164 Arnold et al.: Planting Depth and Mulch Thickness grade in an attempt to prevent trunk leaning or blow-over of the newly transplanted trees. Alternatively, it is sometimes recommended that when planting in heavy clay or poorly drained soils that the top of the root ball be planted slightly above grade to avoid having the root collar or trunk of the tree exposed to wet soil for extended periods of time (Pirone et al. 1988; Arnold 2002). Inadequately compacted fill soil in the bottom of planting holes or decomposition of organic substrates with container-grown stock can result in settling of the root collar below grade after planting. All of these practices may result in trees planted at depths other than that which would have occurred if the tree seedling had naturally germinated and grown on the site. Much more is known about the effects of mulching (Green and Watson 1989; Watson and Himelick 1997; Duryea et al. 1999; Carlson 2002). Most studies report beneficial effects relative to bare soil treatments, such as increased moisture retention in the soil beneath the mulch, improved weed suppression, temperature modification, or improved soil quality (Ashworth and Harrison 1983; Stinson et al. 1990; Greenly and Rakow 1995). However, Carlson (2002) warns of the adverse effects of excessive mulch applications and encourages the concept of varying mulch thickness and textural composition with increased need for oxygen penetration in heavy, poorly drained soils. More is not necessarily better, and there is no evidence to support the widespread practices of piling “mulch volcanoes” around the trunks of newly transplanted trees. Placing mulch too deeply against trunks can result in damage to bark and phloem tissues (Ball 1999). Other concerns with mulches are variation in nutrient content, allelopathy, and decomposition properties (Duryea et al. 1999; Foshee et al. 1999). Likewise, pine bark mulch can result in greater reflectance of longwave radiation from mulch surfaces than from bare soil or turfgrasses, creating a greater heat load in the canopy (Zajicek and Heilman 1991). Alternatively, pine bark mulch may help to buffer soil temperature fluctuations (Montague and Kjelgren 2004). In preliminary results with bougainvillea goldenraintree and green ash, Arnold and McDonald (2002) found surpris- ingly strong differential responses in growth and survival during the first year after transplanting to both planting depth and mulch thickness on a heavy clay soil. Gilman and Grabosky (2004) reported mixed water stress responses during the first growing season for live oaks (Quercus virginiana Mill.) transplanted at various depths. Aside from these short-term reports, no research information on the interactions among these two important tree establishment practices was found. Thus, the purpose of this research project was to (1) document the potential for interactions among planting depths and the thickness of mulch applica- tions on the establishment of container-grown trees, and (2) compare responses to varied planting depths between a flood-tolerant and a flood-intolerant species. ©2005 International Society of Arboriculture MATERIALS AND METHODS Two tree species, Koelreuteria bipinnata (hypoxia intolerant) and Fraxinus pennsylvanica (hypoxia tolerant), were chosen for their differential responses to soils with seasonally poor drainage (Arnold 2002). Trees of both species were pro- duced on site at the Texas A&M University Nursery/Floral Field Laboratory (College Station, Texas, U.S.) to ensure that the root collars were at the surface of the container sub- strate. Seedlings were grown in an outdoor container nursery utilizing a commercial pine bark–based substrate (3 pine bark:1 peat moss:1 coarse builders sand, by volume) in 9.3 L (#3) black plastic containers (Lerio Corp., El Campo, TX). The substrate was amended with 6.8 kg m–3 (12 lb yd–3 18N-3P-8.3K controlled-release fertilizer (18-7-10, Sierrablen, Scotts Corp., Marysville, OH), 3.4 kg m–3 yd–3 m–3 Fredericksburg, TX), and 0.68 kg m–3 ) of gypsum (Standard Gypsum Corp., (1.5 lb yd–3 (6 lb ) of dolomite (Vulcan Materials Co., Tarrant, AL), 1.7 kg (3 lb yd–3 ) of micromax micronutrients (Scotts Corp.). Irrigation water for the nursery was injected with concentrated sulfuric acid (Scholle Corp., Northlake, IL) to lower water pH to 6.5 and with a 24N-3.5P-13.2K (24-8-16, 7.19% ammonium nitrate, 7.21% urea, and 9.60% nitrate, Scotts Corp.) water-soluble fertilizer to yield a concentration of 50 mg L–1 (50 ppm) N. Trees were staked and trained to a central leader. Trees were transplanted to adjacent field plots on 27 April 2001 (K. bipinnata) or 1 May 2001 (F. pennsylvanica). At transplanting, K. bipinnata averaged 88 cm (35 in.) in height and 15.3 mm (0.60 in.) in trunk diameter at 15 cm (6 in.) above the root collar, while F. pennsylvanica averaged 126 cm (50 in.) and 14.9 mm (0.55 in.), respectively. Koelreuteria bipinnata (84 trees) and F. pennsylvanica (120 trees) were established on 0.91 m (3 ft) within-row and 3.1 m (10 ft) between-row spacings in Brazos County, Texas. Field plots contained a Boonville Series, Boonville fine sandy loam, fine, montmorillic thermic ruptic-vertic albaqualfs (pH 9.1, bulk density 1.51 g cm–3 , 61% sand, 11% clay, 28% silt) underlain at a 15.2 to 30.5 cm (6 to 12 in.) depth with a hard clay pan. To ensure uniformity of disturbed soil volume within the planting holes, the holes were dug to accommodate the deepest planting depth possible (from the root collar to bottom of the root ball plus 7.6 cm [3 in.]) using a 45.7 cm (18 in.) diameter auger mounted on a Dingo® compact utility loader (The Toro Co., Bloomington, MN). This procedure also ensured that the clay pan was punctured to the same depth in all holes. Sides of the holes were scarred to avoid glazing and the backfill tamped firmly to achieve the desired planting depths. Final planting depths placed the root collars 7.6 cm (3 in.) below grade, at grade, or 7.6 cm (3 in.) above grade. The excised native soil was used as backfill during planting. Soil water potential was monitored using tensiometers (Model 2725 JetFill Tensiometers, Soil Moisture Equipment Corp., Santa Barbara, CA) inserted to a )
July 2005
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