The Alpine Treeline

Since, by definition, the alpine belt is naturally treeless, the mechanisms by which trees are restricted from growing beyond a certain altitude are key to any understanding of alpine ecosystems. The so-called treeline marks the upper limit of the life-form 'tree' irrespective of the tree species involved (see Alpine Forest). Generally, species which form treelines are Pinus, Picea, Abies, Juniperus, and Larix among conifers, and Betula, Alnus, Erica, Polylepis, Sorbus, Eucalyptus, and others among non-coniferous families. Because tree occurrence does not stop abruptly, and trees gradually get smaller and finally become crippled, any definition of'a line' is a convention. The forest line or timberline represents the edge of the closed upper montane forest (note, 'montane' is the biogeographic term for the next lower belt, not to be confused with 'mountain'), the zone of gradual forest opening near the treeline is often termed treeline parkland, and the uppermost position where tree species can survive as small saplings or shrubs among other low-stature vegetation is called the tree species line, with the 'treeline' holding a middle ground, used for the line connecting the uppermost patches of trees >3m. The whole transition zone from montane forest to alpine heathland is termed treeline eco-tone, across which alpine vegetation gains space yielded by the thinning forest. The altitudinal range of the treeline ecotone may be 20-200 m, often <50 m.

Where moisture is permitting tree growth at these altitudes (a minimum of 250-300 mm of precipitation per year), the position of the natural climatic treeline matches with a mean growing season temperature of 6.6 ± 0.8 °C worldwide. The duration of the growing season may vary from 10 weeks at high latitude to a full year in the tropics and its onset and end are defined by a weekly mean air temperature of 0 °C (corresponding to c. 3 °C in 10 cm soil depth, where most roots occur). This isotherm sets the lower climatic threshold for alpine vegetation, which can be close to 5 °C in dry subtropical mountains and 7.5 ° C in cool temperate mountains, which is a surprisingly narrow range, given the great difference in season length across latitudes.

It is very important not to confuse this climatic (physiological) limit of trees with a multitude of other natural or anthropogenic causes for the local absence of trees such as fire, avalanches, logging and pasturing, loose or missing substrate, waterlogging, or the regional lack of cold-adapted tree species (as is the case for instance in Hawaii or New Zealand). In the last case, the treeline observed is a specific tree species line, not representative of the climatic limit of the life-form tree, as can easily be demonstrated by the success of introduced tree species which grow well at much higher altitudes in such regions. Open 'alpine-looking' grassland and shrubland may thus occur several hundreds of meters below the climatic tree-line; among the most famous of these are the Andean Paramo grasslands with their spectacular giant rosette plants (Figure 2).

Figure 2 Fire and grazing (both naturally and under human influence) can replace the montane forest, leading to 'alpine-looking' vegetation below the climatic treeline. Here an example of the Ecuadorian paramos at 3600 m altitude, c. 400-500 m below the potential climatic treeline (Paramos El Angel). Giant rosettes of the genus Espeletia are the prominent feature of this landscape, with similar vegetation also found in African highlands.

Figure 2 Fire and grazing (both naturally and under human influence) can replace the montane forest, leading to 'alpine-looking' vegetation below the climatic treeline. Here an example of the Ecuadorian paramos at 3600 m altitude, c. 400-500 m below the potential climatic treeline (Paramos El Angel). Giant rosettes of the genus Espeletia are the prominent feature of this landscape, with similar vegetation also found in African highlands.

10 Ways To Fight Off Cancer

10 Ways To Fight Off Cancer

Learning About 10 Ways Fight Off Cancer Can Have Amazing Benefits For Your Life The Best Tips On How To Keep This Killer At Bay Discovering that you or a loved one has cancer can be utterly terrifying. All the same, once you comprehend the causes of cancer and learn how to reverse those causes, you or your loved one may have more than a fighting chance of beating out cancer.

Get My Free Ebook


Post a comment