Effectiveness of hybrid South American mesquite in modifying landscape microclimates in an arid city

Vegetative cover density has been shown to be negatively correlated with mean air temperatures in residential neighborhoods of arid climates. Further resolution of this relationship was undertaken to elucidate the role of tree shade and ground surface cover type on microclimate within the undercanopy layer during summer and winter seasons in Phoenix, Arizona, USA. Micro-meteorological stations were configured to continuously record diel patterns of air (Ta, 2-m height) and surface temperatures, global radiation, and net radiation in a factorial matrix of open full sun exposure or mature hybrid South American mesquite (Prosopis alba × Prosopis chilensis) tree shade and Bermuda grass (Cynodon dactylon) turfgrass lawn or decomposing granite (DG) surface mulch cover in co-joining residential yardscapes of similar size and tree and shrub frequencies, but dissimilar landscape designs. Data were recorded at 30-min intervals during summer 2010 (Julian days 165 to 179 and 216 to 241) and winter 2011 (Julian days 4 to 20). During summer, mean Ta was generally about 1 to 2°C lower in the turf yardscape than the DG yardscape. In the turf yardscape, pre-dawn Ta was about 1 to 2°C lower in the full sun location than the shaded location underneath the mesquite tree. In the DG yardscape, daytime Ta in full sun location was about 1 to 2°C higher than in the shaded location. During winter, Ta was consistently about 2 to 3°C lower in the turf yardscape than the DG yardscape. These pattern differences were associated with temporal patterns of surface temperatures, net radiation and upward long wave radiation losses and irrigation practices.