Microsite characteristics of Muhlenbergia richardsonis (Trin.) Rydb., an alpine C4 grass from the White Mountains, California

C-4 plants are uncommon in cold environments and do not generally occur in the alpine tundra. In the White Mountains of California, however, the C-4 grass Muhlenbergia richardsonis is common in the alpine zone at 3,300-3,800 m, with the highest population observed at 3,960 m (13,000 feet) above sea level. This is the highest reported C-4 species in North America and is near the world altitude limit for C-4 plants (4,000-4,500 m). Above 3,800 m, M. richardsonis is largely restricted to southern slope aspects, with greatest frequency on southeast-facing slopes. In open tundra, M. richardsonis formed prostrate mats with a mean height of 2.5 cm. Neighboring C-3 grasses were two to three times taller. Because of its short stature, leaf temperature of M. richardsonis was greatly influenced by the boundary layer of the ground, rising over 20degreesC above air temperature in full sun and still air and over 10degreesC above air temperature in full sun and wind velocity of 1-4 m s(-1). Thus, although air temperatures did not exceed 15degreesC, midday leaf temperatures of M. richardsonis were routinely between 25degreesC and 35degreesC, a range favorable to C-4 photosynthesis. At night, leaf temperature of M. richardsonis was often 5-12degreesC below air temperature, resulting in regular exposure to subzero temperatures and frosting of the leaves. No visible injury was associated with exposure to freezing night temperatures. The presence of M. richardsonis in the alpine zone demonstrates that C-4 plants can tolerate extreme cold during the growing season. The localization to microsites where leaf temperatures can exceed 25degreesC during the day, however, indicates that even when cold tolerant, C-4 plants still require periods of high leaf temperature to remain competitive with C-3 species. In this regard, the prostrate growth form of M. richardsonis compensates for the alpine climate by allowing sufficient heating of the leaf canopy during the day.