174 Gilman et al.: Impact of Tree Size and Container Volume at Planting, Mulch, and Irrigation McDonald 2009), or no impact (Singer and Martin 2009). There is limited testing of mulch application on large nursery stock. This study was designed to test the influence of initial nursery stock size (in containers of AAN standard volume, 2004) and post-planting irrigation management on red maple (Acer rubrum L.) growth and anchorage six years after in- stallation. Impacts of post-planting mulch application on growth attributes were also evaluated. Red maple was cho- sen due to the popularity of trees in the Acer genus in temper- ate regions in the northern hemisphere (Iles and Vold 2003). MATERIALS AND METHODS Treatments and Planting In late February and early March 2006, 64 red maples (Acer ru- brum L. ‘Florida Flame’) propagated from cuttings were planted 4.6 m apart, in four rows of 16 trees, into field soil from black, smooth-sided plastic containers of four different volumes (16 trees from each): 11 L (28 cm wide × 24 cm tall; trees 2.5 cm caliper, 2.4 m tall), 103 L (60 cm wide × 46 cm tall; trees 6.8 cm caliper, 4.6 m tall), 230 L (78 cm wide × 59 cm tall; trees 9.3 cm caliper, 5.1 m tall), and 983 L (147 cm wide × 66 cm tall; trees 15.7 cm caliper, 7.6 m tall). Tree dimensions were in compliance with ANSI Z-60 National Standards (AAN 2004). Four trees from each container volume were randomly assigned to a position in one block of 16 adjacent trees. Field soil was Millhopper fine sand (loamy, siliceous, hyperthermic Grossar- enic Paleudults) with less than 2% organic matter and a bulk density of 1.51 g/cc in USDA Hardiness Zone 8b in Gaines- ville, Florida, U.S. Wire baskets were installed into the root balls of the 983 L containers three years prior to landscape planting to facilitate lifting. Holes 10 to 15 cm wider than the root balls were dug with straight sides and flat bottoms and adjusted so the top of the undisturbed root ball was about even with the landscape soil surface. One person packed the bottom of holes by foot in an effort to standardize settling. Water was added to settle backfill soil and soil was packed firmly by foot to standardize compaction of backfill soil. No berm or water ring was constructed around the root balls and no mulch was applied. Weeds were kept clear in the plot with periodic (3 to 4 annually) applications of glyphosate (isopropylamine salt, 41%). Trees were irrigated daily through 2 (11 L), 3 (103 L), 4 (230 L), or 6 (983 L) Roberts Spot-Spitters (Roberts Irrigation Products, Inc. San Marcos, Idaho, U.S.) positioned at the edge of the root ball directed toward the trunk through May 8, 2006 (57 L per irri- gation the first three weeks followed by 26 L thereafter for 983 L containers, 19 L for 230 L containers, 13 L for 103 L containers, and 9 L for 11 L containers). This was followed by approximately two weeks of no irrigation in order to evaluate xylem water po- tential under water deficit conditions. Irrigation resumed to every other day May 24 with 983 L containers receiving 82 L, 230 L containers receiving 34 L, 103 L containers receiving 23 L, and 11 L containers receiving 11 L; volume was increased because the weather remained dry, which was normal for the region in that season. In April 2007, half the trees for each container volume in each block were irrigated Monday, Wednesday, and Friday; the other half received no additional irrigation. In May 2007 (14 months after planting), half the irrigated trees from each container size and irrigation treatment in each block received an 8 cm thick ©2013 International Society of Arboriculture layer of shredded hardwood mulch to within several centimeters of the trunk from line-clearing operations. Mulch was applied to a 4.6 m × 4.6 m square around each tree. A 5 cm layer was added at the beginning of year three to make up for decomposition. There was one tree in each block that received each of the 16 treat- ment combinations. Trees were not fertilized at or after planting. Tree Measurements Two wooden stakes were driven into soil 90 cm east and west of the trunk in undisturbed soil to monitor tree subsidence or settlement following planting. A string was stretched tight from the top of each stake so it rested against the trunk. A visible line was drawn on the trunk to mark string position at planting March 2006 and two growing seasons later in October 2007. Abnormal vertical cracks developing on the lower 2 m of trunks possibly due to transplant water stress were counted in October 2007. Irrigation was withheld during eight periods of dry weather (May 2006 through April 2007) during the first year after planting to induce short periods of sub-lethal water stress. Xylem water potential was measured 12:00 to 14:00 hr on sunny or most- ly sunny days with a pressure chamber (Soil Moisture Inc., Santa Barbara, California, U.S.) on all 64 trees. Termi- nal portions (10 cm long) of current year twigs growing in full sun about half way up the southern side of the crown were immediately placed in the pressure chamber for mea- surement. Trunk diameter at 30 cm above the ground and total tree height were measured at planting and each Sep- tember except in 2010 when only diameter was measured. Evaluating Anchorage The Alachua County, Florida, soil survey was used to determine the amount of water to add (757 L) and amount of time to wait (6 hours) to bring a 2.4 m × 2.4 m × 1.2 m deep volume of soil around each tree to field capacity. The actual amount of water added was 1.5 times the amount needed (757 L × 1.5 = 1135 L), to ensure soil saturation consistency. Water was applied through four low-profile sprinkler heads. Each tree was winched 6.0 to 6.5 hours after irrigation ceased thus allowing water to percolate into soil and drain, bring- ing soil to field capacity prior to evaluating tree anchor- age. Water application simulated a large volume rain event often associated with storms, and standardized soil mois- ture conditions among replicates due to reported influ- ence of soil moisture on anchorage (Kamimura et al. 2012). The researchers did not expect mulch to influence anchor- age so non-mulched trees were not winched. All 32 trees in the mulched treatments were winched with a steel cable and electric winch (Model 40764; Chicago Electric Power Tools, Inc. Camarillo, California, U.S.), August 4–16, 2011, in the 350 degree Azimuth (from north) direction to evaluate lat- eral tree stability (anchorage). This direction was chosen so tree crowns would not touch one another as they were winched. There was no prevailing wind direction at the site. Mulch was gently raked from the surface to better observe RP movement during winching. An electronic inclinometer (model 3DM-GX1, Microstrain Inc., Vermont, U.S.) was se- cured with screws to the trunk base 15 cm from soil surface, immediately above the swollen flare. A 3,629 kg capacity load cell (SSM-AF-8000; Interface Inc., Scottsdale, Arizo-
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