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the rise in global levels

The CO₂ used in photosynthesis is obtained almost entirely from the atmosphere, where its concentration has risen from approximately 280 μl l⁻¹ in 1750 to about 411 μl l⁻¹ as we write (2018) and is still increasing by 0.4–0.5% per year (see Figure 21.22).

variations beneath a canopy

Concentrations also vary spatially. In a terrestrial community, the flux of CO₂ at night is upwards, from the soil and vegetation to the atmosphere; on sunny days above a photosynthesising canopy, there is a downward flux. Nonetheless, above a vegetation canopy, the air becomes rapidly mixed. The situation is quite different, however, within and beneath canopies. Changes in CO₂ concentration in the air within a mixed deciduous forest in summer, in Sapporo, Japan, are illustrated in Figure 3.14. Throughout the day, there was a gradient of decreasing concentration from the ground (0.5 m) to the upper canopy (24 m), reflecting the shifting balance between its production through respiration and its utilisation in photosynthesis. Indeed, an earlier study by Bazzaz and Williams (1991) had recorded levels as high as 1800 μl l⁻¹ near the ground, as a result of rapid decomposition of litter and soil organic matter. The gradient was steepest, and concentrations generally higher, during the night than during the day, presumably because the respiration of decomposers, especially, is relatively insensitive to the diurnal cycle. In winter, in the absence of leaves, there was no detectable variation in CO₂ concentration with height.

Figure 3.14 The change in atmospheric CO₂concentration with height in a forest canopy in Sapporo, Japan at night and in the day. The bar is the maximum SE.

Source: After Koike et al. (2001).

That CO₂ concentrations vary so widely within vegetation means that plants growing in different parts of a forest will experience quite different CO₂ environments. Indeed, the lower leaves on a forest shrub will usually experience higher CO₂ concentrations than its upper leaves, and seedlings will live in environments richer in CO₂ than mature trees.

variations in aquatic habitats

In aquatic environments, variations in CO₂ concentration can be just as striking, especially when water mixing is limited, for example during the summer ‘stratification’ of lakes, with layers of warm water towards the surface and colder layers beneath. Some examples from a study in Estonia are shown in Figure 3.15. At one extreme was the shallow Lake Äntu Sinijärv, which is supersaturated with CO₂ (CO₂ concentration higher than would result from equilibration with atmospheric CO₂) as a result of the high concentrations of bicarbonate ions in the water flowing into it. Here, there was usually virtually no vertical stratification of CO₂. Lake Saadjärv was deeper and thermally stratified and also had very high CO₂ concentrations, but in this case there was strong CO₂ stratification in the deeper layers. And finally, Lake Peipsi was a very large lake compared with the others (3555 km² compared with <10 km² for the others), similar in depth to Lake Äntu Sinijärv, but with very much lower CO₂ concentrations overall. In this case, vertical stratification of CO₂ concentrations led consistently to levels in the upper layers where the concentrations were lower than saturation, such that the lake was a sink for atmospheric CO₂, whereas the other two lakes were net CO₂ emitters.

Figure 3.15 Concentrations of CO₂vary, variably, with depth in Estonian lakes. The profiles with depth of CO₂ concentration over a number of days (different in each case) in three lakes in Estonia, as indicated. Note that the colour‐coding varies between the lakes to reflect their different concentration ranges, and that their depths are different.

Source: After Laas et al. (2016).

In aquatic habitats, especially under alkaline (high pH) conditions, dissolved CO₂ tends to react with water to form carbonic acid, which in turn ionises, such that 50% or more of inorganic carbon in water may be in the form of bicarbonate ions. Indeed, the Estonian lakes highlight how, in many cases, overall concentrations of CO₂ may be highly supersaturated. This may seem to suggest that aquatic plants will only rarely be limited by the availability of CO₂, but in fact they commonly are limited, due to the low rates of CO₂ diffusion in water, and around half of aquatic plants are able to use bicarbonate ions as an alternative source of dissolved organic carbon. However, since bicarbonate must ultimately be reconverted to CO₂ for photosynthesis, this is likely to be less useful as a source of inorganic carbon, and in practice, many plants will be limited in their photosynthetic rate by the availability of CO₂. We see an illustration of this in Figure 3.16. Ten species of aquatic plants, all capable of using bicarbonate as a source of CO₂, were grown in two culture conditions, both with the same overall concentration of dissolved organic carbon (0.85 mM). In one case (low‐C) the water was in equilibrium with the surrounding air (saturated) and so the contribution of CO₂ itself to this was small (0.012 mM). But in the other case (high‐C) the initial concentration, largely from bicarbonate ions, was much lower (0.40 mM) but gaseous CO₂ was continually added to the water, supersaturating it, and raising the overall concentration to the low‐C level. All 10 species grew faster under the high‐C conditions (Figure 3.16a), apparently as a result of elevated growth efficiencies at higher concentrations of CO₂, since the low‐C plants were investing more in, for example, leaf nitrogen (Figure 3.16b), enabling them to make more of the limited CO₂ resource available to them. Even for these bicarbonate users, bicarbonate is good but CO₂ is better.

Figure 3.16 Aquatic plants may be limited in their photosynthetic ability by the availability of CO₂. (a) The relative growth rate (RGR, rate of growth per unit weight) for 10 species of aquatic plants, as indicated, when water was at equilibrium with the surrounding air with respect to CO₂ (low‐C) such that the contribution of CO₂ to dissolved inorganic carbon (compared with bicarbonate) was relatively small, and when CO₂ was continually passed into the water (high‐C) such that the contribution of CO₂ was large. In a two‐way analysis of variance, the effects of species and treatment were both significant (respectively, F = 11.6, P < 0.0001 and F = 52.9, P < 0.0001). (b) The leaf nitrogen content of the same 10 species in the same treatments. Again, the effects of species and treatment were both significant (respectively, F = 9.1, P < 0.0001 and F = 101.4, P < 0.0001). Means and SEs are shown in both parts.

Source: After Hussner et al. (2016).