Arboriculture & Urban Forestry 36(6): November 2010 Arboriculture & Urban Forestry 2010. 36(6): 281-291 281 Effect of Tree Size, Root Pruning, and Production Method on Root Growth and Lateral Stability of Quercus virginiana Edward F. Gilman and Forrest J. Masters Abstract. This research aimed to evaluate impact of slicing the outer edge of container root balls, initial tree size at planting, and root ball composition on post-planting tree stability in a simulated wind storm. One-hundred twenty Cathedral Oak® live oak were planted in March 2005. Thirty field-grown trees were transplanted, and 60 trees of similar size were planted from 170 L containers. Root ball sides on 30 containers were sliced prior to planting. Thirty smaller trees from 57 L containers were planted without slicing. Trees were pulled with an electric winch, and blown with a hurricane simulator in 2007. Slicing the root ball had no impact on root growth, bending moment, or bending stress. More bending stress was required to pull field-grown trees than trees planted from containers of either size. Growing trees in containers for three years prior to landscape planting changed root morphology compared to field-grown trees, which corresponded to reduced stability. Trees planted from small containers were as stable as those from larger containers. Root cross-sectional area windward correlated the most with bending stress required to tilt trees with a winch and cable. Bending moment scaled to the 3.4 power of trunk diameter. Key Words: Bending Stress; Container-Grown; Field-Grown; Root Cross-Sectional Area; Root Diameter; Root Number; Saturated Soil; Trunk Diameter; Wind. Wind causes tree overturning which damages trees, adjacent build- ings, and associated structures. Thousands of trees were overturned by hurricanes in the southeastern United States between 1989 and 2004 (Duryea et al. 2007). In at least some cases, the original root ball periphery was clearly evident as young trees lay horizontal on the soil surface. Periphery of root balls were defined by roots deflected by the container wall or branched where roots were cut when transplanted from the field nursery. Causes of root failure in wind are poorly documented for nonconiferous, young shade trees. Tree size, age, root form, and soil attributes influence stability of well established trees. Nursery production method can also in- fluence tree stability. Most studies were conducted on trees plant- ed from small propagation-sized (5 cm diameter) containers used in reforestation. Robert and Lindgren (2006) showed no differ- ence in stability 3 to 10 years after planting lodgepole pine (Pinus contorta Douglas ex Louden) from 5 cm diameter containers with defective root form compared to naturally regenerated trees. However, naturally regenerated Scots pine (Pinus sylvestris L.) were more stable during winching tests than trees planted from 5 cm diameter containers, probably due to a combination of more root cross-sectional area, better root symmetry, and increased number of straight roots (Lindstrom and Rune 1999). Root spi- raling was more severe and noticeable on young (7–9 year old) planted Scots pine than older (19–24 year old) planted trees. Bending moment required to pull trunks of naturally regen- erated Scots pine to 10 degrees tilt was significantly greater than for planted trees of similar size; in agreement with Nich- ols and Alm (1983), the difference was less pronounced for older planted trees. Both concluded and others agreed (Coutts et al. 1999) that the difference in stability between naturally regenerated and container-grown trees decreased over time as roots grow in strength to compensate for irregular root dis- tribution. However, internal problems remaining include ab- normal fiber orientation, compression wood, and inferior root strength from bark inclusions (Krasowski and Owens 2000). Douglas-fir [seudotsuga menziesii (Milb.) Franco] from bare root and from 5 cm diameter containers produced similar root systems as soon as five years after planting (Sundstrom and Keane 1999). Crossed and circling roots resulting from container production on this species simply grafted together. Root deformation from containers may impact stability more on trees such as pines that are not able to form adventitious roots or graft together (Halter et al. 1993). Quercus virgin- iana Cathedral Oak® trees capable of producing adventitious roots can generate new straight roots above root deformations when young (Gilman et al. 2008). However, as trees grew older than about two years, Cathedral Oak lost capacity to generate adventitious roots. This suggests that some trees planted from large containers such as those used in the landscape profes- sion could have many of their roots deflected by the contain- er wall, even on trees capable of forming adventitious roots. Straight horizontal roots on forest trees up to 21-years- old result in better stability following planting (Ortega et al. 2006), than trees with root deflections. Slicing (Gilman et al. 2008) or shaving (Gilman et al. 2010a) root balls when shift- ing from one container size to another can increase total number of roots and number of straight roots in the root ball. Ortega et al. (2006), Lindstrom and Rune (1999), and others showed that descending roots resulting from deflection by container walls on trees planted from 5 cm diameter liner pots reduced stabil- ity. Little is known about impacts on tree stability from plant- ing trees from larger containers common in the landscape trade. ©2010 International Society of Arboriculture
November 2010
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