Montane habitats are at the top of and above the treeline, where the last wind-blasted trees and shrubs grow, below the even colder alpine habitats above where only small grasses and lichens live.

Being high on a mountain may be pleasant, but going up a hundred feet is like taking giant strides towards the North Pole in terms of the temperature decrease. This creates increasingly filtered bands of habitats at each height range, which is called altitudinal zonation.
As it gets colder and higher, fewer and fewer plants can survive, until there is nothing growing but snowmen.

In healthy, diverse Montane habitats, there is a mix of plants known as Montane Scrub. On Great Britain, these areas are mostly found in Scotland, where the primary tree-shrub is Willow.
Willow Montane Scrub used to be everywhere at altitude in Scotland, but is now confined to conservation areas.

This Herald article has some nice pics that indicate why the health of these habitats are so important for bees and therefore fruit pollination: without the stunted trees and shrubs, there isn’t much except grass and moss.

Where there is Willow Scrub, you have caterpillars for birds to eat, and pollinating insects, with several bee species.
Most are relatively rare solitary bees, the rest are bumblebees; you would not expect feral honeybees up here, but nature is wild, so.

Where there is basically just grass, lichen and moss due to generations of grazing by deer and sheep with no wolves to keep them on their toes, there are no bees to speak of. There is little cover from wind, and almost nothing for them to eat.

Therefore, if your high altitude fruit trees are surrounded by healthy Montane Willow Scrub around and above them, it should be easy to welcome the local bees into your garden and quickly boost their numbers.
All they need to thrive are interconnecting windbreaks of hedges and screening trees, which are the norm anyway for gardens facing strong wind.

As well as Willows, which are mostly wind pollinated and have light, fluffy fly-away seeds, there is plenty of Juniper, which has more valuable flowers and fruit for birds and bees.
The main willow species at altitude are Downy Willow, Salix lapponum, Grey Willow, S. cinerea, Tea-Leaf Willow, S. phylicifolia, and in some areas the Woolly Willow, S. lanata.

At the lower end of the Montane zone, you will also find wind-whipped downy birch and aspen, and the occasional leaning, battered Scots pine, Sessile oak, or rowan crouching by itself in a relatively less-exposed corner.
Sheltered by the shrubby trees, a range of other, smaller plants can thrive, providing the diversity of flowers that bees need to survive, such as Vaccinium myrtillus, the billberry or blaeberry (close relative of the larger blueberry) and of course the heather.