Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 372

Insect pollination is problematic for alpine plants, because low temperatures confine
insect activity to brief periods during the summer (Billings 1974a). Some insect pollin-
ators, such as bumblebees, are active only on sunny days when the air temperature is
10°C (50°F) or higher (Macior 1970). As a result, at altitudes above 4,000 m (13,200 ft)
in the Himalayas, bee-pollinated flowers disappear and are replaced by those pollinated
by other insects (Mani 1962). Hummingbirds (or sunbirds in East Africa) are important
in tropical mountains (Cruden 1972; Carpenter 1976) and, to a limited extent, midlatit-
ude mountains where daytime temperatures exceed a minimum physiological threshold.
The most important pollinators in mid- and high-latitude mountains are bees, flies, but-
terflies, and moths, but some ant species also act as pollinators (Puterbaugh 1998).
The comparatively large flower size and range of colors commonly associated with
flowering alpine plants (relative to their small stems and short stature) may have sev-
eral explanations, but it is at least partially a function of their ability to attract insects
(Fig. 7.11c). An exception to this tendency occurs in New Zealand, where the alpine
flowers are small, flat, and white and yellow in color. These traits appear to be related to
pollination by flies and other small insects in the absence of long-tongued bees in New
Zealand (Heine 1937; Mark and Dickinson 1997). Although most alpine plants are in-
sect pollinated, wind and self-pollination become increasingly important under extreme
environmental conditions. This alternative pollination strategy requires less energy ex-
penditure to support elaborate adaptations among the plants and insects involved in in-
sect pollination (Hocking 1968; Macior 1970; Heinrich and Raven 1972; Hickman 1974)
and helps to compensate for the decreasing number of pollinators as environmental con-
ditions become more extreme. A negative consequence of wind and self-pollination is
less genetic diversity, which may be important for long-term fitness of the species (Bliss
1962), a tendency partially offset by a higher frequency of plants with multiple sets of
chromosomes, or polyploids (Löve and Löve 1967).
Viable seed production following pollination depends largely on environmental con-
ditions. Severe freezes or snowstorms during blooming can reduce seed set or inhibit
germination until conditions become more favorable. Consequently, most alpine tundra
plants have alternative methods of reproduction, the most common being through
rhizomes, root-like stems that extend away from the center of the plant and are capable
of growing new shoots. This process can result in single plants occupying several square
meters, giving the appearance of many individual plants. This adaptation is advantage-
ous where frost activity and mass wasting can lead to the partial destruction of the plant
but leave the remainder to reproduce and spread.
Several other methods of vegetative reproduction, including layering, stolons, apo-
mixis, and vivipary, are prevalent in the alpine tundra. These modes of reproduction
become more prevalent as seed production and germination become less reliable with
increasingly severe environmental conditions (Billings and Mooney 1968). Plants rely-
ing on vegetative reproduction may continue to produce seeds but rely on a variety of
different reproductive strategies.
PLANT PRODUCTIVITY
Despite living under extreme environmental conditions and their reduced stature,
alpine plants are highly productive during the growing season. Above-ground biomass