Oceanic mountain environments

Paradoxically, the greatest danger to mountain floras from rising temperatures is probably not to be found in continental mountains, as in the central Swiss Alps, but in mountains with oceanic climates. It might have been expected that oceanic mountains would be buffered against climatic change due to a reduction in annual temperature range. The relatively species-poor mountain flora of the Scottish Highlands and southwest Norway is considered to be due, at least in part, to the mild periods of winter weather that encourage premature spring growth when there is still a risk of exposure to spring frosts. In Norway, the mountain species (Table 10.2) that are absent from more oceanic mountains have been described as south-west coast avotders (Dahl, 1951, 1990).

The conflicting conditions of oceanicity versus continentality create differing upper and lower limits for the altitudinal range of species distributions. This interaction has been examined in a detailed study centred at 63° N on the Norwegian west coast and extending 135 km inland to cover the southern Fennoscandian mountain range - the southern Scandes (Holten, 2003). When species richness is plotted on a two-dimensional vertical projection of elevation and distance from the coast the maximum number of 70-80 arctic-alpine species is found in the Drividalen Knutsho Mountains (Fig. 10.20) and from there it decreased in all directions. The two-dimensional plot shows that the local maximum number of species

Table 10.2. A selection of species described as south-west coast avoiders, grouped by their varying ability to extend their distribution south and west

Boreal species that reach the Baltic Northern European and alpine species but are restricted to high altitudes reaching the Alps and/or the British

Isles but not extending to the Iberian Peninsula, Italy or the Balkans

Aconitium septentrionale Rubus arcticus Epilobium hornemanni E. lactiflorum Petasites frigidus Glyceria lithuanica Carex disperma C. tenuiflora C. loliacea

Salix myrtilloides S. phylicifolia S. starkeana Stellaria calycantha Betula nana Rubus chamaemorus Polemonium caeruleum Tofeldia pusilla

Data from Dahl (1998).

Fig. 10.20 Vertical projection of species richness of alpine vascular plants along a coast-inland transect. Note that although the number of species increases with distance from the sea the height of the mountains also increases. Thus, in addition to increasing continentality in the climate there will also be a Massenerhebung effekt (mass elevation effect; see Section 10.3.3. (Reproduced with permission from Holten, 2003.)

Fig. 10.20 Vertical projection of species richness of alpine vascular plants along a coast-inland transect. Note that although the number of species increases with distance from the sea the height of the mountains also increases. Thus, in addition to increasing continentality in the climate there will also be a Massenerhebung effekt (mass elevation effect; see Section 10.3.3. (Reproduced with permission from Holten, 2003.)

on steep coastal mountains is only half of that on inland mountains. It can also be seen that altitude difference between the upper and lower limits for the distribution of the arctic-alpine species decreases in moving towards the coast. An analysis of the species composition reveals two gradients: a humidity gradient that decreases on moving inland, and a temperature gradient that decreases with altitude. Furthermore, many of the species of the central Scandes show a negative correlation with winter temperatures as suggested by Dahl (1951) in the concept of south-west coast avoiders. Once again we have a clear indication that oceanic climates, especially when they impose mild winter temperatures, are generally unfavourable for alpine plants in boreal and temperate zones (see Chapter 1).

0 0

Post a comment