Recent development of high-altitude Pinus sylvestris scrub in the Northern Cairngorm mountains, Scotland

Scots pine Pinus sylvestris has been colonizing the Northern Corries area of the Cairngorm mountains, following a reduction in grazing and browsing, and a natural subalpine scrub zone appears to be developing. The area was surveyed to assess the density and rates of tree colonization, and identify the factors affecting the process. The pattern of colonization was uneven. Although relationships between pine establishment and environmental variables were complex, it was possible to identify the main factors affecting pine colonization. They were: distance from the forest (the main seed source), altitude, vegetation (type and height), drainage and soil organic horizon depth. Combining the effects of all these factors, pine establishment was greatest close to the forest, at altitudes around 600–700 m, in Calluna vulgaris moor or lichen-rich dwarf Calluna heath under 20 cm tall, on well-drained mineral soils with at most a shallow organic horizon. Establishment at low altitudes tended to be restricted by deep, wet peat and tall vegetation, and at high altitudes mainly by climate. The age structure of the population indicated a sudden increase in colonization around 1960, possibly related to reduced deer browsing, but colonization decreased after about 1970. Some possible reasons for this pattern are given. Comparisons with other tree-line populations suggest that, provided grazing/browsing pressure remains low, the area may develop a natural tree-line similar to the only other extensive natural tree-line presently known in Scotland, at Creag Fhiaclach, also in the Cairngorms. Some suggestions are made of suitable management to accelerate this process.