in the form of dead mass. Hence, the biomass to dead mass ratio tends to increase with
evolution. This theoretical expectation is supported by the fossil record. The Early Proterozoic
microbial mats of a few cyanobacterial species produced a disproportionately
large dead mass preserved as shungites. They also contributed to accumulation of banded
iron ores amounting to about 20% of contemporaneous sedimentation. The subsequent
Late Proterozoic mat communities were much more diverse, yet they left a relatively
modest dead mass (Hofmann, 1976; Bauld, 1981).
The appearance of terrestrial plant life entailed a multifold increase in the total biospheric
biomass that was further elevated with the terrestrial vegetation expanding from
wetlands over drylands. Large trees appeared already in the Late Devonian but they
were telomic rather than leafy, thus producing a much smaller above-ground biomass
than modern trees of a comparable size. On evidence of telomic mats, the arboreal
archaeopterids were shoot-droppers (IV.3). In addition, most Devonian plants were rhizomatic,
and even in the Carboniferous arboreal wetlands a contribution of positive geotropism,
forming massive stigmaria, was considerably greater than in the extant ecological
analogues.
Fossil fuels and kerogen-rich deposits are accumulations of buried dead mass. The
biomass of extinct biotic communities can be approximately assessed by analogy with
their extant ecological equivalents (as in the Cretaceous example, VIII.2.3). When an
ecological equivalence of extinct and extant communities is problematic, as in the case
of Carboniferous peatland vegetation, comparisons can be made on the basis of arboreal
to non-arboreal plant ratios, tree size and diversity as the biomass correlates.
Thus, the Carboniferous wetland forests were sources of huge coal measures, indicating
high dead mass production rates, while their extant analogues accumulate a relatively
small dead mass. They were inferior to the present-day tropical rainforests both in
the tree/herb ratio (about 50% against 70%) and tree size (up to 40 m against 60 m),
indicating an altogether smaller biomass. The Carboniferous assemblages from a single
locality, typically accumulating plant debris from about 1 hectare of tree stands (Krassilov,
1972a), are oligodominant with no more than 10 tree species, whereas the extant