Ecology. Michael Begon

crowded canopies (climax species). Bars are SEs. Succ. and Can. in each case refer to P values indicating the significance in an analysis of variance of the factors ‘Succession’ (climax/pioneer) and ‘Canopy’ (sun/shade), respectively. (a) Carboxylation capacity. (b) Electron transport capacity. (c) Leaf nitrogen content. (d) Leaf phosphorus content. (e) SPAD, a proxy for leaf chlorophyll content.

      Source: After Dusenge et al. (2015).

      3.2.3 Sun and shade plants of an evergreen shrub

A number of the general points above are illustrated by a study of the evergreen shrub Heteromeles arbutifolia. This plant grows both in chaparral habitats in California, where shoots in the upper crown are consistently exposed to full sunlight and high temperatures, and also in shaded woodland habitats, where it receives around one‐seventh as much radiation (Figure 3.10a). The leaves of sun plants have a lower specific leaf area – they are thicker and have a greater photosynthetic capacity (more chlorophyll and nitrogen) per unit leaf area than those of shade plants (Figure 3.10b). Sun‐plant leaves are inclined at a much steeper angle to the horizontal, and therefore absorb the direct rays of the overhead summer sun over a wider leaf area than the more horizontal shade‐plant leaves. The more angled leaves of sun plants, though, are also less likely than shade‐plant leaves to shade other leaves of the same plant from the overhead rays of the summer sun (Figure 3.10c). But in winter, when the sun is much lower in the sky, it is the shade plants that are much less subject to this ‘self‐shading’. As a result, the proportion of incident radiation intercepted per unit area of leaf is higher in shade than in sun plants year round: in summer because of the more horizontal leaves, and in winter because of the relative absence of self‐shading.

Figure 3.10 Variations in the behaviour and properties of sun and shade leaves of an evergreen shrub. (a) Computer reconstructions of stems of typical sun (A, C) and shade (B, D) plants of the evergreen shrub Heteromeles arbutifolia, viewed along the path of the sun’s rays in the early morning (A, B) and at midday (C, D). Darker tones represent parts of leaves shaded by other leaves of the same plant. Scale bars = 4 cm. (b) Observed differences in the leaves of sun and shade plants. Standard deviations are given in parentheses; the significance of differences is given following the analysis of variance. (c) Consequent whole‐plant properties of sun and shade plants. Letter codes indicate groups that differed significantly in analyses of variance (P < 0.05).

      Source: After Valladares & Pearcy (1998).

The properties of whole plants of H. arbutifolia, then, reflect both plant architecture and the morphologies and physiologies of individual leaves. The efficiency of light absorption per unit of biomass is massively greater for shade than for sun plants (Figure 3.10c). Despite receiving only one‐seventh of the radiation of sun plants, shade plants reduce the differential in their daily rate of carbon gain from photosynthesis to only a half. They successfully counterbalance their reduced photosynthetic capacity at the leaf level with enhanced light‐harvesting ability at the whole‐plant level. The sun plants, on the other hand, can be seen as striking a compromise between maximising whole‐plant photosynthesis while avoiding photoinhibition and overheating of individual leaves.

3.3 Water

      Water is a critical resource. Hydration is a necessary condition for metabolic reactions to proceed, and because no organism is completely watertight, its water content needs continual replenishment. Most terrestrial animals drink free water and also generate some from the metabolism of food and body materials. There are extreme cases in which animals of arid zones may obtain all their water from their food.

      3.3.1 Photosynthesis or water conservation? Strategic and tactical solutions

      stomatal opening

For plants, in terrestrial habitats especially, it is not sensible to consider radiation as a resource independently of water. Intercepted radiation does not result in photosynthesis unless there is CO₂ available, and the prime route of entry of CO₂ is through open stomata (see Section 3.4). But if the stomata are open to the air, water will evaporate through them. Indeed, the volume of water that becomes incorporated in higher plants during growth is infinitesimal in comparison to the volume that flows through the plant in the transpiration stream (in through the roots, out through the stomata). If water is lost faster than it can be gained, the leaf (and the plant) will sooner or later wilt and eventually die. In most terrestrial communities, water is, at least sometimes, in short supply. The question therefore arises: should a plant conserve water at the expense of present photosynthesis, or maximise photosynthesis at the risk of running out of water? Once again, we meet the problem of whether the optimal solution involves a strict strategy or the ability to make tactical responses. There are good examples of both solutions and also compromises.

      short active interludes in a dormant life

      Perhaps the most obvious strategy that plants may adopt is to have a short life and high photosynthetic activity during periods when water is abundant, but remain dormant as seeds during the rest of the year, neither photosynthesising nor transpiring. Many desert annuals do this, as do annual weeds and most annual crop plants.

      leaf appearance and structure

      Second, plants with long lives may produce leaves during periods when water is abundant and shed them during droughts (e.g. many species of Acacia). Or they may change the nature of their leaves. Some desert shrubs in Israel (e.g. Teucrium polium) bear finely divided, thin‐cuticled leaves during the season when soil water is freely available. These are then replaced by undivided, small, thick‐cuticled leaves in more drought‐prone seasons, which in turn fall and may leave only green spines or thorns (Orshan, 1963): a sequential polymorphism through the season, with each leaf morph being replaced in turn by a less photosynthetically active but more watertight structure.

      Next, leaves may be produced that are long lived, transpire only slowly and tolerate a water deficit, but which are unable to photosynthesise rapidly even when water is abundant (e.g. evergreen desert shrubs). Structural features such as hairs, sunken stomata and the restriction of stomata to specialised areas on the lower surface of a leaf slow down water loss. But these same morphological features reduce the rate of entry of CO₂. Waxy and hairy leaf surfaces may, however, reflect a greater proportion of radiation that is not in the PAR range and so keep the leaf temperature down and reduce water loss.

      physiological strategies

      Finally, some groups of plants have evolved particular physiologies: C₄ and Crassulacean acid metabolism (CAM). We consider these in more detail in Sections 3.4.1–3.4.3. Here, we simply note that plants with ‘normal’ (i.e. C₃) photosynthesis are wasteful of water compared