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

Abstract

C plants are uncommon in cold environments and do not generally occur in the alpine tundra. In the White Mountains of California, however, the C 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 species in North America and is near the world altitude limit for C 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 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 20°C above air temperature in full sun and still air and over 10°C above air temperature in full sun and wind velocity of 1-4 m s. Thus, although air temperatures did not exceed 15°C, midday leaf temperatures of M. richardsonis were routinely between 25°C and 35°C, a range favorable to C photosynthesis. At night, leaf temperature of M. richardsonis was often 5-12°C 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 plants can tolerate extreme cold during the growing season. The localization to microsites where leaf temperatures can exceed 25°C during the day, however, indicates that even when cold tolerant, C plants still require periods of high leaf temperature to remain competitive with C 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.

Inferences about winter temperatures and summer rains from the late Quaternary record of C4 perennial grasses and C3 desert shrubs in the northern Chihuahuan Desert

“…Most of these studies have focused on low temperatures during the growing season, but our results suggest that winter temperatures may be equally important in limiting C 4 grasses. Some C 4 species can tolerate cold temperatures when dormant (Schwarz and Reaney, 1989) and during the growing season (Sage and Sage, 2002), but a late hard freeze after germination may limit establishment by impacting the chilling-susceptible seedlings in many species. For example, Zea mays exhibits loss of photosynthetic capacity due to photoinhibition during periods of high light intensity and low temperatures.…”

Freezing tolerance in grasses along an altitudinal gradient in the Venezuelan Andes

“…Schwarz and Redmann (1987) and Sage and Sage (2002) suggest further studies for a better understanding of resistance mechanisms of grass species distributed in low temperature environments. Pearcy (2001) and Márquez (2002) also highlight the importance of understanding freezing in plants as a distributional pattern constraint.…”

C 4 grasses in boreal fens: their occurrence in relation to microsite characteristics

“…Some C 4 species do exploit high latitude or high elevation sites, however, indicating that C 4 photosynthesis is not fundamentally excluded from such areas (Long et al 1975;Schwarz and Redmann 1988;Sage and Sage 2002). One possible explanation for their occurrence in colder locations is that their greater water and nitrogen use efficiency (WUE and NUE, respectively) relative to cooccurring C 3 species may offset the deleterious effects of low temperatures.…”

“…Schwarz and Redmann (1988) suggested that boreal C 4 species are limited to microsites warmed by light or geothermal activity, perhaps enabling them to avoid the problems associated with low temperatures. In the alpine zone of the White Mountains of California, Muhlenbergia richardsonis is restricted to the warmest slope-aspects, and takes advantage of the ground boundary layer to attain leaf temperatures that are favourable to C 4 photosynthesis (Sage and Sage 2002). In cool climates, specific microsite characteristics may thus enable the C 4 pathway to persist, despite the fundamental limitation imposed by their low quantum yield and low Rubisco content.…”