Origins and Ecology of the Sierran Alpine Flora and Vegetation

Abstract

The alpine flora of the Sierra Nevada has developed relatively recently and largely in situ from western American sources. The Sierra thus provides a good site for an attempt to answer the question: "How does an alpine flora originate?" The primary study area was a transect from the desert near Bishop, California (1,400 m), to Piute Pass in the Sierra Nevada (3,540 m). Upward along the transect the vegetational gradient is Ephedra nevadensis—Tetradymia spinosa desert shrub, Pinus monophylla—Artemisia tridentata open woodland, Pinus jeffreyi open forest, Pinus murrayana forest, Pinus albicaulis—subalpine herbaceous vegetation, and scattered alpine communities. Only 19% of the alpine species at Piute Pass occur in the Arctic, whereas 38% are held in common with the Rocky Mountains. Species endemic to the Sierra (17%) are in genera predominantly from the Californian or Great Basin floras of lower elevations. A number of species have populations in the desert and also at high elevations near the alpine zone. Most of the alpine flora consists of perennials, but several annual species are also present. Annuals are rare in other arctic and alpine floras. Environmental monitoring stations were maintained at several locations along the transect during two summers. Air temperature at 5 cm above the ground decreased with increasing elevation at a rate of 0.74°C/100 m. Summer precipitation increased nonlinearly with elevation at a rate of 0.16—0.82 cm/100 m. Long—term annual average precipitation for Bishop is 14.6 cm; the annual precipitation for Piute Pass estimated from this study is greater than 78 cm with a strong winter maximum in the form of snow. Soil moisture during the summer is low for all sites. Strong vegetation patterning occurs in both alpine and desert areas along drainageways from snowbanks or perennial streams. Solar, sky, and net radiation at 1 m above the soil are greater for alpine than for desert areas. Air and plant—tissue temperatures near the surface of the alpine soil are higher than those in most other alpine areas. Laboratory experiments showed several physiological responses to be characteristic of Sierran alpine populations. These may be important in plant evolution and migration into an alpine habitat. (1) Germination of seeds from alpine plants occurred maximally between 20° and 30°C. Constitutive dormancy is a minor factor; the low winter temperatures of the alpine zone appear to operate as an exogenous dormancy control. Desert and low—elevation species of this area are predominantly winter—germinators and have maximum germination at low temperatures. (2) Mature alpine plants have strong dormancy control by short photoperiod; dormancy in lower elevation populations may be induced by either short or long photoperiods. (3) Temperatures of the upper photosynthetic compensation point and maximum net photosynthesis are lower in plants of alpine species. (4) Low temperature regimes cause plants to shift dark respiration higher rates when compared with plants of the same species from high temperature regimes; this change appears to have both genotypic and phenotypic causes. (5) Considerable acclimation of dark respiration to a change in temperature can occur in as little as 8—10 hr, beginning 1—2 hr after the change. The speed of acclimation increases with increasing temperature and is genetically based. Populations from higher elevations have faster acclimation rates than those from lower elevations. This characteristic could allow alpine plants to adapt rapidly to changing weather conditions. (6) Acclimation of dark respiration does not appear to be related to changes in leaf water potential, diffusion resistance, or isozymes of three dehydrogenases. It is related to changes in the rate of mitochondrial oxidation (7) Translocation of starch from chloroplasts at low temperatures was impaired in a desert species, but was maintained in an alpine species. Although a number of physiological processes in alpine plants are adapted to low temperatures, there is no indication that any single process, or adaptation to temperature alone, is responsible for the evolution and success of an alpine species. Efficient utilization of a short, clod growing season may be the most important selective characteristic in the origin of an alpine flora. Some genera of lower elevations near the Sierra are already preadapted, in their winter and spring growth patterns, to such a low—temperature regime and may have provided a part of the present Sierran alpine gene pool.