Sage, R.F.*; Sage, T.L.: Microsites Characteristics of Muhlenbergia richardsonis, an alpine C4 Plant.

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 found that the C4 grass Muhlenbergia richardsonis commonly occurs in the alpine zone at 3300 to 3800 m, with the highest population observed 3950 m above sea level. 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 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. Muhlenbergia 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 20�C above air temperature in full sun and still air, and 10�C above air temperature in full sun and wind. As a result, midday leaf temperatures in M. richardsonis were routinely observed to be 25�C to 35�C, conditions favorable to C4 photosynthesis. At night, the high boundary layer in the Muhlenbergia mats allows for 5� to 15�C 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.