Altitudinal zonation and timberlines

3.5.1 Alpine timberlines

The polar margin of arctic forests and the upper margin of subalpine forests (i.e. forests on mountainsides just below the treeless, alpine zone) have cold timberlines. In reality these are facets of the same environmental conditions shown in Fig. 3.15a; the alpine timberline becomes lower in altitude away from the equator until it reaches sea level near the poles. The ultimate factor determining timberlines is decreasing temperature with altitude and latitude (in the northern hemisphere forest ceases to grow when the average temperature of the warmest month drops below 10 °C). The timberlines of subalpine forests, however, are also modified by higher wind speeds related to increasing altitude and snow accumulation (which offers shelter from the cold and wind). Since timberlines on mountains are thus ecologically more interesting, and are more likely to be seen by most people, these are first considered here (Fig. 3.15b). Every forest margin is bounded by an ecotone, azone oftransition between two ecosystems, in this instance alpine forest and open mountainside. This timberline ecotone above the forest limit is described by European ecologists as the kampfzone (struggle zone). In North America the term timber-line or treeline is applied to the uppermost height at which trees reach at least 13 ft (3 m) at maturity, and is normally where the closed canopy forest ends. Natural treelines are usually sharp. Above this, isolated trees may grow until the tree limit is reached. These trees provide refuge for new seedlings and the i

Closed forest

Increasing susceptibility to winter desiccation

Closed forest

Increasing susceptibility to winter desiccation

Timberline ecotone (Kampfzone)

Figure 3.15 Representation of the susceptibility of trees to winter-desiccation on a mountain slope. In both (a) and (b) risk of damage begins at altitude A; above this height conditions become increasingly extreme and the growing season is shortened so there is less time for needles to form and develop cuticular protection adequate for drought resistance. Up to the forest limit a closed stand can gradually develop by natural regeneration. Within the forest winter desiccation is much less severe than on a deforested slope, but above the sharp timberline B, conditions rapidly deteriorate and trees of normal stature occur only on unusually favourable sites. C, the krummholz limit, is determined by the resistance to desiccation of the trees growing at the highest point; all trees near to this point are stunted and deformed. In diagram (a) the arrows above and below B and C indicate the height ranges within which these two limits are found after a series of warm (upward movement) or of cold (downward movement) seasons. The trees shown in diagram (b) of the European Alps are Norway spruce Picea abies and arolla pine Pinus cembra, of which the latter has its lethal limit for winter desiccation at the higher altitude. At Obergurgl, Austria, the timberline for arolla pine is at 2070 m. (Modified from Tranquillini, 1979. Redrawn version from Packham et al., 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall, Fig. 4.13. With kind permission of Springer Science and Business Media.)

rooting of branches sagging to the ground (layering) often resulting in oval patches extending away from the prevailing wind. As trees on the windward side are gradually killed by cold and desiccation of winter winds (causing branches that project above the winter snowline to have a strongly flagged

Figure 3.16 Subalpine fir Abies lasiocarpa forming krummholz in the Canadian Rocky Mountains. The upper part has flagged branches that grow only on the protected leeward side; branches on the windward side are killed by dry, cold desiccating winds which in winter carry ice spicules which wear away the needles' protective wax layer. The skirt of healthy branches around the base are covered by snow in winter and so escape wind and excessive cold. (Photograph by Peter A. Thomas.)

appearance), and new trees establish on the lee-side, these clumps gradually move across the landscape over centuries, literally blown by the wind. The trees at the krummholz ('bent or crooked wood' limit at the top of the kampfzone) are very contorted, often forming multi-stemmed cushions and so low in stature that in winter they are covered and protected by snow. Arno and Hammerly (1984) give a wide-ranging account of the alpine and arctic forest timberlines of North America and elsewhere, providing detailed illustrations of the ways in which tree form is influenced by extreme conditions, particularly cold and wind. Fig. 3.16 illustrates the extreme deformations suffered by trees living high on exposed mountains. Though environmental influences probably predominate, genetic factors may also be involved in this squat tree habit. Not all timberlines are marked by krummholz; elfin woods maintain their closed structure even as the individual trees are dwarfed by poor growing conditions. Good examples of these occur at 40 °S in the Argentine Andes where the southern beech Nothofagus pumilio becomes more and more stunted as it approaches the timberline, but even here forms dense stands with an almost closed front against the alpine level (Ellenberg, 1988).

