The relationship of homeowner practices and carbon acquisition potential of landscape plants to mesic and xeric designed southwest residential landscapes

Mature landscape plants at eight suburban residential home sites in the greater Phoenix metropolitan area, AZ USA, were chosen for study of carbon acquisition potential (Cap) and irrigation practices in relation to mesic and xeric landscape design type. Residential home sites were of similar age and owned by people of similar socioeconomic standing. A total estimate of landscape plant Cap was made using a trapezoidal integration model of seasonal diurnal and monthly maximum carbon assimilation measurements of dominate woody trees, shrubs, and ground covers at four mesic and four xeric designed residential landscapes for the twelve month period, July 1998 to June 1999. Xeric landscapes consisted of a mixture of 10 species of drip-irrigated trees, shrubs, and ground covers, no turf, and decorative decomposing granite surface mulch. Mesic landscapes consisted of a mixture of seven species of drip irrigated trees, shrubs, and ground covers with sprinkler irrigated turf, and had 2.6 times more canopy cover than xeric landscapes. Home residents controlled irrigation frequencies and durations. Rainfall during the study was 210-mm. Water meters recorded irrigation volume at each landscape. The monthly amounts of water applied to mesic landscape were higher than the amounts applied to xeric landscape in April-July; otherwise, the amounts of water applied to landscapes of either design type were similar. Mean annual estimates of Cap were 111.2+12.6 and 100.9+11.4 mol m-2 yr-1 for mesic and xeric landscapes, respectively. There was no relationship between Cap and irrigation volume for either landscape design type (mesic, P=0.26; xeric, P=0.32). Assimilation efficiency (Ae) was calculated as the ratio of Cap to irrigation water volume. There was a negative relationship (P<0.01) between Ae and irrigation water volume (mesic, r=-0.97; xeric, r=-0.98). These data suggest that Cap and homeowner irrigation practices may not be related to planting design in southwest upper middle class suburban neighborhoods. These data suggest that the arrangement and size of plants in the landscape maybe more important factors than arrangement of taxa in regards to landscape water conservation and water use efficiency.