Arboriculture & Urban Forestry 34(4): July 2008 Arizona and composted by Southwest Forest Products, Phoenix, Arizona (www.southwestforestproducts.com). The LTT mulch consisted of fresh landscape tree trimmings generated by DLC Resources Inc., Phoenix, Arizona (www.dlcresources.com) from normal management of residential common areas in the Phoenix metropolitan area and was a bulk mix of shredded branch trim- mings from mostly tree species of acacia (Acacia), eucalyptus (Eucalyptus), oak (Quercus), and elm (Ulmus). These two or- ganic mulches were compared with decomposing granite and soil without a mulch cover (control). The decomposing granite (DG) mulch was a screened 0.9 cm (0.36 in) minus Red Moun- tain Coral crushed granite rock quarried locally from the Salt River drainage in Mesa, Arizona. Table 1 shows some physical characteristics of the three mulch types. Experiment 1 From April 2004 until October 2005, an experiment was con- ducted to determine effects of three landscape surface mulches on soil temperatures and radiant flux properties at an open sky, field research site in Phoenix, Arizona. Soil at the research site was a Rillito series gravelly loam (taxonomic class coarse loam, mixed, superactive, hyperthermic Typic Haplocalcid, bulk density 1.57 g/cm3 [9.2 oz/in3]) with a 0% to 1% slope. Cumulative rainfall and potential evapotranspiration near the re- search site from April 2004 to October 2005 were 499 mm (19.96 in) and 3,371 mm (134.84 in), respectively (Arizona Me- teorological Network 2007). In April 2004, 14 9 m × 9 m (29.7 ft × 29.7 ft) nearly con- tiguous plots were established at the research site by removing all surface debris from each plot area. A completely randomized unbalanced experimental design was used to assign mulch treat- ments to each plot. The three landscape surface mulches, LTT, PPR, or DG, were applied to 12 (n 4) of the plots at a minimum depth of 5 cm (2 in), the minimum depth for landscape mulches stipulated by The Arizona Department of Transporta- tion Landscape and Irrigation Specifications, Section 430.4. The remaining two landscape plots did not receive surface mulch (bare soil control, n2). Surrounding each of the 14 plots was a 2.5 m (8.25 ft) wide strip of nonvegetative soil without mulch. The thickness of the landscape mulch layer within each plot was recorded at the beginning and end of the experiment by making 12 measurements of mulch depth per plot in three north to south transect gradients. Daily patterns of total irradiance (W/m2), net all wavelength radiation (W/m2), mulch and soil surface temperatures were re- corded over the course of 2 weeks during spring, summer, and fall of 2004 and 2005 under clear sky conditions. Total irradiance was recorded with a LI-200S pyranometer (Li-Cor Biosciences, Lincoln, NE) horizontally leveled on the top of the mulch surface at the center of each mulch plot. Net all wave- length radiation was recorded with a Q7_1-L REBS net radiom- eter (Campbell Scientific, Logan, UT) horizontally positioned at 231 the center of each plot at a height of 1 m (3.3 ft) above the mulch treatment plot surface. Using a 21X micrologger (Campbell Sci- entific, Logan, UT), total irradiance and net radiation data were recorded every 300 sec, averaged for each 15 min interval, and integrated for each 6 hr interval. Long wave radiation was cal- culated as the numeric difference between total irradiance and net radiation. Surface temperatures of mulch and bare soil with- out mulch (control) were recorded every 4 hr with a handheld Oakton InfraPro Infrared Thermometer (emissivity 0.94, 7° field of view; Oakton Instruments, Vernon Hills, IL) at ap- proximately 30 cm (12 in) above the mulch surface. Soil temperatures were recorded every 2 hrs from January 2004 (3 months before mulch treatment installation) until No- vember 2005 using Watch Dog model 100 data loggers (Spec- trum Technologies, Plainfield, IL). The Watch Dog data loggers were positioned 5 and 30 cm (2 and 12 in) beneath the soil surface at the center of each mulch treatment plot. Monthly records of soil moisture content were made within each plot with a portable Field Scout TDR 100 Soil Moisture probe (Spectrum Technologies) inserted into the soil to a depth of 5 cm (2 in). Experiment 2 Another experiment was conducted to determine dry bulk den- sity, thermal conductivity, and surface albedo of the PPR, LTT, and DG mulches. Mulch dry bulk densities were calculated as the weight of five oven-dried mulch (72 hrs at 105°C [221°F]) samples divided by the volume occupied by each mulch sam- ple. Thermal conductivity (W/m°C) values of each mulch type were calculated using a modified protocol of Montague and Kjelgren (2004). Surface albedo of the PPR, LTT, and DG mulches were determined at the research site described previ- ously in Experiment 1. For thermal conductivity, three 45 cm × 60 cm (18 in × 24 in) Styrofoam box containers per mulch type were filled witha5cm (2 in) layer of air-dried, screened soil from the research site described in Experiment 1. On top of the layer of soil was placed a 5 cm (2 in) layer of air-dried mulch. A soil heat flux plate (HFT3 Soil Heat Flux Plate; Campbell Scientific) and copper- constantan thermocouple was placed on the soil surface beneath the mulch at the center of each Styrofoam box. The Styrofoam box containers were then placed in a polycarbonate-covered greenhouse (30% light exclusion) maintained at 38°C (100°F)/ 24°C (75°F) ambient daytime/nighttime temperatures and al- lowed to acclimate for 72 hrs. Once acclimated, diurnal records of heat flux and temperature were made every 10 sec by a mi- crologger (Model 23X micrologger; Campbell Scientific) and averaged for 30 min intervals. Midday (1400 HR) mulch surface temperatures in each box were recorded with a handheld Oakton InfraPro Infrared Thermometer positioned approximately 2.5 cm (1 in) above the measurement surface. Midday total irradiance at the mulch surface was 814 W/m2 (LI-200S pyranometer; Li-Cor Table 1. Landscape surface mulch physical characteristics (± SE): particle size, bulk density, thermal conductivity, and albedo of landscape tree trimmings (LTT), ponderosa pine residue (PPR), and decomposing granite (DG). Landscape mulch LTT PPR DG Bulk Mulch particle size grade Approx. 1.9 cm (0.76 in) minus unscreened 1.9 cm (0.76 in) minus screened 0.9 cm (0.36 in) minus screened density (g/cm3) 0.24 ± 0.08 0.25 ± 0.08 1.69 ± 0.07 Thermal conductivity at 38°C (100°F) (W/m°C) 0.05 ± 0.002 0.05 ± 0.002 0.18 ± 0.007 Albedo 0.16 ± 0.003 0.16 ± 0.003 0.20 ± 0.003 ©2008 International Society of Arboriculture
July 2008
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