Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 353

majority of tree behavior at timberline (Wardle 1971, 1974; Körner 1998; Körner and
Paulsen 2004). While acknowledging that no single factor can explain local timberlines
(e.g., Körner 2012), a review of some of the more important factors known to limit tree
growth provides insight into the complexity of the problem and possible avenues to its
solution.
SNOW
Snow has both a positive and a negative influence on timberline. Snow enhances tree
survival at timberline by providing protection from low temperatures and high winds,
and it provides moisture during the growing season. Excessive snow, however, can
smother trees or cause physical damage by promoting avalanches or snow creep. Late-
lying snow harbors molds and fungi that attack trees (Stevens and Fox 1991; Körner
1998), reduces the length of the growing season, and influences local competition
for resources with vascular plants and cryptograms (Moir et al. 1999). The depth of
snowpack may also explain why trees grow preferentially along ridges rather than in
valleys in most midlatitude mountains (Shaw 1909) and how ribbon forests develop
(Billings 1969; Hättenschwiler and Smith 1999; Figs. 7.9, 7.10). Regardless of its ecolo-
gical importance in controlling the position of local timberlines, snow is less significant
in many tropical mountain areas, thus eliminating it as a controlling factor of timberline
at the global scale. Even in midlatitude areas where snowpack strongly influences and
is influenced by local conditions (Hiemstra et al. 2002), it is only one of several import-
ant factors operating at timberline.
WIND
Wind velocity generally increases with altitude and produces high levels of both mech-
anical and physiological stress for trees. Wind can cause significant mechanical damage
by felling or causing the breakage of stems or limbs, particularly when they are coated
with rime, ice, or snow. Wind also acts as an abrasive agent by transporting small min-
eral grains and snow and ice crystals. This results in bark and limb erosion and, when
combined with branch breakage on the windward side, gives rise to flagged trees (Fig.
7.9a). Wind desiccation also shapes primary shoots, encouraging lateral branch growth,
flagged branches, krummholz tree forms, and flat tree canopies (Fig. 7.9b) (Cairns
2001). Finally, winter wind damages young shoots and buds by removing their cuticle,
an impermeable waxy surface (Hadley and Smith 1983), causing accelerated moisture
loss when water is largely unavailable because of the freezing temperatures (e.g., Tran-
quillini 1979).
Seedling regeneration on wind-exposed sites is generally low, because timberline
trees rarely produce viable seeds (e.g., Wardle 1968; Caldwell 1970). Seed supply can
be further compromised by poor seed development in many cones. European spruce
( Picea abies ) in the Alps, for example, produce cones at lower-elevation sites every three
to five years, but only once every six to eight years at higher elevations, and once every
nine to eleven years at timberline (Tranquillini 1979). This behavior suggests that many
tree seeds in alpine areas may be wind-transported from lower-elevation forests. Con-