in common gardens under natural conditions and in the laboratory with different levels of moisture.
Metabolic heat rate and respiratory rate were measured weekly. Both species grew best in early spring but
remained green and metabolically active throughout the summer. Small burnet was much more drought
tolerant than alfalfa. Differences among populations and among cultivers were detected in both common
garden and laboratory conditions [36].
Cryptogamic crusts in deserts all over the world are communities composed of lichens, cyanobacte-
ria, algae, mosses, and fungi found on or near the soil surface. Crusts are very susceptible to physical dis-
turbance but if intact appear to play a role in providing nutrients, especially nitrogen, to higher plants [37].
Crusts, if they are present, also appear to increase the water holding capacity of the soil following infre-
quent precipitation events. Using calorimetric measurements of metabolism, we have learned that expo-
sure of crusts to various levels of relative humidity had no effect, but liquid water caused immediate elon-
gation of algal filaments. The temperature optimum for metabolism is about 15°C, indicating growth of
cryptogamic crusts under cool, moist conditions.
C. Salt
Many desert playas are covered with water in the early spring. As the weather becomes warmer and drier,
water evaporates, increasing the salt content of the soil from 7000 to almost 16,000 mM NaCl. Changes
in respiratory metabolism during the growing season of four halophytes characteristic of cold desert
playas were followed using calorimetry. In order of decreasing salt tolerance and metabolic activity, the
species examined were the forbs Salicornia rubraandS. utahensis, the grass Distichlis spicata, and the
shrubAllenrolfea occidentalis. These species are all well adapted to the environment in which they are
found. Highest metabolic activity was found during May and June with lowest activity during the hot, dry
month of August (Figure 3) [38].
Salicornia utahensiswas grown in growth chambers in concentrations of NaCl ranging from 0 to 1.8
M. Metabolic rates were measured at temperatures from 5 to 45°C. Predicted growth was best at salt con-
centrations greater than 400 mM NaCl. The best growth was at 1000 mM NaCl but only at temperatures
above 40°C (Figure 4).
8 SMITH ET AL.Figure 2 Metabolic heat rate (q,) and respiration rate (455RCO2,) for winterfat (Eurotia lanata{Pursh}
Moq.) seedlings from Pinebluffs, Wyoming measured at different temperatures. The data indicate optimal
growth between 0 and 15°C and stress at temperatures below 0°C and above 18°C.