Читать книгу Ecology - Michael Begon - Страница 79

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 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. High mixing rates in the atmosphere mean that these are changes that will affect all plants. Plants now are experiencing around a 30% higher concentration compared with the preindustrial period – effectively instantaneous on a geological timescale. Trees living now may experience a doubling in concentration over their lifetimes – effectively instantaneous on an evolutionary timescale.

changes in geological time

Putting this in a wider context, though, there is also evidence of large‐scale changes in atmospheric CO₂ over much longer timescales. A range of models suggest that during the Triassic, Jurassic and Cretaceous periods (around 250–70 million years ago), atmospheric concentrations of CO₂ were four to eight times greater than at present (and both lower and very much higher prior to that), falling after the Cretaceous from between 1500 and 3000 μl l⁻¹ to below 1000 μl l⁻¹ in the subsequent Eocene, Miocene and Pliocene, and fluctuating between 180 and 280 μl l⁻¹ during more recent glacial and interglacial periods (the last 400 000 years; Figure 3.20). The steady rise in CO₂ since the Industrial Revolution is therefore a partial return to pre‐Pleistocene conditions, more than 2.5 million years ago.

Figure 3.20 Estimates of atmospheric CO₂concentrations over the past 600 million years (Ma). The red line (with estimated range of error in pink) is the output of the GEOCARB III model, which uses estimates of rates of geological weathering, emission and burial modified by a range of factors including global temperature and continental size. The blue line is the result of averaging four proxies of CO₂ concentrations, including isotopic compositions of minerals and plankton and the distribution of stomata in plant leaves.

Source: After Royer et al. (2004).

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 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 (A_sat), 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 (A_sat) 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 (g_s) 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 (eCO₂) relative to ambient levels (ambCO₂). Data shown are three‐year moving averages. In an analysis of variance, the interaction between year, treatment and C₃/C₄ was significant (F = 7.2; P < 0.01): the difference in response between C₃ and C₄ grasses reversed over time. (b) The effect in the experiment on net nitrogen mineralisation (a measure of nitrogen availability to the plants averaged over successive five‐year periods). Bars are SEs among years. In an analysis of variance, the interaction between year, treatment and C₃/C₄ was significant (F = 4.0; P < 0.05): the difference in response between C₃ and C₄ grasses reversed over time.

Source: After Reich et al. (2018).