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Fig. 1.9 (Above) The Swiss stone pine (Pinus cembra) growing as a timberline tree in the Alps with the Matterhorn in the background. The stone pine is an example of a tree which shows no signs of suffering a carbohydrate shortage even at its highest locations. (Below) Seasonal course of daily carbon balance and the net cumulative carbon exchange of an experimental Pinus cembra tree. The data were collected between 5 November 2001 and 31 December 2002. (Reproduced with permission from Wieser et al., 2005.)

Fig. 1.10 Temperature at alpine treelines for various latitudes. Upper line: mean temperature for the warmest month. Middle heavy line: mean temperature for the growing season. Lower line: mean temperature for the growing season for patches of forest from very high altitudes. Treeline altitudes (and extreme tree limits): 1, Kilimanjaro (3950 m); 2, Mt Wilhelm, New Guinea 3850 m (4100 m); 3, Bale Mts, Ethiopia 4000 m (4100 m only approx. estimates of temperature); 4, Venezuelan Andes 3300 m (4200 m); 5, Polylepis record, northern Chile (4900 m); 6, Khumbu Himal (Everest region) 4200 m (4420 m on sunny slopes, 200 m less on shaded slopes); 7, White Mts, California 3600 m; 8, Colorado 3550 m; 9, Central Alps 2100 m (2500 m); 10, Rocky Mts, Alberta 2400 m; 11, Northern Scandinavia (Abisko) 680m (750m). Island mountains not considered in lines fitted to data: 12, Mauna Kea, Hawaii 3000 m; 13, Craigieburn Range, New Zealand 1300 m; 14, Cairngorms, Scotland 600 m. (Data compiled by Korner, 1999, from various sources and reproduced with permission.)

Fig. 1.10 Temperature at alpine treelines for various latitudes. Upper line: mean temperature for the warmest month. Middle heavy line: mean temperature for the growing season. Lower line: mean temperature for the growing season for patches of forest from very high altitudes. Treeline altitudes (and extreme tree limits): 1, Kilimanjaro (3950 m); 2, Mt Wilhelm, New Guinea 3850 m (4100 m); 3, Bale Mts, Ethiopia 4000 m (4100 m only approx. estimates of temperature); 4, Venezuelan Andes 3300 m (4200 m); 5, Polylepis record, northern Chile (4900 m); 6, Khumbu Himal (Everest region) 4200 m (4420 m on sunny slopes, 200 m less on shaded slopes); 7, White Mts, California 3600 m; 8, Colorado 3550 m; 9, Central Alps 2100 m (2500 m); 10, Rocky Mts, Alberta 2400 m; 11, Northern Scandinavia (Abisko) 680m (750m). Island mountains not considered in lines fitted to data: 12, Mauna Kea, Hawaii 3000 m; 13, Craigieburn Range, New Zealand 1300 m; 14, Cairngorms, Scotland 600 m. (Data compiled by Korner, 1999, from various sources and reproduced with permission.)

can be seriously carbon deficient under specific conditions. When carbon balance is viewed in relation to plants with a large biomass such as trees it is easy to overlook the necessity of examining the vitality of specific organs. The fact that one summer day's photosynthetic activity near the timberline of the Swiss stone pine (Pinus cembra) is equivalent to the total respiration for the entire winter (Wieser, 1997) can easily lead to the conclusion that a carbon deficit is not likely to be a limiting factor. Unfortunately, this whole-plant approach overlooks the fact that in many cases it is certain vulnerable key tissues and not whole plants that are likely to suffer from carbon deficits. The root tips in Sitka spruce (Picea sithchensis) become depleted of carbohydrate in winter, particularly when the soil is flooded or merely water saturated. This is not immediately harmful to the roots, but when air returns in spring, when the water table subsides, the roots show dieback as a result of post-anoxic injury. Reduced carbohydrate availability can also be expected to cause a deficiency in antioxidants and thus render the root tips vulnerable to membrane damage when suddenly re-exposed to oxygen. Again this is not immediately harmful as there are usually sufficient upper roots to keep the tree alive. However, when repeated over a number of seasons the resultant shallow rooting of trees in wet soils makes them highly vulnerable to wind-throw. The spruce tree as a whole is never carbon limited (Jarvis & Leverenz, 1983), yet this type of injury, which can be traced to a localized carbohydrate deficiency, is one of the severest limitations to Sitka spruce cultivation in oceanic conditions. Despite the fact that Sitka spruce is capable of making a net increase in biomass in every month of the year in the British Isles, it is the carbon depletion of the root meristem in warm, water-saturated soils that brings about the eventual collapse of trees (Crawford, 2003). Similarly, intermittent mild winter periods can deplete carbohydrate levels in shoots of Vaccinium myrtillus, and the progressive respiratory loss of cryoprotective

