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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...

Experiments were conducted to examine whether leaf adaptation to light in Fragaria virginiana (Rosaceae) was determined by peak photon‐flux density or by total quanta received during the day. Leaf structure and apparent photosynthesis rates were similar under environments where total energy received was the same even though peak photon‐flux density was different. When peak photon‐flux density was held constant and total quanta varied, significant differences were noted in apparent photosynthesis, leaf thickness, specific leaf weight, mesophyll cell volume, and Ames/A ratio. High total quanta produced high‐light or sun‐type leaves even at relatively low peak intensities. Thus, total light energy received during the day has a greater influence on leaf adaptation to light than does peak photon‐flux density.

Apparent photosynthesis and dark respiration were followed during development in four light environments of leaves of Fragari igniaa Duchesne. Leaf expansion was completed more rapidly the higher the growth photon flux density and leaves senesced more quickly in high light. Maximum photosynthetic capacity coincided with the completion of blade expansion and declined quickly thereafter. Leaves were transferred from high to low and low to high photon flux densities at several stages during expansion. Leaf Experimental Treatments. Two sets of experiments were performed. In the first, the effects of irradiance and leaf age on photosynthetic performance were studied. Leaves were individually tagged as they appeared in each of the four light treatments. Length and width of leaflets were measured to the nearest 0.5 mm at 1-to 3-day intervals until expansion was essentially complete and at longer intervals thereafter. Longevity was estimated by allowing some leaves to die naturally. Apparent photosynthesis and dark respiration were measured on leaves of different ages.A second experiment examined adaptive capacity as a function of leaf age at the highest and lowest growth chamber light levels. Leaves which appeared after the plants had been in the high or low light levels for at least I week were individually tagged. A plant was transferred to the contrasting light level when its tagged leaf was at one of three stages of development (Fig.