12 Gilman: Pruning Acer rubrum at Planting Impacts Structure and Growth pruning) on 1.2 to 1.8 m tall whips of Malus sargentii (Evans and Klett 1984) and Prunus cerasifera ‘Newport’ (Evans and Klett 1985). In contrast, several studies found root growth was reduced with pruning, especially at higher pruning amounts (Head 1967; Fordham 1972). With no consistent advantage of pruning at planting for the purpose of compensating for root loss, focus in the last few decades has shiſted toward the improvement of branch and trunk structure. Many apple and pear orchards began pruning young whips at planting to craſt a struc- ture that allowed for clearance under the tree and developed a structure that supports fruit load (Forshey et al. 1992). Today, trees with large dominant trunks and much smaller branches (i.e., those with small aspect ratios) can be seen in apple orchards across North America (pers. obs.). Millions of trees are now pruned at or soon aſter planting each year in shade-tree nurseries. This is to train to one central leader and for clear- ance of lower trunk (K. Warren, pers. comm., Schmidt’s Nursery, Boring, Oregon, U.S.; M. Mar- shall, Marshall Tree Farm, Moriston, Florida, U.S.). Some tree nut crop managers are also begin- ning to experiment with central leader structure. Pruning one stem of a codominant stem pair to encourage the other stem to grow faster has been referred to as structural or formative pruning (Gilman and Lilly 2008). There are several studies support- ing this concept. One on seedlings grown in an open landscape on California coast live oak (Quercus agrifolia N’ee) and valley oak (Q. lobata N’ee) found that the headed or thinned codominant stem grew more slowly than the non-pruned stem (Downer et al. 1994). Fumey et al. (2011) also showed a shiſt in growth from the pruned to the non-pruned portion of one-year-old apple seedlings. Structural or formative pruning seeks to enhance growth on the non-pruned leader by reducing growth rate on the branches with the largest aspect ratios; these are usually the largest branches on the tree. Recently, studies have begun to evaluate the impact of pruning at planting on larger trees with the intention of improving branch structure in the urban landscape. One showed no reduc- tion in trunk diameter growth rate measured four years aſter planting as a result of pruning at planting to improve structure (Kristofferson et al. ©2015 International Society of Arboriculture 2010; although unreported, communication with the author confirmed no more than about 30% of foliage was removed from any single branch). Despite no reduction in trunk growth rate, branch aspect ratio decreased slightly, but significantly, over a four-year period as a result of pruning. The purpose of this study was to evalu- ate impact on distribution of large branches, aspect ratio, growth, and anchorage from prun- ing to reduce length of the largest primary branches when planting into landscape soil. MATERIALS AND METHODS Acer rubrum L. ‘Florida Flame’ (propagated from cuttings) were pruned twice annually for three years by the same person to craſt a single central leader during nursery production in plastic containers in Gainesville, Florida, U.S., USDA hardiness zone 8b. A single leader was produced in the nursery by re- ducing the length of the largest diameter branches at each pruning using reduction and heading cuts. Trees were planted into field soil in May 2011 from 170 L round plastic containers. Soil was Millhopper fine sand (loamy, siliceous, hyperthermic Grossar- enic Paleudults) with less than 2% organic matter. All finished trees had the lower 1.2 m of trunk clear of branches and met Florida #1 grade (no branches larger than two-thirds trunk diameter in the lower half of the tree) of the Florida Grades and Standards for Nursery Plants (Anonymous 1998). Mean trunk diameter 15 cm from ground (caliper) was 67 mm and tree height was 5.4 m at planting; this size tree is commonly planted in a number of moist-climate regions across the globe. Trees were spaced 2.4 m apart in four rows of 16 trees, with the eastern rows assigned to one block and the western rows to another. All 16 trees in one row of each block were structurally pruned at plant- ing; the other row of 16 trees in each block was not pruned at planting (32 trees were pruned at plant- ing, and 32 were not pruned). Treatments were assigned randomly to the two rows in each block. Structural pruning consisted of reducing the length of the largest diameter primary branches [i.e., those attached to the trunk that were larger than 0.5 aspect ratio (B ÷ A, Figure 1) estimated just distal to the union] that originated in the lower 80% of the crown. Branches originating from the top 20% of the crown were not pruned at plant-
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