Intrazonal variations and pantropical plateauing of alpine treelines

A multitude of abiotic and biotic reasons can cause local advances or depressions of treelines:

avalanches
1 - Avalanches and late lying snow restrict treeline forest to ridges. Mont Noble, Swiss Central Alps, 2300 m.
climatic shelter
2 - Mt. Wilhelm, Papua New Guinea. The treeline ecotone between 3600 and 3800 m. In the tropics, climatic shelter permits the forest to advance to higher elevations in topographic depressions (gullies etc.).