13.2.4  Ontogeny and duration of CO2 enrichment

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Figure 13.2 Weight ratios of plants frown under CO2 enriched to ambient conditions are generally greater than unity, indicating that CO2 enrichment most commonly increases plant growth. Rarely is the ratio less than one, and mostly between 1.1 and 1.8. (Based on Gifford et al. 1984 and data from other sources)

One likely reason for variation in growth responses of isolated plants in Figure 13.2 is variation in growth duration. During early exponential growth any CO2 stimulation of growth is partially invested in increased leaf area. This establishes a positive feedback because increased leaf area is available to intercept light and utilise the increased availability of CO2. This causes a massive amplification of the leaf-level response to doubled CO2 concentration (from 10% to 40%) up to a peak of 150–300% response (based on plant size at a given time prior to canopy closure). Following canopy closure, self-shading and competition for other scarce resources pro-gressively reduce or eliminate this positive feedback. That is, negative feedbacks that relate to plant size offset the positive feedback. Since plants grown with CO2 enrichment grow faster, the negative feedbacks set in sooner and the relative CO2 effect on plant size at a given time progressively declines. In experiments with communities of interacting C3 plants, as in crop stands and pasture swards or natural ecosystems, growth response of the canopy as a whole resembles short-term leaf photosynthetic responses and commonly falls between 10% and 40%.