Vegetational zones and timberlines are usually at higher elevations in large mountain masses than on isolated summits. It may be that wind velocities are lower or that snowfall is greater in such areas as the Rocky Mountains or the European Alps; either would result in more soil water being available for summer growth and less winter damage. As Fig. 3.15a indicates, winter desiccation is much less severe within the forest than on a deforested slope. Conditions within a forest that stretches well up a mountain from a much lower starting point vary greatly, and usually result in an altitudinal zonation in which different tree species dominate particular elevational zones as well as changing their form in response to environmental conditions. Forests of the northeastern USA up to 750 m altitude are usually of the northern hardwood type, dominated by sugar maple Acer saccharum, American beech Fagus grandifolia, yellow birch Betula lutea and sometimes eastern hemlock Tsuga canadensis. Red spruce Picea rubens is increasingly important with increasing altitude and above 850 m the forest is mainly of this species and balsam fir Abies balsamea. Balsam fir becomes ever more important ascending the mountain on which the upper limit for forest growth is in the 1350-1700 m range, with the topmost individuals dwarfed to prostrate or shrubby krummholz forms. Small pockets of shrubs or shrubby trees occur occasionally in protected places in the alpine tundra above the kampfzone. Upper forests in this region frequently show wave regeneration in which mature trees subject to the influence of the prevailing wind die off at the front of the wave (see Section 9.3.2). Other types of disturbance, such as very heavy felling or severe spruce budworm Choristoneura fumeriferana outbreaks, which affect both spruce and fir, frequently give rise to regeneration forests in which the proportion of fir is higher than before.

The lower zones of the European Alps and other mountainous regions are frequently dominated by broadleaved trees with conifers occupying the upper forest zones. High altitude forests in parts of the Austrian Tyrol appear to have been felled in the Middle Ages to provide timber for mining (Tranquillini, 1979). Though the forest has gradually spread up the slopes the kampfzone is still unusually wide; in regions not so disturbed it remains very abrupt as in the Andean example quoted earlier.

In Europe the number and area of true old-growth montane forests are decidedly limited, so the investigations made by Piovesan et al. (2005) into the remnants of beech Fagus sylvatica forest growing near the tree line at 1600-1850 m in the central Apennines, Italy, are of especial interest. These authors used a combination of historical, structural and dendrochronological (i.e. tree ring) approaches in attempts to answer the following important questions. What are the structural attributes and the history of old-growth Fagus forest in Mediterranean montane environments? Which processes determine their structural organization? Are these forests stable in time and how does spatial scale affect our assessment of stability? How do these forests compare with other old-growth forests? The four patches of old-growth beechwood concerned, which escaped logging after World War II, were photographed from the air in 1945, 1954, 1985 and 1994, while the structure of the old-growth region of the forest was investigated using 18 circular and 2 rectangular plots. Living and dead components within this region were both within the range expected for old-growth forests of temperate biomes. Widths of annual rings (see Sections 1.3.2 and 10.1.2) observed in dendrochronological analyses using cores from 32 dominant or co-dominant trees revealed the roles of disturbance, competition and climate in structuring the forest. The identification of a persistent Fagus community in which gap-phase regeneration (see Section 9.2.2) has led to a single-species multi-aged stand at spatial scales of a few hectares, is of particular importance as far as the long-term conservation of such forests is concerned.

Timberlines will certainly move vertically as climate changes with time. The presence of 'fossil stands' of bristlecone pine Pinus aristata (the longest-lived of all organisms - Section 1.3.2) found well above present upper bristlecone timberlines in the White Mountains east of the Sierra Nevada of southern California, indicate that during a warmer climatic period from 2000-4000 years ago the upper margin of the forest was at least 300 m higher than it is now.

As a mountain is ascended conditions become more variable which in itself can be a distinct problem. In exposed parts of the northern Rocky Mountains seedlings of white spruce Picea glauca are killed by stem girdling caused by a few hours of high temperature at ground level, though at night temperatures drop sharply. Partial shading prevents most heat deaths in seedlings of this tree and drought is then the most likely cause of mortality, especially as growth at high altitudes is so slow that seedlings take several seasons to form tap roots long enough to enable them to survive a drought (see Box 3.2 for further discussion on survival at the timberline). Tree roots at the timberline show intensive development of mycorrhizas (Section 5.4.1). Almost all short roots of arolla pine Pinus cembra in central European timberlines are mycorrhizal, and mycorrhizal fungi have evolved high-altitude strains adapted to the high daytime temperatures encountered. Species as diverse as Nothofagus solandri, Eucalyptus pauciflora, Picea engelmannii, Pinus contorta, P. flexilis and P. hartwegii will establish near timberlines only if mycorrhizas are present.