Table 1.1. Definitions of ecological concepts used in discussion of the nature of margins

Concept

Definition

Reference

Biome Competition

Deprivation indifference Ecological release (competitive release) Ecotone

Fitness: immediate and long-term

Isomorphism Koppen's Rule

Life history traits /

strategies

Limes convergens

Limes divergens

Metapopulation

Montgomery effect Niche

A biological division of the Earth's surface that reflects the ecological and physiognomic character of the vegetation.

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 this pre-empted space.

The ability to survive a temporary absence of an essential resource.

The expansion of the range of a species when competition for its niche is removed.

A narrow transition zone between two or more different communities. Such edge communities are usually species rich.

The ability of a given genotype to contribute to subsequent generations. Populations that maintain a high degree of variability can be considered to have long-term fitness as they are pre-adapted to environmental alteration.

Apparent similarity of individuals of different race or species.

The low temperature limit for tree growth is reached when the mean temperature of the warmest month of the year is less than 10 °C.

Major features of an organism's life cycle most directly related to birth and death rates.

A well-defined boundary zone (Latin limen, threshold defining the boundary between two major habitat types) A diffuse boundary zone (cf. above) where one major habitat changes gradually into another.

A network of subpopulations isolated in habitat patches. The long-term persistence of the species depends on local (patch) extinction and recolonization and on net gene flow between subpopulations.

The ecological advantage conferred by low growth rates for survival in areas of low environmental potential. The requirements that the environment has to meet to allow the persistence of a species or population.

Allaby, 1998

Calow, 1998 Harper, 1977

Grubb, 1977

Crawford, 1989

Wilson, 1959 Allaby, 1998

Allaby, 1998

Lincoln et al, 1998 Crawford, 1999

Lincoln et al., 1998 Koppen, 1931

Calow, 1998

Allaby, 1998

Allaby, 1998

Levins, 1970 (see Hanski, 1999)

Montgomery, 1912 Calow, 1998

Concept

Definition

Reference

Pre-adaptation

Refugium (plural refugia)

Relict species (climatic and evolutionary)

Resource Ruderal species

Tolerance: physiological and ecological

An organism may be described as pre-adapted to a new situation where pre-existing morphological structures or physiological adaptations have been inherited from an ancestor for a potentially unrelated purpose.

An area that has escaped major climatic changes typical of a region as a whole and acts as a refuge for biota previously more widely distributed.

Persistent remnants of a formerly widespread fauna or flora existing in isolated areas or habitats.

Any component of the environment that is consumed by an organism.

A species of open, disturbed conditions usually resulting from human activity (Latin rudus, rudera, rubble, rubbish dump)

The range of environmental factors in which an organism can survive. In absence of competition: physiological tolerance; in natural surroundings and exposed to competition: ecological tolerance.

Allaby, 1998

Lincoln et al., 1998

Lincoln et al., 1998

Section 3.1

Grime, 2001

Walter, 1960

sugars renders the shoots sensitive to frost damage, a phenomenon also observed in Picea abies (Ogren, 1996).

