Terrestrial Palaeoecology and Global Change

222 Valentin A. Krassilov. Terrestrial Palaeoecology
Land biota plays primarily a stabilizing role maintaining the balance of atmospheric
composition through the multiple negative feedbacks of pCO 2
fluctuations. From 1/2 to
2/3 of CO 2
emitted to the atmosphere sinks to biota (Cao & Woodward, 1998; Schimel et
al., 2001). The biotic uptake fluctuates on a decadal scale, with an increase in the 1900’s
owing mainly to physiological feedbacks (a longer growing season, fertilization by CO 2
),
fire prevention, forest management and reclamation of abandoned lands over the northern
extratropical areas (Schimel et al., 2001).
Total phytomass production tends to increase with atmospheric pCO 2
, in particular
over the mid- to high latitudes (Adams et al., 1990). It seems possible to at least partly
compensate for the industrial CO 2
by afforestation (Fang et al., 2001). With a doubling of
atmospheric CO 2
, agricultural crop production, if not limited by a deficit of soil nitrates
and phosphates, would potentially increase by about 10% binding an appreciable amount
of carbon. Plant species widely differ in their responses to atmospheric CO 2
levels (Wilsey,
2001), an elevation of which impose a selection pressure for acceleration of growth
rates.
However, an enhancing effect of additional CO 2
on photosynthesis is partly negated
by an increase in respiration from leaf canopies (Specht et al., 1992) and soil (Cao &
Woodward, 1998). Experimental data suggest a rapid adaptation of annual plants to the
current atmospheric CO 2
concentration level, with a negative effect of further elevation
on their physiological efficiency (Bunce, 2001). Anyway, an acceleration in growth rates
under an elevated CO 2
concentration (Heil et al., 1988) may not be a long-lasting effect,
for it is slowed down by a side effect on nitrogen turnover (Centritto et al., 1999; Marriott
et al., 2001). More nitrogen is consumed by the rapidly growing plants and less is
released from their litter which has a relatively high C:N ratio (Tateno & Chapin, 1997;
Cao & Woodward, 1998). An increase in the C:N ratio in foliage stimulates herbivory, in
particular root feeding (Wilsey, 2001), with adverse effects on CO 2
consumption by vegetation.
Greenhouse not only increases plant growth rates but also accelerates reproduction,
shifting flowering to an earlier developmental stage/smaller size (Morse & Bazzaz, 1994).
An elevated CO 2
concentration promotes early flowering typical of early seral stages
that expand at the expense of the later stages (Roden et al., 1997), and reverse at a
depressed CO 2
level. Owing to the parallelism of seral and evolutionary sequences
(VIII.4), the latter effect might have occurred during plant evolution as well.
The carbon content of lignified plant tissues tends to increase with evolutionary advance.
In the Carboniferous wetland forests, it increases from tree ferns (Psaronius) to
lepidophytes to medullosan gymnosperms (Baker & DiMichele, 1997). A replacement
of lepidophyte wetlands by gymnosperm wetlands at about the Permian/Triassic boundary
would have then had a CO 2
depressing effect similar to that of the hardwood/softwood
replacements over the Late Cretaceous and Cenozoic.
A specific physiological response is provided by the CCMs, or CO 2
concentrating
photosynthetic mechanisms, such as the Crassulacean Acid or C 4
Dicarboxylic Acid