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Values arranged in ascending order

Fig. 3.20 Molecular ratio of carbon dioxide fixed (net photosynthesis) to respiration in shoots of Saxifraga oppositifolia of two distinct ecotypes. Triangular symbols, measurements made on semi-erect plants taken from a dry beach ridge; solid circles, measured at same time from an adjacent low beach late, wet site with prostrate creeping plants. Note the semi-erect plants from the early dry site are carbon savers and the plants from the late, wet site are carbon spenders.

Values arranged in ascending order

Fig. 3.20 Molecular ratio of carbon dioxide fixed (net photosynthesis) to respiration in shoots of Saxifraga oppositifolia of two distinct ecotypes. Triangular symbols, measurements made on semi-erect plants taken from a dry beach ridge; solid circles, measured at same time from an adjacent low beach late, wet site with prostrate creeping plants. Note the semi-erect plants from the early dry site are carbon savers and the plants from the late, wet site are carbon spenders.

Fig. 3.21 Ratio of d.wt of green to non-green tissues in plants of the semi-erect and prostrate forms of Saxifraga oppositifolia inhabiting respectively dry warm ridge sites and late, wet shore sites at Kongsfjjord, Spitsbergen. Note the greater proportion of green tissue in the prostrate form that developed during the peak of the growing season which will compensate in part for the shortness of the growing season on the wet shore site.

2nd Week 2nd Week 3rd Week July August August

Fig. 3.21 Ratio of d.wt of green to non-green tissues in plants of the semi-erect and prostrate forms of Saxifraga oppositifolia inhabiting respectively dry warm ridge sites and late, wet shore sites at Kongsfjjord, Spitsbergen. Note the greater proportion of green tissue in the prostrate form that developed during the peak of the growing season which will compensate in part for the shortness of the growing season on the wet shore site.

particularly when computer-based models are used to predict the effects on plants of environmental change. Different biomes can be typified for comparative studies by grouping plants into different functional types. For arid zones, functional types can be defined by categorizing plants in relation to their responses to drought. In a large-scale study of global distribution of ecosystems in relation to fire, angiosperm and gymno-sperm forests can be grouped functionally according to whether or not they are resistant to fire. Similarly, grasslands can be categorized depending on whether their dominant species are C3 or C4 plants. This latter categorization has been suggested as evolving in relation to fire some 6-8 million years ago when fire was already a major factor in the spread of C4 grasslands into former forested areas long before there was any human-induced forest burning (Bond et al., 2005; see Figs. 2.10-2.11).

In other situations different categories can be employed. Studies on climate change responses in arctic plants have found it convenient to consider the various species grouped into deciduous or evergreen shrubs, sedges, and herbs, while in herbivory studies

Table 3.1. Ecological definitions related to competition studies r-selection Selection favouring a rapid rate of population increase, typical of species that colonize short-lived environments or of species that undergo large fluctuations in population size.

K-selection Selection producing superior competitive ability in stable unpredictable environments, in which rapid population growth is unimportant as the population is maintained at or near the carrying capacity (K) of the habitat.

Consumptive competition (Exploitative competition) The simultaneous demand by two or more organisms or species for a resource that is actually or potentially in limited supply.

Interference competition The detrimental interaction between two or more organisms or species. Pre-emptive competition The ability to occupy an open site and thus pre-empt space.

Niche The ecological role of a species in a community; the multidimensional space of which the co-ordinates are the various parameters representing the species' conditions of existence to which it is restricted by the presence of competitor species.

Fundamental niche The entire multidimensional space that represents the total range of conditions within which an organism can function and which it can occupy in the absence of competitors or interactive species; pre-interactive niche; ecospace, cf. physiological tolerance.

Realized niche That part of the fundamental niche actually occupied by a species in the presence of competition or interactive species; realized ecospace; post-interactive niche; cf. ecological tolerance. Absolute competition Productivity in monoculture minus productivity in mixed culture. Relative competition (Productivity in monoculture minus productivity in mixed culture)/Productivity in monoculture.

species can be usefully placed into functional types in relation to their palatability, the essential factor being that species within each grouping share a combination of physiological, morphological and life-span characteristics. Such an approach is valid if the variability in key characteristics is greater between than within the defined types (Smith et al., 1997). It has to be noted, however, that scaling up or aggregation runs counter to normal scientific method which usually demands that investigations should proceed from the particular to the general, i.e. bottom-up and not top-down. It is nevertheless understandable that the complexities of biology, and in particular ecology, often cause this logical sequence to be reversed or ignored in response to the need for expediency in arriving at decisions necessary for conservation and political action.

The premise that the variability of key characteristics is greater between than within vegetation types cannot always be justified particularly in marginal situations. Ecotypic variability within species and even neighbouring populations of the same species is very common in marginal situations, with different ecotypes frequently having opposing and incompatible adaptations, e.g. flooding versus drought tolerance, shade versus high light requirements etc. In such cases grouping these populations together as one functional type ignores basic ecological differences.

Equally, at the species level, minor variations within one vegetation type can be occupied by closely related species that exploit different survival strategies in relation to their physiological and reproductive processes. Examples of such differences can be found in the use of crucial resources such as water. There are even differences between sexes of the same species, as has been shown in the higher growth rate and greater ability in female plants to conserve water in arctic willows (see Section 4.11; Crawford & Balfour, 1983; Dawson & Bliss, 1989a). Similarly, in male and female boxelder trees (Acer negundo) female plants show higher carbon assimilation rates than males, both when well watered and when droughted (Fig. 3.22). The significance of such differences is ignored when scaling-up

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