CO2 Assimilation and Plant Growth Plant growth is generally considered as a consequence of CO2 assimilation through photosynthesis. However, growth at the level of an individual cannot be predicted from photosynthesis alone, simply because most of the other growth determinants (such as seed quality and seed germination, carbon investment in root exudation and mycorrhiza formation, plant maturation and senescence, competition for resources, nutrient recycling etc.) are quantitatively or even qualitatively unknown. In particular, phenological controls (such as floral initiation, fruiting etc.) are poorly understood, but they are of key importance for all other growth determinants. Growth requires resources other than CO2 and their availability, and in particular, their pool size also cannot be expected to increase proportionally along with enhanced CO2 (Körner, 1996). Therefore, the most common initial response of plants to CO2 fertilization is overshooting of assimilate levels in leaves (mostly starch) reflecting limitations to export of these photosynthetic products from chloroplasts to the plant parts (Ehret and Joliffe, 1985; Körner et al., 1995; Rey et al., 1997). Although growth enhancement in tree seedlings grown under elevated CO2 have been reported, CO2-induced net photosynthesis has been found to be greater for slower-growing species than for faster-growing species (Tjoelker et al., 1998). Species-specific and environmental characteristics may therefore determine the long-term response to elevated CO2.
It is assumed that elevated CO2 will induce an increase of Leaf Area Index (LAI) (Leith et al., 1986; Long and Drake, 1992; Tjoelker et al., 1998). The basis for this assumption is the reduction of the light compensation point of photosynthesis under elevated CO2. Leaves living under limiting light conditions can be expected to profit relatively more from CO2 enrichment, although the absolute carbon gain may still be small. However, when mineral nutrient availability is limiting growth which may be the case even in moderately fertile soils, it appears that plants in dense communities could respond in a different way (Körner, 1996). Despite improved carbon balance in their shaded leaves, the nitrogen trapped in these leaves could be invested much more efficiently in the fully sunlit crown, where the relative stimulation by CO2 enrichment may be smaller but the absolute carbon gain due to elevated CO2 is much greater. However, this does not apply to understorey plants which are bound to the lowest canopy level (Körner, 1996).
Plants grown under elevated CO2 have almost always been found to produce tissues that contain more carbon and less nitrogen, even when subtracting starch accumulation (e. g. DeLucia et al., 1985; Curtis et al., 1989; Wong, 1990; Körner and Miglietta, 1994). It has been assumed that this CO2-induced increase in C/N ratio (and possibly increased lignin content) will lead to reduced rates of decomposition, and thereby facilitate increased carbon sequestration to soils (Van Veen et al., 1991). There is another pathway by which CO2 fertilization may influence the soil environment namely through priming effects of increased rates of turnover in fine roots and exudation of low molecular weight organic compounds (sugars, amino acids) to the rhizosphere. This may enhance microbial activity that could result in increased rates of litter decomposition leading to decreasing soil carbon pool. These two avenues of carbon into the soil may have quite contrasting effects on the soil carbon pool, and currently, it is still uncertain in which direction soil carbon pool actually will work.
Relationship of Plant Growth Rate and Ecosystem Carbon Pool Plant growth stimulation is almost always seen as a self-evident indicator of enhanced carbon sequestering, which is not true. The carbon pool of an ecosystem is not correlated with its growth rate, as can be seen when old-growth and early successional forests are compared (Körner, 1996). Despite a limited carbon pool, the speed of carbon cycling in early successional forests is higher compared to old growth forests which is why they grow faster, mature earlier and also die earlier. However, the mean residence time of carbon in the ecosystem is of greater importance than the speed of carbon cycling through the system to contain excessive carbon in a CO2-enriched environment. In forests, which form the only significant biomass carbon pool of the biosphere, the speed of closure and reopening of gaps, to large extent, determine the carbon pool per unit area. There is no evidence that the carbon residence in biomass is likely to increase in a CO2-enriched world (Körner, 1996). Rather the reverse may be true, an idea which found recent support in the observation that tree turnover in tropical forests has been increased significantly in recent decades (Phillips and Gentry, 1994).
Elevated CO2 and Effects of Extreme Temperatures Limited data from studies with herbaceous species suggest that the combined effect of CO2 and temperature are not necessarily additive and are, therefore, difficult to predict from knowledge of their individual effects. Increased atmospheric temperatures are assumed to induce increased respiratory rates by organisms and thereby could mitigate CO2 effects on photosynthesis. To make any anticipatory statement regarding the vegetation response to climate warming, one must consider the species-specific effects of temperature in combination with elevated CO2.