Ecology. Michael Begon

      roots as foragers

Water that arrives on a soil surface does not distribute itself evenly down the soil profile. Instead, it tends to bring the surface layer to field capacity, with further rain extending this layer deeper and deeper. This means that different parts of the same plant root system may encounter water held with quite different forces. Similar variations can occur as a result of heterogeneities in soil type – clay soils with small pores can hold far more water than sandy soils with large pores. As a root passes through such heterogeneous soil (and all soils are heterogeneous seen from a ‘root’s‐eye view’), it typically responds by branching freely in zones that supply resources, and scarcely branching at all in less rewarding patches (Figure 3.13a). That it can do so depends on the individual rootlet’s ability to react on an extremely local scale to the conditions that it meets. Strategic differences in developmental programmes can be recognised between the roots of different species (Figure 3.13b), but it is the ability of root systems to override strict programmes and be opportunistic, depending both on local conditions and their overall level of resource availability, that makes them effective exploiters of the soil (de Kroon et al., 2009).

Figure 3.13 Roots as foragers. (a) The root system developed by a plant of wheat grown through a sandy soil containing a layer of clay. Note the responsiveness of root development to the localised environment that it encounters. (b–j) Profiles of root systems of plants from contrasting environments. (b–e) Northern temperate species of open ground: (b) Lolium multiflorum, an annual grass; (c) Mercurialis annua, an annual weed; and (d) Aphanes arvensis and (e) Sagina procumbens, both ephemeral weeds. (f–j) Desert shrub and semishrub species, Mid Hills, eastern Mojave Desert, California.

      Source: (a) Courtesy of J.V. Lake. (b–e) From Fitter (1991). (f–j) Redrawn from a variety of sources.

      The root system that a plant establishes early in its life can determine its responsiveness to future events. Where most water is received as occasional showers on a dry substrate, a seedling that puts its early energy into a deep taproot will gain little from subsequent showers, but in an environment in which heavy rains fill a soil reservoir to depth in the spring, followed by a long period of drought, that taproot may guarantee continual access to water. Indeed, it seems that the placement of roots with respect to water and especially nutrient availability is most important in the earlier stages of a plant’s life. Later there is much greater reliance on stored resources in overcoming local or temporary shortages (de Kroon et al., 2009).

3.4 Carbon dioxide

      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