In the Cairngorms National Park, altitude is an obvious and key factor controlling habitats and species found. It has had the effect of dividing the Park into two ecological zones:
- the sub-arctic (or arctic–alpine) tundra zone;
- the temperate, boreal forest zone.
The transition between these zones can be seen well in Glen Clunie (by Glen Shee) at the foot of Meall Odhar.
Around the Cairngorms, the point of transition varies from 600 to 900 m, depending on snow cover, exposure and the kind of ground. A change in the plants and animals found often occurs at about 750 m.
In addition, at around 675 m, we can separate upland and mountain soils. Above this level, podsolisation ceases to be a dominant soil-forming process. The actual altitude varies with aspect, exposure and local relief. On exposed summits, podsols disappear at around 600 m; in sheltered basins they may persist to 750 m.
The relict temperate boreal forests of Scots pine and birch, in and around the Park, make up the biggest area of ancient semi-natural forest found in Britain today.
Altitude affects the limit of tree growth. Trees also decrease gradually in height with altitude. Only in very few areas of Scotland can you still see a natural upper limit of the boreal forest. The best surviving example in the Park (and in Scotland) is at 615 m on Creag Fhiachlach, a spur facing Aviemore. Here, a group of gnarled and wind-clipped (krumholz) Scots pine, mixed with juniper, gradually merge into open heather moorland.
Generally, across Scotland and the Cairngorms National Park the present tree line is around 600–700 m above sea level.