Effect of Landscape Mulches on Desert Landscape Microclimates

Abstract

Effects of three landscape mulches: 1) composted ponderosa pine residue; 2) uncomposted shredded landscape tree trimmings; and 3) screened decomposing granite, were compared over the course of 2 years (2004 to 2005) for their ability to modify air and soil landscape microclimates in Phoenix, Arizona, U.S. Temperatures at the surface of the two organic mulches were generally higher during the day and lower at night than at the surface of either decomposing granite or soil without a mulch cover. During nighttime hours, decomposing granite mulch or soil without a mulch cover emitted more long wave radiation than the two organic mulches. Conductive heat transfer through the organic mulches was generally lower than through decomposing granite. Daytime temperatures of soil at 5 and 30 cm (2 and 12 in) depths were generally lower beneath the two organic mulches than under decomposing granite mulch or soil without any landscape mulch cover. Soil covered with organic mulch evaporated less water than soil without mulch. Under desert conditions, the two organic mulches were more effective at moderating heat gain and water loss from soil than decomposing granite mulch because of an increased resistance to heat transfer and evaporation.

Investigating the Role of Vegetation on Mid-Afternoon Microclimates in a Residential Courtyard

“…The shaded sites, cooled by both shade and ET, had the largest VCIs and provided similar cooling effects within the 2m vertical air temperature profile. On warmer days with increased radiation, the shaded garden site provided the largest VCI, possibly because the mulched surface held in moisture from the morning irrigation, which meant water was still available in the afternoon to cool the air through evaporation (Singer & Martin, 2008). In contrast, the shaded lawn may have lost moisture earlier on hot days (when temperatures were above the average of 30.75 °C), as the cooling effect of irrigation over lawns has been found to disappear by noon during elevated heat events (Cowles, 2014).…”