Rock associations, root depth, and temperature tolerances for the “rock live-forever,” Dudleya saxosa, at three elevations in the north-western Sonoran Desert

At three elevations in the north-western Sonoran Desert, Dudleya saxosa (Crassulaceae) occurred predominantly where the ground surface was covered >90% by rocks. Its roots were extremely shallow, with a mean depth of only 46 mm and essentially none below 120 mm, for a north-facing dolomitic rock outcropping at an elevation of 300 m, exposed quartz veins at 825 m (Agave Hill), and a north-facing granitic outcropping at 1395 m. For a mild spring day at Agave Hill, the maximum ground surface temperature of a quartz vein was 11 °C cooler than that of the adjacent bare soil, which was a consequence of the high thermal conductivity coefficient (8.2 W m−1 °C−1) and low shortwave absorptance (0.23) of the quartz. The leaves and the roots of D. saxosa had only modest low-temperature tolerances and acclimation, as is typical of succulent plants. Although its root cortical cells were relatively tolerant of high temperatures, D. saxosa from Agave Hill could not tolerate shoot temperatures above 60 °C, whereas eight other succulent species endemic to the north-western Sonoran Desert can. The average increase in the temperature killing 50% of their shoot cells as the ambient temperature is increased by 10 °C was 2.4 °C for D. saxosa but averaged 5.2 °C for the eight other species. Thus, the relatively poor high-temperature tolerance and acclimation ability of its mesophyll cells may cause D. saxosa to favor cooler north-facing microhabitats, such as when it occurs on granitic outcroppings with their high shortwave absorptance (0.73).