High altitude forest outposts: where and why?

Isolated tall trees or patches of forest far above the current high elevation treeline are found in many parts of the world. There are two explanations:

1. Relict outposts (the common case) indicate the true treeline position in an otherwise deforested region. A classical example is Polylepis, a Rosaceae elfin wood in the high Andes which has survived millennia of burning and land use because of topographic shelter from fire or growth on block fields.
2. True outposts in favourable microhabitats. A certain topography and local air flow may create atypical microclimatic conditions (e.g. Pinus cembra patches at 2550 m near Zermatt, Switzerland, 250 m above treeline).

1 - Relict outposts

3 - Pinus cembra patches at 2550 m near Zermatt, Switzerland, 250 m above treeline

3 - True outposts