Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 347

ciduous larch ( Larix spp.) is represented by several species in North America and Euras-
ia. Juniper ( Juniperus communis ) is also found in most northern hemisphere mountains,
but usually grows as a scattered and low shrub (Wardle 1974).
Broadleaf deciduous trees form the timberline in some areas, especially in Scand-
inavia, eastern Asia, and parts of the Himalaya (Troll 1973). The dominant genus is
birch, but other genera such as aspen, alder, and beech also occur. The upper treeline
occurrence of broadleaf genera usually occurs in the absence of competing conifers.
Broadleaf deciduous trees such as willow, birch, alder, and maple may also be present
near timberline in areas experiencing slope instability and avalanches. These trees are
well adapted for unstable slopes because of their ability to resprout from their root sys-
tems and because their flexible stems resist breakage by moving snow.
Timberline areas in tropical and southern hemisphere mountains are typically smal-
ler but more floristically diverse than in the northern hemisphere (Wardle 1974). Tim-
berline occurs along the axis of the Andes in South America, but is limited to the tropical
mountains in Africa and exists as scattered enclaves in Australasia, with the exception
of New Zealand. Broadleaf evergreen species, particularly Nothofagus, dominate tim-
berline throughout the southern hemisphere. The timberline species of Nothofagus are
generally evergreen, but can also occur in deciduous form in the drier and more contin-
ental areas of Patagonia, Tasmania, and New Zealand. Coniferous species of Araucaria,
Podocarpus, Libocedrus, and Papuacedrus may also occur at timberline but are subdom-
inant to Nothofagus. In Australia, upper timberline is dominated almost entirely by the
snow gum ( Eucalyptus niphophila ) (Costin 1959).
Members of the heath family (Ericaceae) dominate the timberline vegetation of many
tropical mountains, forming an “ericaceous belt” (Hedberg 1951; Troll 1968; Wesche et
al. 2000). The ecological significance of Ericaceae in tropical mountains is unknown, but
many species are fire resistant and able to reproduce quickly following fire and other
types of disturbance (Sleumer 1965; Wesche et al. 2000). Ericaceous species typically
grow as evergreen shrubs and trees with tough and leathery leaves. Dominant genera
include Vaccinium, Rododendron, Befaria, Pernettya, Erica, and Philippia. In the trop-
ical Andes, a rosaceous genus, Polylepis, grows as shrubby trees on steep rocky slopes
above the cloud forest. Polylepis tomentella grows to >4,800 m (15,800 ft), the highest
elevation of tree growth in the world (Hoch and Körner 2005).
In the high mountains of tropical Africa, ericaceous woodlands of Erica and Philippia
grow to elevations of 3,500-4,000 m (11,500-13,200 ft). Tussock grasslands occur
above this, along with giant Senecios and Lobelias. Equivalent plant forms grow in
the high mountain grasslands of the tropical Andes ( Espeletia spp.), as well as on the
highest reaches of Hawai'i ( Argyroxiphiurn spp.). The presence of similar life forms in
these widely separated areas is generally interpreted as representing convergent evol-
ution under similar conditions. Although they grow to heights of several meters and are
tree-like, these plants are tall, columnar, massive, and almost unbranched herbs. The
confusion as to whether to classify them as trees or shrubs has made the delimitation of
timberline in these areas somewhat problematic (Hedberg 1951; Troll 1973).
Timberline Patterns