scarcely influences the rate of photosynthesis of C4 plants but it increases the rate for C3 plants. As atmospheric concentrations continue to rise, therefore, it is no surprise that there has been considerable interest in the effects of higher CO2 concentrations on the productivity of individual plants and of whole crops, and natural communities including tropical rainforests (Lewis et al., 2009). Earlier studies often used open‐top chambers into which CO2 was blown before escaping through the top, but increasingly and now predominantly, use is made of free air CO2 enrichment facilities (FACE – Figure 3.21) in which a ring of pipes release CO2, at a range of heights, into an otherwise unconstrained body of plants much larger than an open top chamber (often 8–30 m in diameter). A computer‐controlled system is used to regulate the flow so as to maintain the target CO2 concentration in the FACE facility, typically 475–600 ppm.
Reviews of FACE studies have consistently shown increases in photosynthetic rates in response to elevated CO2 concentrations (Figure 3.21a), and these responses have been markedly greater in C3 than in C4 plants (Figure 3.21b), as predicted. Elevations in photosynthetic rates have also often been translated into increases in yield, though it is striking that such effects are more marked under the more natural conditions of a FACE facility than in open top chambers (Figure 3.21c).
Figure 3.21 Photosynthetic activity is increased by enhanced CO2concentrations in FACE experiments. (a) Mean responses from meta‐analyses of light‐saturated CO2 uptake (Asat), a measure of photosynthetic activity, to enhanced CO2 concentrations ([CO2]) in free air CO2 enrichment facilities (FACE) experiments, for a variety of plant groups. Red symbols are from the current meta‐analysis, brown symbols from previous meta‐analyses. (b) Mean responses from these meta‐analyses of light‐saturated CO2 uptake (Asat) to enhanced CO2 concentrations ([CO2]) according to whether plants were C3 or C4. (c) Mean responses from a meta‐analysis of the yields of various crops, as indicated, to enhanced CO2 concentrations ([CO2]) in FACE experiments and open top chambers (OTCs). Bars are 95% CIs in all parts.
Source: (a, b) After Ainsworth & Long (2005). (c) After Bishop et al. (2014).
Such responses are in part simply a reflection of a greater availability of resource (CO2), but there is likely to be an additional effect, especially in C3 plants, resulting from a reduced need for stomatal opening and the consequent reduction in water loss. This reduction in stomatal conductance has indeed been observed, at least in crops and especially under drought conditions (Figure 3.22a). This in turn suggests that such increases in yield at higher CO2 concentrations should themselves be higher when there is reduced availability of water, since this is when the benefits of conserving water will be greatest. This, too, has been confirmed (Figure 3.22b).
Figure 3.22 Stomatal conductance is decreased by enhanced CO2concentrations, leading to increased yields especially when water is scarce. (a) Mean responses from the meta‐analysis in Figure 3.21c of stomatal conductance (gs) to enhanced CO2 concentrations ([CO2]) in free air CO2 enrichment facilities (FACE) experiments with crop plants, classified according to different additional treatments. Bars are 95% CIs. (b) The significant negative relationship of the yield increases in Figure 3.21c to the level of water availability (y = 1.36 – 0.0005x; r2 = 0.24; P = 0.009).
Source: After Bishop et al. (2014).
We should not be tempted by these general patterns, however, into thinking that responses to elevated CO2 concentrations will be straightforward and predictable. The results of a 20‐year FACE experiment, in which the responses of C3 and C4 grasses were compared, are shown in Figure 3.23. For the first 12 years of the experiment, the results were as might conventionally have been predicted. There was a marked increase in biomass in C3 plots compared with those experiencing ambient CO2, but no such response in C4 plots. However, this pattern was reversed in the subsequent eight years (Figure 3.23a). The explanation for this reversal is uncertain, but it is associated with a shifting balance in the effect of CO2 enhancement on the availability of nitrogen in the soils of the contrasting plots. The effect was positive initially in the C3 plots (more nitrogen available) but became increasingly negative, and was negative initially in the C4 plots but became increasingly positive (Figure 3.23b). And overall in the experiment, positive effects of CO2 enhancement on nitrogen availability gave rise to positive effects on biomass. The authors of the study admit, frustratingly, that the underlying basis of these nitrogen cycling responses remains an open question. But these results do at least remind us, first, that short‐term responses may be misleading in predicting the consequences of what are likely to be long‐term trends, and also, that the changing availability of one resource may, within the community as a whole, give rise to altered levels of other resources with equally profound consequences.
Figure 3.23 Effects of elevated CO2concentrations on C3and C4grasses are reversed over the longer term. (a) Changes in biomass over a 20‐year period in a free air CO2 enrichment facilities (FACE) experiment comparing the responses of C3 and C4 grasses to enhanced CO2 concentration