Disturbance caused by fire can locally depress the level of upper timberlines or raise that of the lower timberline, often operating in conjunction with winds that remove snow needed to protect tree seedlings in a harsh continental

Box 3.2 Influence of altitude on forest dynamics in northern Sweden

Figure 3.17 illustrates the influence of increasing altitude on the population dynamics of trees growing in the Vallibacken Forest, northern Sweden. This forest has storm gaps of a similar type but smaller size than those of the boreonemoral forest of Fiby urskog (see Section 2.4.2). Cumulative age distributions in two forest plots are shown for Norway spruce and downy birch as a semi-logarithmic diagram with 10-year age classes. Each cumulative number curve was created by cumulatively adding up the age of all the trees of the species concerned within the plot. The curve commences with the number of trees in the oldest age class, the numbers of trees in successively younger age classes were then added in turn until the youngest age class (0-10 years) was reached. Provided there is at least one good seed year with acceptable germinability during each 10-year period, and the mortality in each age class is constant, such curves can be accepted as

What Altitudinal Zonation

Figure 3.17 Cumulative age distributions of all individuals of downy birch Betula pubescens and Norway spruce Picea abies in two forest plots of similar area in the Vallibacken Forest, North Sweden with the vertical axis on a log scale. For clarity, the two species at plot 3 are shown separately. Plot 2 is at an altitude of 460 m and plot 3 at 335 m. The coniferous forest limit is at 580 m in this area. (From Hytteborn et al., 1987. Vegetatio 72, Fig. 5. With kind permission of Springer Science and Business Media.)

Figure 3.17 Cumulative age distributions of all individuals of downy birch Betula pubescens and Norway spruce Picea abies in two forest plots of similar area in the Vallibacken Forest, North Sweden with the vertical axis on a log scale. For clarity, the two species at plot 3 are shown separately. Plot 2 is at an altitude of 460 m and plot 3 at 335 m. The coniferous forest limit is at 580 m in this area. (From Hytteborn et al., 1987. Vegetatio 72, Fig. 5. With kind permission of Springer Science and Business Media.)

static survivorship curves (Deevey, 1947) which convey an impression of how a typical generation of young saplings would fare over time. This is true of plot 3, of which 140 m2 was a storm gap and the remaining 260 m2 were covered by tree canopy. The mortality of both species in this lower plot is initially high, but after 70 years birch reaches a plateau that continues to 110 years. After initial high mortality in the very first age classes spruce died off more slowly. Its mortality rate increased later on, but not as fast as birch in the same higher age classes. The form of this curve reflects the ability of spruce to survive as dwarf trees, with poorly developed leading shoots and well-formed lateral branches. The survivorship curve for spruce in a plot from Fiby urskog containing storm gaps showed this effect even more clearly (Hytteborn and Packham, 1987).

There was no evidence of typical storm gaps in the considerably higher plot 2 or its surroundings, though the plot was open with a crown cover of only 15%. The cumulative age distribution of Norway spruce here cannot be interpreted as a survivorship curve because the number of cones produced at this altitude and latitude (67 °N) is both low and irregular, while mature seeds are produced in any quantity only in those rare years when the mean temperature between June and September exceeds 10 °C. Thus in plot 2 spruce growth and regeneration are dominated by environmental factors, while in plot 3 the biotic influence of the mature trees is overriding.

climate. Such a case developed over a century ago at 2290 m in the upper Larch Valley, Banff National Park, Canada. After fire levelled a stand of tall alpine larch Larix lyalli the area reverted to tundra; so far there has been only a slow re-invasion of krummholz conifers.

Forests can also have natural lower timberlines where they usually give way to dry grassland, sage-brush, tall shrubs or scrub oaks. This boundary is usually the result of inadequate moisture and termed a drought-caused timber-line. The elevational distance between this and the alpine or cold timberline is only 600 m on the semi-arid Lost River Range in Idaho, USA. Double timber-lines are found in western USA where some of the ranges are so dry that only a few species of drought-tolerant trees can survive, and then only within narrow elevational bands. When the elevational distributions of these trees do not coincide two separate forest belts occur; the lowest is typically of pinyon pine and juniper (Pinus edulis and most commonly the Utah juniper Juniperus osteosperma), commonly referred to as PJ. This is separated by a belt oftreeless sage-brush above which is a narrow zone of bristlecone and limber pine Pinus flexilis extending from about 2740 to 3500 m.

In Iceland and the British Isles extensive clearing, heavy grazing and the burning of heath which took over forest land as a result of disturbance, have caused the loss of any semblance of a natural timberline. Remnants of subalpine Scots pine Pinus sylvestris forest still remain in Scotland, where the natural limit of this tree is between 610 and 700 m in the Cairngorms, though human activity has generally lowered it to around 500 m. Although British woodlands do not reach such high altitudes as their continental counterparts some, such as Wistman's Wood, which at 380-410 m on Dartmoor is amongst the highest oakwoods in Britain, are of remarkable interest. The Dartmoor oakwoods experience an oceanic or Atlantic climate which is both cool and wet; they are remarkable for their very extensive lower plant communities (Box 1.3). The sprawling and stunted form of the oaks present in Wistman's Wood at the beginning of the twentieth century, though undoubtedly influenced by grazing, was mainly caused by extreme exposure to wind and the very low amount of shading from older trees and other vegetation. This exposure was particularly severe during the 'Little Ice Age' of the seventeenth to mid-nineteenth centuries.

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