248 Giblin et al.: Effects of Planting Depth on Windthrow, Stability, and Growth (Quercus virginiana), noting that both extremely shallow and deep planting of rooted cuttings in containers can have ad- verse growth effects in this species. Fare (2005) also examined the effects of four different planting depths on five common containerized landscape trees, finding that planting depth had no significant effect on growth in four of the five species tested. With regard to nursery container stock, the ANLA standard (U.S.) simply states, “[a]ll container grown nursery stock shall be healthy, vigorous, well rooted, and established in the container in which it is growing. Container grown nursery stock shall have a well-established root system reaching the sides of the container to maintain a firm ball when the container is removed, but shall not have excessive root growth encircling the inside of the container.” There is no mention of planting depth in this standard. A study was initiated in 2002 to examine the effects of planting depth in containerized nursery production on four tree species containerized at various depths in con- tainers as they related to tree growth, lean, and windthrow. MATERIALS AND METHODS Four trees species were selected for use in this study: Whitespire birch (Betula platyphylla var. japonica ‘Whitespire’), green ash (Fraxinus pennsylvanica), Snowdrift crabapple (Malus × ‘Snow- drift’), and bicolor oak (Quercus bicolor). All trees were har- vested as field-grown, bare root nursery stock. The birch and oak were 2 to 3 cm in stem caliper, 1.8 m in height, and were graded as “branched” by nursery production standards. Crabapples were also 2 to 3 cm in stem caliper but were 1.5 m in height and were graded as “heavy-branched.” Ash varied from 1.5 to 2.5 cm in stem cali- per, were 2.1 m in height, and were graded as “unbranched whips.” From June 6 to June 8, 2002, trees were root pruned to fit into Grip Lip GL4000 containers (#10 trade size: top diameter 36.2 cm, height 36.2 cm, bottom diameter 33 cm, volume 34L) and to remove any turned or broken roots (as per standard prac- tice for finishing nursery stock) in containers at Bailey Nurseries (Hastings, Minnesota, U.S.). After root pruning, root volume was ©2011 International Society of Arboriculture However, some research has shown that planting trees deep is beneficial. During wind loading events, Parry (1974) found that apple trees planted with 15.24 cm of soil over the first main-order root were more stable than trees planted at the same depth as the production nursery. In traditional forestry applica- tions, Stroempl (1990) found that deeply planting some pines and spruce protected tender stems from heat-induced mortality. Currently, there are specific ANSI specifications (U.S.) for planting depth in field-grown nursery stock, including balled- and-burlapped (B&B) stock, in the current American Stan- dard for Nursery Stock (ANLA 2004), “[d]epth of the ball is measured from the top of the ball, which in all cases shall be- gin at the root flare... Soil above the root flare...shall not be in- cluded in ball depth measurement, and should be removed.” In contrast to the large volume of research examining plant- ing depth in nursery field production and landscape settings, little research has examined the effects of planting depth in nurs- ery-grown container stock. Recently, Bryan et al. (2010) stud- ied the effects of planting depth on growth in container-grown Ulmus parvifolia. They observed significant growth benefits when planting roots at grade or above grade in the first stage of container production. Gilman and Harchick (2008) exam- ined the effects of deep planting in container-grown Cathedral Oak® measured by water displacement (Young and Werner 1984), and stem caliper was measured 15 cm above the first main-order root attachment for each tree. Each species was then subjected to four different planting depth treatments, with 0, 5, 10, or 15 cm of ad- ditional substrate placed over the first main-order root. The con- tainer substrate consisted of sphagnum peat (40% by volume), pine bark (30% by volume), compost (20% by volume), sand (10% by volume), and also incorporated (2.9 kg/m3 ) slow-release fertilizer (Polyon 19-4-8, Harrell’s LLC, Lakeland, Florida, U.S.). The research plot was located at a Bailey Nurseries pot-in-pot production facility. The experimental design was a completely randomized block design with three subsamples of each spe- cies and depth combination randomly placed in five different blocks. Weekly data included frequency of windthrow and the frequency and extent of lean. A windthrow event was defined as an event that resulted in more than 50% of the plant’s roots be- ing tipped out of the container substrate. Lean was recorded as centimeters off plumb, measured using a plumb bob held at the stem 30.5 cm above a meter stick placed on top of the container. During the first four weeks of the study, all trees were straight- ened after the lean data were recorded. From weeks 5 to 17, the extent of lean was recorded, but trees were not straightened, as per standard practice at Bailey Nurseries (Timothy Bailey, pers. comm.). Trees were watered using automatic micro-irrigation via individual spray stakes in each container. In general, stan- dard nursery practices were followed except that trees were not staked to correct or prevent lean and/or windthrow. Air temperature, wind speed, and wind direction were measured using a WatchDog Model 525 weather station and data log- ger (Spectrum Technologies, Inc., Plainfield, Illinois, U.S.). On October 4, 2002, final stem caliper and shoot new growth measurements (using three randomly selected branches) were made on each tree, and the study was terminated. Root systems were then cleaned of container substrate using compressed air and water. This technique was tested on several trees to ensure that roots were not lost in the cleaning process. After cleaning, final root volumes were measured using water displacement as previously described. Descriptive statistics and univariate Analysis of Variance for stem caliper increase, root volume increase, and shoot new growth between treatments were determined using the general linear model and Tukey’s HSD post hoc (P < 0.05) functions of SPSS 16.0. The effects of time after planting and planting depth on lean frequency and average lean were examined us- ing repeated measures analysis of variance in the GLM func- tion of PASW (SPSS) 17.0. In cases in which the assumption of sphericity was violated, the Greenhouse-Geisser correction was used to adjust degrees of freedom and calculate a valid F- statistic for repeated measures Analysis of Variance. Post hoc tests were computed using the Bonferroni correction (P < 0.05). RESULTS & DISCUSSION Windthrow Windthrow was not a major problem for any of the species or treatments (data not shown). Three oak and one birch tree chronically windthrew. Other windthrow events occurred dur- ing the study but were infrequent and random. There was no statistically significant reduction in the occurrence of windthrow when these tree species were planted with their
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