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  • Author or Editor: Linda B. Stabler x
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Urban sprawl of the greater Phoenix metropolitan area is rapidly replacing agricultural and non-irrigated desert vegetation with an irrigated urban forest comprised of a mixture of woody ornamental plant materials. Our objective was to estimate and compare the carbon acquisition potential (CAP) of residential landscape plants to the dominate plant species found in adjacent agricultural and desert sites. Maximum shoot and leaf gas exchange measurements were made at monthly intervals for one year (Aug. 1998 to July 1999) using a portable photo-synthesis system. Concurrent diel gas exchange measurements were made seasonally. Gas exchange measurements were made on alfalfa at agricultural sites, blue palo verde, creosote bush and bur sage at desert sites, and on a mixture of 19 different woody ornamental tree, shrub and ground cover species at residential sites. A trapezoidal integration model was used to estimate daily CAP at each site based on maximum assimilation flux values and seasonally adjusted diel assimilation patterns. Annual landscape CAP was then calculated as the summation of estimates of daily CAP. Calculated annual CAP was highest at agricultural sites (159.0 mol/m2 per year), lowest at desert sites (35.3 mol/m2 per year), and intermediate at residential landscape sites (99.3 mol/m2 per year).

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Growth and water use efficiency (WUE) of two Southwest landscape plants under various regimes of irrigation frequency was studied in a greenhouse experiment. Red bird of paradise (Caesalpinia pulcherrima L.) and blue palo verde (Cercidium floridum Benth. ex A. Gray) were grown at three levels of irrigation frequency intended to mimic a range of watering practices determined via survey data from the Phoenix, Ariz., metropolitan area. During two irrigation cycles, measurement of mid-day water and osmotic potentials, lysimetric whole-plant transpiration (T), and mid-day shoot gas exchange was made. Irrigation frequency treatments affected Cercidium more than Caesalpinia. Frequent irrigations increased Cercidium shoot length and dry weight. For both species, infrequently irrigated plants showed patterns of osmoregulation in response to drying soil. Transpiration (T) was consistently highest for infrequently irrigated plants. WUE was affected by treatment for Cercidium, but not Caesalpinia. Gas exchange was unrelated to plant growth or T. Instantaneous transpiration efficiency (ITE) was negatively correlated to the ratio of intracellular CO2 to ambient (CICA) in all treatments, suggesting that under well-watered conditions, WUE might be reduced by negative feedback effects of high internal CICA ratios.

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Combustion of fossil fuels in urban areas might increase local atmospheric CO2 concentrations and could result in an urban to rural CO2 concentration gradient. Our objective was to ascertain if such a CO2 gradient exists and to characterize seasonal patterns of amplitude and distribution of atmospheric CO2 concentrations in the Phoenix, Ariz., metropolitan, area. Atmospheric CO2 concentration was measured along a series of gradients that transected the greater Phoenix metropolitan area in June 1999, in Dec. 1999, and Jan. 2000. Carbon dioxide concentration was measured with a portable infrared gas analyzer in open system mode from a mobile vehicle traveling at a constant rate of speed. All measurements were made around 0500 and 1500 HR on days when weather conditions were clear and calm. The CO2 intake port was located above the vehicle at a height of 2.5 m. Data were categorized based on distance from the Phoenix urban core, defined as the intersection of Central Avenue and Van Buren Street. Gradients of high to low CO2 concentration existed from city center to outlying rural areas. Carbon dioxide concentrations were highest during winter and varied most during the afternoon. Mean CO2 concentrations in central Phoenix were 12% higher than surrounding rural areas during summer, but were up to twice as high as rural areas during winter. We conclude that there is a potential for atmospheric CO2 fertilization of plants in the Phoenix area, particularly of urban landscape plants that are biologically active during winter.

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