Physiognomic And Ecological Differentiation Of Mountain Timberline

As has been shown in the previous chapter mountain timberlines may be abrupt, gradual or occur as a transition zone bordering treeless vegetation such as dwarf shrubs and grasses (e.g., European Alps), dwarf shrub-lichen heath (fjell, Fennoscandia), mountain steppe (arid zone) or tussock grassland (e.g., in the tropics, New Zealand). On a global scale, the spectrum of trees species represented at timberline is very large and covers tree species of many genera and families. Locally, timberline is usually formed by one to three tree species but even more may occur (e.g., Wardle, 1971, 1974; Hope, 1976; Loffler, 1979; Corlett, 1984; Richter et al., 2008).

4.1 Tree species at timberline

Which and how many tree species occur at timberline depends on the climate zone and on the history of floral development. If timberline is formed by more than one species, the different ecological properties and requirements of the species (e.g. shade tolerant and intolerant, pioneer or climax species, animal or wind mediated seed dispersal, etc.) may play an important role as to the development of the tree stands, and also in respect of structure, physiognomy and climatically induced shifts of timberline. Competitive species such as beech (Fagus sylvatica), Norway or Engelmann spruce (Picea abies, Picea engelmannii), for example, often form dense stands and abrupt timberlines (cf. Photo 20), while in the case of less competetive species such as larch, most pines or juniper forests are comparatively open giving way gradually to grassland or other alpine vegetation (e.g., Walter, 1968; Armand, 1992; see also Photos 13 and 14). Some authors call such forests 'open forests' (e.g., Kessler, 1995; Miehe et al., 1998).

4.1.1 Influence of geological and floral history

Floristically, timberlines are more closely related to each other in the northern than in the southern hemisphere, which has to be ascribed to the geological history of the continents and the resulting geographic orientation of the mountain systems. While in the northern hemisphere, contiguous

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