3.12 Field capacity and the permanent wilting point in soil in relation to pore size and pressure. The status of water in the soil, showing the relationship between the diameter of soil pores that remain water‐filled and the pressure created by the capillary action of those pores that opposes the tendency of water to drain away under the force of gravity. Pressure values are negative because they describe the process of suction. The size of water‐filled pores may be compared in the figure with the sizes of rootlets, root hairs and bacterial cells. Note that for most species of crop plant the permanent wilting point is at approximately −15 bars, but in many other species it reaches −80 bars, depending on their ability to extract water from the narrowest pores.
roots and the dynamics of water depletion zones
As a root withdraws water from the soil pores at the root’s surface, it creates water‐depletion zones around it – another example of the RDZs described in Section 3.2.1. These determine gradients of water potential between the interconnected soil pores. Water flows along the gradient into the depleted zones, supplying further water to the root, but this simple process is made much more complex because the more the soil around the roots is depleted of water, the more resistance there is to water flow. Thus, as the root starts to withdraw water from the soil, the first water that it obtains is from the wider pores because they hold the water with weaker capillary forces. This leaves only the narrower, more tortuous pathways, and so the resistance to water flow increases. Thus, when the root draws water from the soil very rapidly, the RDZ may become very sharply defined, because water can move across its boundary only slowly. For this reason, rapidly transpiring plants may wilt in a soil that contains abundant water.
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
