Site Choice and Sample Collection

Two sites were selected for this study, an urban site, located at the Desert Botanical Garden in Phoenix, Arizona, and a desert site located at Usery Mountain Park (desert site), Mesa, Arizona. The urban site is located in the Lower Colorado River Valley Subdivision of the Sonoran Desert, the driest and most arid region in the Sonoran Desert with creosote bush (Larrea tridentata (Sess and Moc. ex DC.) Coville) and white bursage (Ambrosia dumosa (A. Gray) W.W. Payne) plant communities among the most abundant and important in this region. The desert site is located in the Arizona Upland Subdivision, which receives higher amounts of precipitation and supports a diverse amount of leguminous trees, perennial shrubs and cacti. Records of precipitation and temperature data during the study at nearby weather stations previously installed were obtained. Total precipitation at the urban site and the desert site were 24.10 cm and 33.53 cm respectively for the study period (July 1, 2004 to June 30, 2005). Monthly precipitation levels were similar with higher levels of rainfall at the desert site during the months of July through October of 2004, December 2004 through March 2005, April, and June of 2005. Monthly precipitation levels were higher at the urban site during the months of November and March. There were no recorded levels of precipitation at either site during the months of June of 2004 and May of 2005. Mean daily temperatures at both sites were similar with higher temperatures at the desert site during the months of October through December and higher temperature at the urban site during the months of February through June 2005. During the months that growth was measured (February through June 2005), the mean weekly high and low temperatures were variable within a few degrees with the urban site usually warmer.

General soil analysis was performed using a subsample from a mixture of six randomly collected soil samples from each site (IAS Laboratories Phoenix, Arizona). Soil from the urban site and the desert site were classified as a sandy loam. Soil pH at the urban site and the desert site was 8.3 and 7.5 respectively. Sand, silt and clay was 68%, 24% and 8% at the urban site and 66%, 25%, and 9% at the desert site. Total phosphorus, total carbon and total nitrogen were 640 ppm, 3800 ppm; and 250 ppm at the urban site and 960 ppm, 15000 ppm, and 770 ppm at the desert site.

Study plant and Benomyl treatment

Data were collected from Encelia farinosa, a perennial shrub found growing throughout the Sonoran and Mojave Deserts as well as cultivated for planting as a landscape plant throughout the Phoenix metropolitan area. It is a drought deciduous plant that can have glaborous winter leaves that are dropped in response to heat and drought stress. Summer leaves are smaller and highly pubescent. E. farinosa plants located at the urban site were nursery grown plants transplanted from 3.8-liter containers in April 2004. E. farinosa plants located at the desert site were randomly selected from within the existing plant community. Fourteen E. farinosa plants were selected at each site for a total of twenty-eight plants sampled for this study.

A benomyl treatment (Benlate SP, E.I. du Pont de Nemours and Company, Agricultural Products, Wilmington, DE) (.24 grams of benomyl/Liter of water) was applied to half of the plants at each site using the method described by Dhillion and Gardsjord (2004). From July 2004 to May 1, 2005, two liters of benomyl solution was applied to these plants every five weeks with one liter of solution applied to the above ground foliar and stem tissue and one liter applied as a drench to the soil area under the shrub canopy. Soil was not evaluated for the depth of moisture penetration.

Plant responses and measurements

During February 2005, six months after the beginning of benomyl application, plants were pruned to a uniform cube with a height and width of 80 cm. Plant height and width measurement data were collected prior to pruning. Ten random leaves were collected from intersection points formed from an imaginary 3x3 grid placed over the top of the plant, with one additional leaf collected from the center of the plant and placed in self-locking plastic bags in October 2004, December 2004, February 2005 and June 2005. The areas of the collected leaves were measured using a CID inc. CI-203 Laser Area Meter with conveyor attachment (Vancouver, Washington, USA).

Several plant productivity measurements were taken during mid June 2005 at the end of the spring growing season. The number of flower clusters and seed heads, when present, were collected, counted, dried, and weighed to calculate reproductive effort (Koide 1991). Seeds were cleaned by first passing through a column air blower followed by tossing and trapping the seeds in a 1.40 mm sieve (USA Standard Sieve Series Sieve No. 14, Newark, NJ). Measurements were taken of plant height and two perpendicular widths. Measurements of two perpendicular widths were used to calculate canopy diameter (Housman et al. 2002), height and two widths were used to calculate growth index (Martin and Ruter 1996). Plants were then clipped back to their starting volume. The plant tissue removed from each plant was bagged individually, dried for 86 hours at 70º C (Koide 1985) and weighed to determine biomass accumulation during the growing season. Vegetative and reproductive biomass was used to determine sexual reproductive effort (SRE) (Werk and Ehleringer 1983; Wilson and Hartnett 1997).

Soil water content was determined in the rhizosphere of each plant (Brady and Weil 2002). Twenty-eight soil samples were collected August 2004, October 2004, January 2005 and July 2005 from a depth of 10 cm under the canopy of each study plant and were stored in self-locking plastic bags. Soil fresh weight and soil dry weight (oven dried at 104º C for 2 days) was ascertained from a 25-cm3-sub sample from each soil sample.

Determination of root colonization

Twenty-eight soil samples containing E. farinosa roots were collected for analysis of colonization by AM fungi and dark septate endophytes (DSE) in April of 2005, eight months after benomyl application began. Soil and root samples were collected from beneath the canopy of each plant to a depth of 15-20 cm. Fresh roots were separated from soil and fixed in 50% ethanol, cleared of cytoplasm and stained with trypan blue (Koske and Gemma 1989) and Sudan IV stain (Barrow and Aaltonen 2001). Percent colonization by AM and DSE fungi was ascertained using the modified intercept method (McGonigle et al. 1990).