The desire to have a common currency for measuring potential distribution limits in physiological terms can obscure the realization that even along a simple environmental gradient many factors operate on plant survival that cannot be quantified under the common currency of carbon balance. In the case of the treeline cited above, time for resource utilization in growth rather than carbon acquisition may be the overruling limit to survival. A further disadvantage of using resource acquisition in general as a measure of potential limits to distribution comes from the fact that most species have broadly similar resource requirements (Grubb, 1977). Where plant species do differ is in their relative tolerance of adverse factors. Earlier works on plant ecology in the last century stressed ecological tolerance (see Table 1.1) as one of the most important factors in plant ecology (Walter, 1960). When the objective of the study is assessment of the productivity of a community such as a stand of forest trees, then no objection can be made to assessing carbon balance. However, if the aim of the study is to deter mine the causes of distribution limits, then the investigation should be free of any concept of yield or productivity and concentrate on the ultimate evolutionary criterion, namely survival.

1.3.3 Resource access and conservation in marginal areas

Limits to distribution are determined not just by the ability to acquire resources, but also by their utilization and conservation. Prolonged snow cover, flooding, drought or disturbance can impede the ability of plants to access and conserve resources. Many of the adaptations that allow plants to live in marginal areas have evolved as means of overcoming limited access to resources. One of the commonest solutions for survival on minimal resources, including carbon, is to reduce demand, which has similarities with the concept of deprivation indifference mentioned above. In an earlier study of North American cereals the concept of the ecological advantages of low growth rates was encapsulated in what has become known as the Montgomery effect, which states that 'in areas of low environmental potential ecological advantage is conferred by low growth rates' (Montgomery, 1912).

Species with widespread distributions such as the circumpolar polar arctic-alpine species Oxyria digyna and Saxífraga oppositifolia achieve their geographical spread in part by altering their size and form, both phenotypically and genotypically, and thus reducing the need for resources as measured in absolute terms of carbon sequestration. Some arctic species are so adapted to conserving resources that they can survive under continuous snow cover for two to three years in succession. Populations that live in areas where

Fig. 1.11 Examples of species that can survive prolonged and continuous snow cover for more than one growing season. (Above) The dwarf buttercup (Ranunculus pygmaeus; scale divisions — 1 cm). An extreme dwarf species of Ranunculus of circumpolar arctic and subarctic distribution that grows among moss and beside streams and often beside glaciers and snow drifts and therefore in some years remains buried for an entire growing season. (Below) The blue heath (Phyllodoce caerulea), a species in danger of disappearing from the mountains of Scotland due to the lack of winter snow.

this hazard is relatively frequent usually consist of diminutive specimens of certain widespread species such as Polygonum viviparum, Oxyria digyna, Salix polaris, and Ranunculus pygmaeus (Fig. 1.11).

Plants in marginal habitats, as in coastal habitats, semi-deserts and in the Arctic, frequently show a high degree of polymorphism. No one form or ecotype is continually favoured as environmental conditions are continually oscillating, causing populations to consist of a mixture of different forms. Such polymorphic populations aid survival in fluctuating environments by providing a number of ecotypes usually with slightly different habitat requirements (see Table 1.1). The existence of stable assemblages of interbreeding, yet distinct ecotypes, is usually described as a balanced polymorphism. Balanced polymorphisms confer immediate fitness by increasing the ecological tolerance of the species as a whole. They also enhance long-term fitness as the existence of a range of adaptations in different yet interfertile ecotypes can pre-adapt a species to long-term climatic oscillations (Crawford, 1997a). Examples of this phenomenon are readily visible at high latitudes (see Chapter 2) where the constant risks of disturbance and environmental stress maintain a pronounced degree of polymorphism in arctic plant populations. These polymorphisms can be seen in the exposed ridge and snow bank forms of Dryas octopetala as well as in the different forms of Saxifraga oppositifolia found in High

Arctic coastal sites on the beach ridges and low shores (Crawford, 2004). Both these species have evolved forms that are adapted to short and long growing seasons in the same geographical area (Fig. 1.12).

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