Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 356

continental regions. However, in areas outside the tropics, it is difficult to separate the
effects of clouds from those of wind, precipitation (especially snow), and temperature.
BIOTIC FACTORS
Species interaction among high-elevation trees is a relatively neglected but important
factor in determining the composition and spatial pattern of upper treelines. Dullinger
et al. (2005), for example, note that the presence of pine cover in the Austrian Alps has
a negative effect on the recruitment (germination success and growth into larger size
classes) of neighboring spruce and larch. This and similar competitive interactions in-
fluence both the local dominance and future composition of these treeline communities.
Other research by Maher et al. (2005) in the Rocky Mountains found that tree and herb-
aceous plant cover increased seedling survival and improved photosynthesis rates relat-
ive to seedlings growing in open conditions, except during periods of low-water stress.
These results suggest that tree seedlings benefit in a facilitative way similar to other
types of plants growing under alpine and other extreme environmental conditions (e.g.,
Callaway et al. 2002; Callaway 2007) and are consistent with tree recruitment patterns
occurring above treeline (e.g., Cuevas 2000) and within subalpine forests (Callaway
1998).
Seed, seedling, and tree predation by animals can also limit tree growth at high el-
evations, as noted at some locations in Switzerland after the successful reintroduction
of the ibex ( Capra ibex ) to the Alps in the 1920s (Holtmeier 1973). Insects and diseases
can also cause considerable destruction of trees at timberline. Bark beetle ( Dendro-
cotonus engelmannii ) and spruce budworm ( Choristoneura occidentalis ) are both ser-
ious pests at timber-line in the Rocky Mountains (Johnson and Denton 1975; Schmid
and Frye 1977), as is white pine blister rust caused by the exotic pathogen Cronartium
ribicola (Tomback and Resler 2007; Resler and Tomback 2008; Smith et al. 2011). In
the Alps and Scandinavian mountains, the Oporinia autumnata and Zeiraphera grisena
moths periodically damage high-altitude forests (Holtmeier 1973). Insects also suppress
growth rates of eucalyptus in the subalpine forests of the Snowy Mountains, Australia
(Morrow and LaMarche 1978). The various blights, rusts, and other pathogens that live
in snow also cause considerable damage to timberline species (Cooke 1955; Habeck
1969). In the Alps, needle rust ( Chrysomyxa rhododendri ) infects spruce, inhibiting pho-
tosynthesis and consequently reducing needle biomass (Bauer et al. 2000) and, presum-
ably, tree growth.
Temperature
Low temperature has long been considered the primary cause of timberline. This con-
clusion, however, is largely based on the coincidence of treeless conditions in arctic
and alpine climates, rather than on its demonstration through experimentation. Non-
etheless, there seems to be a strong correlation between temperature and timberline
(Körner and Paulsen 2004), especially in mesic (moderately moist) regions (see Lloyd
1997).
Low winter temperatures are less critical than summer temperatures, which control
primary metabolic and growth processes. The exact mechanism whereby low temper-