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A multitude of abiotic and biotic reasons can cause local advances or depressions of treelines:
- For instance, avalanches depress treelines on steep slopes, except on ridges, in the temperate and boreal zone (see Fig. 1).
- In contrast, climatic shelter permits treeline to advance in depressions in the tropics (see Fig. 2).
- Cloudiness, and thus reduced solar radiation, depress the treeline in some humid equatorial regions (e.g. SE-Asia, Ruwenzori, part of the tropical Andes).
- Clear sky, and thus strong insolation and surface warming leads to record highs of treeline position in the northern semi-arid subtropics (e.g. N-Chile, Tibet).
- Loose gravel on volcanoes or frequent fires make tree establishment impossible and depress treeline.
- Human land use can convert forest to grassland anywhere, including the montane belt and the treeline ecotone.
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