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Specific Leaf Area

Specific leaf area characterises the relation of projected leaf area to leaf weight (m2 kg-1) and it is a sensitive indicator of several ecological processes and adaptations. The specific leaf area of organs for assimilation differs by a factor of 5. Annual crop plants reach 23.6 m2 kg-1 leaf area in contrast to 4.1 m2 kg-1 in evergreen conifers. These differences are:

• determined by evolution, i.e. in competition with other species; during the course of evolution plants developed ever greater leaf areas per unit of invested dry mass, thus increasing light capture, and supplied these thin leaves with optimal amounts of N (high N concentrations);

• adaptively determined, i.e. the habitat, particularly the water and nitrogen supply together with changes in light conditions, result in modifications to the specific leaf area.

The specific leaf area has several ecologically important consequences and effects:

• elasticity of leaves and their ability to endure drought and other stress situation; this is not to say that ecological factors determine the structure of the leaf or whether species with certain leaf structures occupy a specific site,

• packaging density (concentrations) of nutrients, particularly nitrogen (38.4 mg N g_1 in crop plants and 11 mg N g_1 in conifers);

• relation of assimilating cells to transpiring surface (A/E relation).

In general, species with high specific leaf area show a higher metabolic activity than species with lower specific leaf area. Therefore, there is differentiation of vegetation on the earth: Species with high specific leaf areas are opportunists (ruderal plants), settling in new sites and those with a large supply of resources. In contrast, plants with small specific leaf area occupy less favourable sites. Both groups occupy their respective site, not because they would not grow on other more or less favourable sites (exceptions are stress-tolerant species: e.g. obligate ha-lophytes), but because they are generally "pushed out" from sites outside their niche by competition. Other mechanisms, e.g. shade tolerance and germination, result in plants which reproduce less effectively (see Chap. 4.3).

Orians and Solbrig (1977) formalised this observation in a cost-benefit analysis of the adaptation of plant to a gradient of increasing drought (Fig. 2.4.5). The model starts with the assumption that all species are able to live at a well-watered site. Species which assimilate large quantities of C02 are less stress tolerant, i.e.

Fig. 2.4.5. The dependence of C02 assimilation on soil water content for different functional plant types that are sensitive or resistant to drying. Sensitive types have a much higher rate of C02 assimilation than resistant types when water is abundant. Sensitive types reduce C02 exchange early as the soil dries, so that under this condition resistant types dominate. (After Orians and Solbrig 1977)

Fig. 2.4.5. The dependence of C02 assimilation on soil water content for different functional plant types that are sensitive or resistant to drying. Sensitive types have a much higher rate of C02 assimilation than resistant types when water is abundant. Sensitive types reduce C02 exchange early as the soil dries, so that under this condition resistant types dominate. (After Orians and Solbrig 1977)

they have a large leaf area per dry weight and are thus sensitive to being dried out. In those plants, C02 assimilation decreases with very little soil drying. In contrast, species with low rates of C02 assimilation usually possess leaves with a smaller leaf area per dry weight and are thus able to endure drought stress to a greater degree. However, under optimal conditions, these species do not achieve such large rates of C02 assimilation as the less stress-tolerant species, but are still able to assimilate C02 when stress-intolerant species can no longer photosynthesis e.

Obviously, specific leaf area is an important parameter in response to favourable and unfavourable conditions, as well as a constitutive characteristic of species (herbaceous annuals in comparison to conifers), which plays a large role in determining the rate of C02 assimilation. The relation between specific leaf area (m2 kg-1) and nutrient (nitrogen) concentration in the leaf is linear (Fig. 2.4.6 A; Schulze et al. 1994). Simultaneously, there is a close relationship between C02 assimilation and N concentration (Fig. 2.4.6 B) where, corresponding to the model of Orians and Solbrig (1977), species with large maximum rate of C02 assimilation react more sensitively towards N deficiency than species with less photosynthetic capacity. Obviously, the investment to overcome N deficiency (high dry weight per leaf area) reduces physiological efficiency.

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