Woody Plants Beyond The Treeline

Beyond the limits for the survival of forest, woody plants in shrub form exist as viable and even major components of ecosystems. Plants with lignified stems and branches can be found on mountains, moors, coastal heaths (Fig. 9.1), as well as across the tundra to the very north of Greenland and south to remote sub-Antarctic islands. The woody shrub or bush form is highly flexible and can be almost of tree stature as in krummholz pine or diminutive as in some arctic heather species (Figs. 9.2-9.3). Every possible variation exists in the extent of tissue lignification from bushes that are entirely woody, as in the dwarf willows, mountain alders and birches to plants where the shoots are only partly woody. The minimal woody stem condition, where only the roots and the base of the stem are lig-nified (defined botanically as suffrutex), is not discussed here, as the shoots of these plants are essentially herbaceous. The versatility of woody species in both size and habit probably contributes much to their success in marginal areas whether it be in hot or cold deserts or on wind-swept oceanic heaths.

The presence of woody plants, and the canopy they create, whatever their size, alters the temperature,

Fig. 9.2 Versatility of woody shrub form habit. Krummholz pine (Pinus sylvestris) at the treeline at Creag Fhiaclach at 620 m a.s.l. in the Cairngorm Mountains, Scotland.

Fig. 9.3 A marginal woody plant of diminutive size. The clubmoss mountain heather (Cassiope lycopodiodes), native to Alaska, Kamchatka and Japan. (Photo approx. 2 X life size.)

Fig. 9.2 Versatility of woody shrub form habit. Krummholz pine (Pinus sylvestris) at the treeline at Creag Fhiaclach at 620 m a.s.l. in the Cairngorm Mountains, Scotland.

Fig. 9.3 A marginal woody plant of diminutive size. The clubmoss mountain heather (Cassiope lycopodiodes), native to Alaska, Kamchatka and Japan. (Photo approx. 2 X life size.)

light, and evapotranspiration regimes around the leaves in summer, and the degree of exposure to adverse climatic conditions in winter. Branches can be erect, giving the shrub a narrow outline fastigiate), or they can be spreading at wide angles (divaricate). In between these extremes many forms exist. When the branches are procumbent, they can be stoloniferous and spread as a mat over the surface of the ground as in the arctic bearberry (Arctostaphylos alpina; Fig. 9.6),

The abandonment of the single pole form in favour of the many-stemmed bush habit also facilitates regeneration after damage, whether from climatic adversities such as drought or frost or from disturbance by grazing or fire. These properties serve to make woody shrubs long-lived and major components of a wide variety of plant communities.

Highly exposed sites at sea level can support very tenacious heath communities. Although heather (Calluna vulgaris) and crowberry (Empetrum nigrum) may suffer erosion as a result of strong winds they nevertheless manage to survive by slowly migrating in terrace formation before the wind (Fig. 9.4). In more sheltered sites behind islands the natural succession is to dune heath (Fig. 9.5) provided the sand is not rich in seashells and the sea salts are leached out by rain to leave a soil pH value of 6 or less.

Fig. 9.4 Maritime heath on the west coast of Orkney. (Top) Wind terracing with heath ridges eroding on their downhill side and recolonizing the ground on the upper side. (Above left) Detail ofadvance ofheath species onto bare ground. The crowberry (Empetrum nigrum) (bright green) is particularly successful in putting out new roots around edges of turf and spreading across the open ground in areas with high wind exposure. (Above right) Detail oferosion on windward side and ability ofheath to recolonize on the downwind face of the eroding terrace.

Fig. 9.4 Maritime heath on the west coast of Orkney. (Top) Wind terracing with heath ridges eroding on their downhill side and recolonizing the ground on the upper side. (Above left) Detail ofadvance ofheath species onto bare ground. The crowberry (Empetrum nigrum) (bright green) is particularly successful in putting out new roots around edges of turf and spreading across the open ground in areas with high wind exposure. (Above right) Detail oferosion on windward side and ability ofheath to recolonize on the downwind face of the eroding terrace.

Fig. 9.5 Lowland dune heath dominated by bell heather (Erica cinerea — see inset) in north-east Scotland. Active grazing by sheep and cattle control the further spread of Scots pine (Pinus sylvestris).

Woody shrubs can be aggressive invaders, as can be seen in the spread of Rhododendron ponticum in the British Isles and Calluna vulgaris in New Zealand. Asian tamarisks (saltcedars, Tamarix spp.) were first imported into the United States in the nineteenth century as ornamental plants and then used for erosion control. This has now led to the invasion of almost all watercourses and other wetland habitats throughout the south-west, taking over more than one million acres of wetland. In New Zealand heather (Calluna vulgaris) can invade burnt surfaces more rapidly than the native New Zealand species, with the result that it has displaced much of the lower and subalpine natural shrub and tussock vegetation (Wardle, 1991). In western Europe, the removal of much native forest has led to the downhill migration of the heather from its natural lower-alpine habitat to create extensive heathlands where once trees flourished. These many-stemmed woody invaders from the lower-alpine zone have survived 5000 years of fire and grazing, presumably because the conditions under which they evolved above the treeline produced a plant form that was pre-adapted to the burning, grazing and slashing regimes that were imposed on woody plants with the Neolithic development of agriculture.

The ecology of woody plants in relation to climate change can therefore be examined with examples ranging from the north of Greenland (83° N) to the southernmost outpost for woody plants on New Zealand's Southern Oceanic Auckland and Campbell Islands (c. 51° S).

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