Limits To Distribution

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In many demographic studies, edge effects or boundaries have traditionally been regarded as merely 'nuisances'. Nevertheless, they can be modelled and investigated for the dynamic properties of populations at the edge of their distribution. Such studies can be used to provide graphic demonstrations that the distribution patterns of individuals at population edges have distinctive properties due to the influence of space on population margins. Processes occurring at the margin ofpopulations are likely to have an influence

Fig. 1.6 Aerial view of wetland plant community zonation. Boundaries between plant communities on the shores of Loch of Kinnordy, Angus, as photographed in 1969. (Photo J. K. S. St Joseph.)

on the extension and distribution, particularly when populations are small and localized (Antonovics etal, 2001).

1.3.1 Physiological boundaries

Figure 1.8 is an attempt to divide the physiological limitations to species distribution into a number of discrete categories. Physiological requirements for plant survival are considered under the heading of resource availability, which involves biological properties and can therefore be separated from purely physical limits to viability such as heat and cold tolerance. As modelling studies suggest, individuals at the margins of populations may be exposed to special spatial features resulting from a different relationship with their neighbours which can be influenced by a variety of factors including seed production, pollen dispersal, gene flow and the availability of potential sites for establishment (Antonovics et al., 2001). The margin is also a region where environments oscillate. For sedentary organisms, particularly if they require several years to develop before they can reproduce, this poses a particular problem. First and foremost, the long-term survival of individuals will depend on their physiological capacity to survive in a fluctuating environment.

Whatever their location in a population, be it marginal or at the core, all individuals if they are to survive have to attain a positive carbon balance and secure adequate supplies of nutrients, light, and water. This does not have to be steady-state existence. For some species access to certain resources can be interrupted for prolonged periods without causing any serious injury. Consequently, in the study of limits to distribution, particular attention has to be paid to the timing of environmental stresses and the frequencies of extreme events. Ultimately, the survival of any individual or population is related to the relative resource needs of the species in question as compared with their competitors. This, however, cannot be observed in any one growing season. Short-term experiments (e.g. 3-4 years), which can include raising the ambient temperature with shelters or giving additional nutrients, therefore have limited value. Species that succeed in capturing resources to the detriment of a less vigorous competitor, in what for these dominant plants may have been a series of favourable growing years, may ultimately be excluded from the habitat if they cannot survive at other times when resource levels are reduced. The species with the smaller demand may have therefore a greater long-term viability (Section 3.6.2).

1.3.2 Resource availability

In ecophysiological studies it is often considered desirable to have some common unit for assessing the relative viability of plant populations as they approach limits to their distribution. Given that plants may be limited by different resource deficiencies at various stages in their life cycle, carbon balance is commonly used as an appropriate currency for measuring success in resource acquisition as it is the investment of carbon that makes possible the acquisition of all other plant resources. In recent years the acquisition of resources as affected by environmental factors has been much studied, possibly because carbon acquisition either in individual plants or whole communities is readily monitored by recording carbon dioxide flux from

Fig. 1.7 Boundary zone marking the interface between limestone and acid Torridonian sandstone in the Scottish north-west Highlands. View looking north to Elphin with cliffs of Durness limestone on the right (east) supporting a calcicole flora that contrasts with the bog and acid mountain vegetation of the Torridonian sandstone and Lewisian gneiss lying to the north-west.

Fig. 1.7 Boundary zone marking the interface between limestone and acid Torridonian sandstone in the Scottish north-west Highlands. View looking north to Elphin with cliffs of Durness limestone on the right (east) supporting a calcicole flora that contrasts with the bog and acid mountain vegetation of the Torridonian sandstone and Lewisian gneiss lying to the north-west.

individual leaves, whole plants, or even forest canopies (Lee et al., 1998). In this approach, limits to plant distribution can be viewed in terms of carbon balance with the potential theoretical physiological limit for any species or community being reached only when carbon gain is no longer greater than expenditure. Current interest in climate change and the desire to be able to predict through modelling future limits to plant distribution makes the use of a general metabolic currency, such as carbon, a potentially attractive proposition.

Physiological Limits to Plant distribution

Potentially limiting resources

— Carbon dioxide

Water shortage

— Nutrient deficiencies

— Oxygen deprivation (flooding, ice-encasement)

Physical and Chemical limits _I

— Thermaltime (day degrees)

— Growing season length Frost t duration intensity

I-Wind

— Toxic minerals

— Salt Heavy metals

— Hydrogen ion concentration (pH)

Fig. 1.8 A selection of physiological factors that can impose limits on plant distribution.

If such a generally applicable method could be found to monitor energy resources it might serve to detect potential carbohydrate impoverishment before the plants in question showed any other signs of decline in viability.

In some cases it is possible to demonstrate a relationship between carbon balance and limits to distribution and physiological viability. Drought and cold both reduce the potential for carbon acquisition. In addition, certain stressful environmental situations can further deplete carbon reserves by causing an increase in carbohydrate consumption. The depletion of carbohydrate reserves by high respiratory activity at warm temperatures has often been suggested as a factor that limits the southward extension of northern species (Mooney & Billings, 1965; Stewart & Bannister, 1973). Various case histories relating plant distribution to carbon balance are discussed in Chapter 3. Although in certain cases carbon starvation is associated with failure to survive it is not a universal method that can be applied crudely to all plant forms in any situation for determining their distribution limits as there is remarkably little evidence that carbon starvation is the primary cause for either woody or herbaceous species failure at the cold end of their ecological distribution.

The most thoroughly examined aspects of the ecology of plants in relation to temperature are the altitudinal and latitudinal limits for the survival of trees. Many studies have sought to determine whether or not the low temperature regimes of high latitudes and altitudes cause trees to come into a carbon balance deficit. Physiologically, this would appear a simple and logical explanation of the effect of low temperatures on tree survival. It might be expected that as woody plants devote a considerable part of their resources to the formation of non-productive trunks and stems they may be unable to support such a growth strategy when growing seasons are cool and short. However, an extensive worldwide study of the carbon balance in trees at their upper altitudinal boundaries has shown the converse, namely, that tree growth near the tim-berline is not limited by carbon supply (Fig. 1.9) and that it is more probable that it is sink activity and its direct control by the environment that restricts biomass production of trees under current ambient carbon dioxide concentrations (Körner, 2003).

A worldwide study looking at numerous thermal indicators found that a growing season mean soil temperature of 6-7 °C provided the best generalized indicator of montane treelines from the tropics to the boreal zone (Körner & Paulsen, 2004). The soil temperature provides an approximate indicator of thermal conditions at the treeline and suggests that an edaphic thermal summation of growing season conditions could replace the older Koppen's Rule that the limit to tree growth coincided with the 10 °C isotherm of the warmest month of the year (Fig. 1.10). This modification from a measurement indicating maximum warmth to a temperature mean for the entire growing season reflects a realization that the altitudinal limits to tree growth are not directly related to the ability to make a net carbon gain. Instead, the treeline is more likely to be related to the length of growing season that is needed for the production and development of new tissues (Körner, 1999). However, in any discussion between cause and effect it is necessary to remember that mean temperatures do not exist in nature and therefore should be considered merely as indicators and not causal factors. The same is true for mean soil temperatures (Holtmeier, 2003).

Although overall carbon starvation is not a feature of large woody plants at their upper limits of distribution, it is nevertheless possible that certain organs

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