Mat muhly (Muhlenbergia richardsonis), at almost 4000 meters in eastern California's White Mountains, holds the highest elevation record for a C4 species in North America. Archival herbaria and climate data indicate this species has been moving upslope in the last 10-20 years, apparently as a result of recent climate change. A comparative study of foliar characteristics was done along a 3000-3800 meter elevational gradient with M. richardsonis and three co-occurring C3 graminoid species. In all species, we examined elevation-dependent variation in stomatal density (SD), leaf nitrogen content (%N), and leaf tissue d 13C. These foliar traits generally vary with elevation in C3 species but have not been examined across elevation in C4 species. Leaf SD, %N, and d 13C all increased with elevation as expected in the C3 plants. Leaf SD and d 13C were unchanging with elevation in M. richardsonis but %N increased with elevation as it did in the C3 species. We interpret the C3/C4 contrasts in SD and d 13C data to mean that C4 photosynthesis is not limited by the low partial pressures of atmospheric CO2 (pCO2) present at high elevations. C3 plants are limited by low pCO2 at high elevation and shifts in SD and d 13C reflect compensatory responses to cope with the thinning atmosphere. The similar trend in %N for C3 and C4 tissues suggests this is a response to altitudinal variables other than pCO2 (e.g. temperature or precipitation regimes) that affects C3 and C4 plants similarly. C4 plants are rare at high elevations for reasons that are not well understood. This study indicates C4 plants are not excluded from high elevations by low pCO2. Rather, in this respect, C4 plants appear to be pre-adapted to the thin atmospheres of alpine habitats.
