Document: ROW-3-99-143

Microsite characteristics of the C4 grass, Muhlenbergia richardsonis, in the alpine zone of the White Mountains, California.

SAGE, R.F.* and T.L.SAGE

University of Toronto, ON M5S3B2 Canada ¹

Abstract:
C₄ plants are uncommon in cold environments at high latitude and altitude, and are not generally thought to occur in the alpine tundra. In the White Mountains of California, we have observed that the C₄ grass, Muhlenbergia richardsonis, commonly occurs in the alpine zone at 3300 to 3800 m, with the highest population observed at 3950 m above sea level (13,000 feet). This is the highest reported C₄ plant population in North America, and is near the world altitude limits (4000 to 4200 m) reported for C₄ plants in central Asia and the Andes. At its highest distribution (3600 to 3950 m), M. richardsonis is restricted to southeast and southwest facing slopes, with greatest frequency on southeast faces. M. richardsonis plants form low mats with mean height of 2.7 cm, while co-occurring C₃ grasses form bunches that are on average two to three times taller. As a result, M. richardsonis leaves are more affected by the boundary layer of the soil than nearby C₃ grasses. Temperature of M. richardsonis leaves was over 30C above air temperature in full sun and still air, and 20C above air temperature in full sun and wind. As a result, midday leaf temperatures in M. richardsonis were routinely observed to be 25C to 35C, conditions favorable to C₄ photosynthesis. At night, the high boundary layer in the Muhlenbergia mats allows for 5 to 15C reduction of leaf temperature below air temperature, resulting in regular frosting of leaves. These results indicate that M. richardsonis requires daytime heating for ecological success, and has evolved the capacity for freezing tolerance at night and early in the day. The ability to withstand regular freezing suggests that minimum daily temperature alone cannot account for the general absence of the C₄ pathway in cold environments. Instead, a feature associated with the daytime heat requirement of the C₄ pathway may be more important.

Keywords: Muhlenbergia richardsonis

This abstract is being presented at: 2:30 PM in session:
Oral Session #33: Plant Demography.