Factors unlikely to be responsible for the global treeline phenomenon

The position of treelines is not driven by

frost damage
1 - Late frost damage in spring (note: the current season flush has been killed and turned brown).
 1
fungal infections
2 - Fungal infections. Swiss treeline at 2250 m.
 2
winter desiccation
3 - Winter desiccation. Austrian treeline at 2100 m.
 3
krummholz
4 - Eucalyptus pauciflora, 2050 m, Mt. Perisher, Snowy Mts., Australia.
 4
krummholz
5 - Polylepis tarapacana, 4810 m, Sajama, Bolivia.
 5
krummholz
6 - Pinus flexilis, Abies lasciocarpa, Picea engelmannii, 3400 m, Niwot Ridge, Colorado, Rocky Mts. Front Range.
 6
krummholz
7 - Tree islands of Picea engelmannii, 3350 m, Medicin Bow Mts., Rocky Mts., Wyoming.
 7
krummholz
8 - Pinus mugo, 1900 m, Lechtal Alps, Austria.
 8
Captions
Stress and pathogens in the treeline ecotone
  1. Late frost damage in spring (note: the current season flush has been killed and turned brown).
  1. Fungal infections. Swiss treeline at 2250 m.
  1. Winter desiccation. Austrian treeline at 2100 m.
"Krummholz" above treelines
  1. Eucalyptus pauciflora, 2050 m, Mt. Perisher, Snowy Mts., Australia.
  1. Polylepis tarapacana, 4810 m, Sajama, Bolivia.
  1. Pinus flexilis, Abies lasciocarpa, Picea engelmannii, 3400 m, Niwot Ridge, Colorado, Rocky Mts. Front Range.
  1. Tree islands of Picea engelmannii, 3350 m, Medicin Bow Mts., Rocky Mts., Wyoming.
  1. Pinus mugo, 1900 m, Lechtal Alps, Austria.

In summary, not all regions of the world have treeline species which can survive at the climatic treeline. In regions which have such taxa, the position of the treeline is determined by a globally common factor (temperature) plus regionally variable (modulating) factors (other climate components, soil or biotic factors). The climatic treeline is found at a mean growing season temperature of c. 6.4 ± 0.7 °C.