The response of plants to changing atmospheric concentrations of CO2

Of all the various resources required by plants, CO2 is the only one that is increasing on a global scale. This rise is strongly correlated with the increased rate of consumption of fossil fuels

Figure 3.14 (a) The percentage of native C4 dicot species in various regions of North America. (b) The relationship between the percentage of native C4 species in 31 geographic regions of North America, and the mean summer (May-October) pan evaporation -a climatic indicator of plant/water balance. Regions for which appropriate climatic data were unavailable were excluded, together with south Florida, where the peculiar geography and climate may explain the aberrant composition of the flora. (After Stowe & Teeri, 1978.)

Figure 3.14 (a) The percentage of native C4 dicot species in various regions of North America. (b) The relationship between the percentage of native C4 species in 31 geographic regions of North America, and the mean summer (May-October) pan evaporation -a climatic indicator of plant/water balance. Regions for which appropriate climatic data were unavailable were excluded, together with south Florida, where the peculiar geography and climate may explain the aberrant composition of the flora. (After Stowe & Teeri, 1978.)

and the clearing of forests. As Loladze (2002) points out, while consequential changes to global climate may be controversial in some quarters, marked increases in CO2 concentration itself are not. Plants now are experiencing around a 30% higher concentration compared to the pre-industrial period - effectively instantaneous on geological timescales; trees living now may experience a doubling in concentration over their lifetimes - effectively an instantaneous change on an evolutionary timescale; and high mixing rates in the atmosphere mean that these are changes that will affect all plants.

There is also evidence of large-changes in geological scale changes in atmospheric CO2 time over much longer timescales. Carbon balance models suggest that during the

Triassic, Jurassic and Cretaceous periods, atmospheric concentrations of CO2 were four to eight times greater than at present, falling after the Cretaceous from between 1400 and 2800 |ll l-1 to below 1000 |ll l-1 in the Eocene, Miocene and Pliocene, and fluctuating between 180 and 280 |ll l-1 during subsequent glacial and interglacial periods (Ehleringer & Monson, 1993).

The declines in CO2 concentration in the atmosphere after the Cretaceous may have been the primary force that favored the evolution of plants with C4 physiology (Ehleringer et al., 1991), because at low concentrations of CO2, photorespiration places C3 plants at a particular disadvantage. The steady rise in CO2 since the Industrial Revolution is therefore a partial return to pre-Pleistocene conditions and C4 plants may begin to lose some of their advantage.

what will be the consequences of current rises?

When other resources are present at adequate levels, additional CO2 scarcely influences the rate of photosynthesis of C4 plants but increases the rate of C3 plants. Indeed, artificially increasing the CO2 concentration in greenhouses is a commercial technique to increase crop (C3) yields. We might reasonably predict dramatic increases in the productivity of individual plants and of whole crops, forests and natural communities as atmospheric concentrations of CO2 continue to increase. In the 1990s alone, results from more than 2700 studies on free-air CO2 enrichment (FACE) experiments were published, and it is clear that, for example, doubling CO2 concentration generally stimulates photosynthesis and increases agricultural yield by an average of 41% (Loladze, 2002). However, there is also much evidence that the responses may be complicated (Bazzaz, 1990). For example, when six species of temperate forest tree were grown for 3 years in a CO2-enriched atmosphere in a glasshouse, they were generally larger than controls, but the CO2 enhancement of growth declined even within the relatively short timescale of the experiment (Bazzaz et al., 1993).

Moreover, there is a general tendency for CO2 enrichment to change the composition of plants, and in particular to reduce nitrogen concentration in above-ground plant tissues - around 14% on average under CO2 enhancement (Cotrufo et al., 1998). This in turn may have indirect effects on plant-animal interactions, because insect herbivores may then eat 20-80% more foliage to maintain their nitrogen intake and fail to gain weight as fast (Figure 3.15).

CO2 enhancement may also reduce concentrations in plants of other essential nutrients and micronutrients (Figure 3.16) (see Section 3.5), contributing in turn to 'micronutrient malnutrition', which diminishes the health and economy of more than one-half of the world's human population (Loladze, 2002).

CO2 and nitrogen and micronutrient composition

10 20 Larval age (days)

Figure 3.15 Growth of larvae of the buckeye butterfly (Junonia coenia) feeding on Plantago lanceolata that had been grown at ambient and elevated CO2 concentrations. (After Fajer, 1989.)

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