Rhizosphere, surface, and air temperature patterns at parking lots in Phoenix, Arizona, U.S

Parking lot landscape surfaces can affect thermal microenvironments where parking lot trees grow. We studied temperatures at parking lots in Phoenix, Arizona, U.S. with expansive asphalt paving surrounding narrow parking lot landscape medians covered with decomposing granite rock (DG) mulch and also within adjoining landscaped areas covered with turfgrass or DG mulch. We recorded temperatures at two Phoenix residential landscapes that were covered with either turfgrass or DG mulch and one remnant Sonoran Desert site in the city that had a bare soil surface. Rhizosphere [30 cm (12 in.) depth] temperatures were seasonally highest under asphalt and lowest under turfgrass, and occurred 4 to 8 hr after highest air temperatures. Rhizosphere temperatures under asphalt as close as 1 m (3.3 ft) from parking lot landscape medians were above 40°C (105°F) long enough to damage any tree roots growing outside of landscape medians. Ground surface temperatures were also highest for asphalt and lowest for turfgrass, but surface temperature patterns more closely followed patterns of insolation intensity rather than air temperature. These data show that supraoptimal rhizosphere temperatures around narrow parking lot landscape medians might be one important reason why parking lot trees in the desert Southwest grow poorly when compared with trees away from asphalt surfaces.