What is montane scrub?

Trees can grow in the most unpromising situations. Their roots spread out to find nourishment and water, and act as an efficient anchor and prop, supporting the tree against the battering of gales. But no trees can grow near the tops of Scotland's highest mountains.

It is simply too windy and exposed, and the growing season is too short for trees to establish. A range of 'dwarf shrubs' are the only woody plants that can survive at these altitudes. They include ling heather (Calluna vulgaris), crowberry (Empetrum nigrum), bilberry or blaeberry (Vaccinium myrtillus), bog bilberry (V. uliginosum) and other, more localised species.

What happens between the woodland zone on the lower slopes and the dwarf shrubs high on the hill? It is wrong to imagine that there is a precise altitude at which tree growth is no longer possible and the forest simply ends. Trees can creep higher where a gully offers shelter than they can on a wind-exposed hillside, so the upper margin of the forest is naturally 'feathered'. Straight upper edges of forests arise only where fences to exclude grazing animals are constructed insensitively.

As trees approach their altitudinal limits, their growth is severely retarded by long periods of low temperature. Pruning by icy winds makes their branches grow outwards, rather than upwards. The result is low-growing, crooked trees. Their twisted timber is of little value to traditional forest industries, so foresters avoid planting trees here.

Even at lower altitudes, the forest does not consist solely of tall forest trees like sessile oak (Quercus petraea), downy birch (Betula pubescens) and Scots pine (Pinus sylvestris). Mixed with them are lower-growing, more shrubby species, such as rowan (Sorbus aucuparia), juniper (Juniperus communis) and willows ( Salix species). As the high forest trees begin to decline towards their altitudinal limit, these smaller species become more prevalent and eventually replace them completely at altitudes above which the forest trees can no longer survive. Some shrubby species are high altitude specialists, although several of these are now rare in Scotland.

Montane Heath

Montane Scrub

Forest

There is, therefore, a zonation up the hillside (see figure 1 below), although the gradation from one zone to the next is gradual, with indistinct boundaries. The forest zone is the extensive area at low altitude in which trees grow tall and upright, and generally produce good timber.

At a particular altitude (which will vary with climate and exposure), trees can no longer maintain this upright growth form. That marks the Timberline. Some trees may survive for a hundred metres or more above this altitude, but they grow progressively shorter and more twisted.

Continental botanists call this zone ‘krummholz’, which means literally 'twisted wood'. Then an altitude is reached at which the climate becomes so severe that even krummholz can no longer survive. This is the treeline.

Specialist montane scrub species

Dwarf juniper (Juniperus communis nana)
Dwarf birch (Betula nana) *
Montane willows:-
  • Mountain willow ( I Salix arbuscula /I ) *
  • Dwarf willow ( I S. herbacea /I )
  • Woolly willow ( I S. lanata /I ) **
  • Downy willow ( I S. lapponum /I ) *
  • Dark-leaved willow ( I S. myrsinifolia /I )
  • Whortle-leaved willow ( I S. myrsinites /I )*
  • Tea-leaved willow ( I S. phylicifolia /I )
  • Net-leaved willow ( I S. reticulata /I ) *
  • **rare ("Red Data Book") species
    * scarce species

Figure 1: An idealised zonation up a mountain as the trees of the forest zone are replaced by low-growing scrub including stunted, twisted plants of Scots pine and then by dwarf shrubs on the high tops. A slightly different terminology is used on the continent (right hand column).

Tree Species (mainly Scots pine and birch)

Montane scrub species (mainly juniper and willows); In Scotland these can occur throughout the range, becoming patchily above the natural treeline.

How high is the treeline?

The altitude at which trees die out varies between species (Scots pines, for example, will grow higher up the hill than oaks), and with climate (the treeline is considerably lower in the more exposed north-west and on the islands). It was also higher during milder climatic periods in the past. Today, Scots pine grows as high up the hills as any forest tree. At Creag Fhiaclach, the pine treeline is almost 650 metres (2,100 feet) above sea level, although the timberline is 150 metres.

Above the treeline, montane scrub species may continue some distance further up the mountainside, but eventually they too die out, at the scrubline. Above this, only lowgrowing dwarf shrubs grow in montane heath, although on the most exposed hills even these shrubs eventually give way on the windswept tops to a zone of mosses, lichens and sedges.

The tree and tall plant community between the timberline and the scrubline is the montane scrub zone to which this booklet is dedicated.