The concept of heaths is of ancient origin. The word itself can be traced back to Old Saxon hetha and Middle High German Heide, meaning wasteland. The people who lived on the heath were the heathens (Gothic haithi) in the same way that plants that grow on the heath. The genera that constitute the heaths all belong to the Ericaceae (e.g. Erica, Calluna, Vaccinium, Empetrum (Figs. 9.4-9.8), Arctostaphylos, Cassiope, Andromeda, Ledum, and Loiseleuria). All these genera of dwarf woody shrubs that grow on the oligotrophic soils are commonly referred to as heaths or heath species. As a vegetation formation, heaths are found worldwide from the Arctic to the Fynbos of South Africa (see Chapter 2) and the mountain ranges of the Himalaya (Rhododendron spp.) to the raised bogs and moors of Patagonia and the Falkland Islands (Empetrum rubrum, Gaultheria spp.) and the moorlands of the Atlantic coasts and islands of western and north-western Europe dominated by heather (Calluna vulgaris).
Heathlands are typically communities that flourish in oceanic climates both at altitudes above the treeline and at sea level. Where trees fail or are removed by disturbance (fire, grazing, etc.) heaths can migrate downhill and flourish to an extent that is not seen in the high altitude sites. Any study of the factors which control the distribution of species in oceanic climates has to take into account the dual limitations of both summer and winter temperatures. Predicting plant distribution in terms of seasonal temperature regimes demands correlative observations from a range of variables. An objective manner of comparing species distribution with the interaction of winter and summer temperatures has been described and examined for a number of European species (Crawford et al., 2003). This method enables the production of maps which compare the probability of the present occurrence of a species with that which would be expected under a range of specified winter and summer temperature combinations. Using this map system it is instructive to view the probable changes in the distribution in heath species in relation to varying degrees of summer and winter warming (Figs. 9.21-9.24).
(1) Heather (Calluna vulgaris) under present conditions is more oceanic in its distribution than V. myrtillus even though the temperature-based probability distribution suggests that its presence in Scotland is marginal. Calluna vulgaris also occurs on dry and hot sites above the forest limit in the relatively continental Central Alps (e.g. Otztal, Upper Enga-dine). In northern Germany, Calluna has been favoured by human impact, probably the same factors as in Scotland. When grazing is removed, pine, birch, and also juniper invade Calluna heath as the light-demanding Calluna becomes rapidly outcompeted. In the model (Fig. 9.21) summer warmth appears from these predictions to be a limiting factor, as summer warming would expand the range, while winter warming would cause a further retreat from the west and advance eastwards. As might be expected, summer cooling by 4 °C below the present temperature levels would cause a south-western migration which would not take place if it were the winter temperatures that had been reduced.
(2) The bilberry (Vaccinium myrtillus) is a plant of northern Europe but with a much wider distribution southwards and eastwards than Cassiope hypnoides. In the Alps Vaccinium myrtillus occurs widely in pine and spruce forests on acidic soils. Above the forest limit in the Alps it is restricted to sites with a not too shallow snow pack and is not found on snow-free sites. The probability map for its present
P= Calluna vulgaris >0.7
>0.7-0.55 >0.55-0.35 I >0.35-0.15 ■ >0.15-0.05 <0.05
Probability of occurrence at 1961-1990 temperatures
Warmer winters Warmer summers
Mean annual warming 0 °C
Fig. 9.21 Calluna vulgaris and possible responses to seasonal differences in relation to climatic warming. Probability density plots show the possible ranges in relation to changing summer and winter temperatures. The colours represent bands of increasing probability of the different combinations of winter and summer temperatures being suitable for the species. Red is most suitable (see inserted scale). In (a) the scenario is shown for a higher winter temperature of + 4 °C while the summer temperature is reduced by —4 0 C. The reverse conditions pertain in (b) where the winter is —4 ° C colder and the summer 4 ° C warmer. In relation to annual mean temperature this represents no change from the 1961—1990 temperatures as given in the CRU Global Climate Dataset (New et al., 1999, 2000). Note the prediction of a retreat for this species from the British Isles and Scandinavia with warmer winters and cooler summers. (For further details see Crawford & Jeffree, 2007). N.B. These maps are model predictions based on the present distribution of the species in terms of potential range in relation to temperature and not on present geographic distribution.
distribution based on temperature classifies its western extension in the British Isles, western France and northern Spain as marginal (Fig. 9.22). Summer warming would appear to lead to a major decline in this species while winter warming would result in a western retreat and an eastward expansion. Climatic cooling, if it took place largely through a reduction in summer temperatures, would cause a retreat from the northernmost habitats and an expansion southwards and eastwards. Winter cooling, however, would have a different effect and would be likely to reduce the marginality of its presence in western Europe.
(3) The dwarf arctic heath (Cassiope hypnoides) is a species of transatlantic distribution. Under the present climatic regime it is found from eastern Canada where it is confined from Labrador northwards to 750 N (Fig. 9.23). It is widespread in Greenland to 750 N on the west coast and 800 N on the east coast. The species also occurs in central and northern Iceland and in the more northern montane regions of Scandinavia, the Kola Peninsula, and
Vaccinium myrtillus p=
Probability of occurrence at 1961-1990 temperatures
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