As might be expected in an upland region, the distribution
of plant communities is determined largely by low
temperatures, in particular by the degree and frequency of
frosts below -5°C, glazing storms, and ice-bearing winds.
The most significant single index which is readily available
is probably the mean summer temperature. A mean temperature
of 10°C for the warmest month has been accepted as the indicator
of the tree line (Daubenmire 1954). Most of the
Central Plateau above 1040m is thus in this category
