Arboriculture & Urban Forestry 35(4): July 2009 secondary phloem fibers were slightly shorter than those in stem bark (Trockenbrodt 1995). Of particular relevance to the char- acteristics of the root shank, the taproot bark near ground level (less than 40 cm deep) was similar to mature, thick stem bark and exhibited a distinct rhytidome, or outer bark. Lateral roots, on the other hand, did not form a rhytidome. Research is meager in this area, and studies directly applicable to root shank ques- tions, especially winter hardiness considerations, are essential. When liners are planted in the field for growing to saleable size, many growers purposely plant the liners too deep for sev- eral reasons: to reduce the need for staking, to control sprouts from rootstocks, to protect the bud union, or to hide the crook in the stem that resulted from the grafting and cutback process. In addition, nursery maintenance and harvesting practices may contribute to deep root systems. Unlike the forest where the soil is protected from drying by leaf litter “mulch,” frequent drying of bare nursery surface soils may inhibit root develop- ment near the surface. Cultivation in nurseries for weed control can also discourage root growth near the soil surface and may mound soil around the base of trees. If not removed before harvesting, this soil can bury structural roots in the root ball. When field-grown bare root liners are planted into containers for growing to saleable size, the structural roots can easily be too deep, sometimes resulting in reduced growth (Fare 2005). Fine roots originating from the structural root system can proliferate in the container substrate above the structural roots (Fare 2005), making transplanting “at grade” difficult unless these roots are sev- ered. In addition, deep planting during container production may result in poor quality trees. Gilman and Harchick (2008) found that planting live oak deep in the container during production produced circling roots such that they did not meet the minimum quality of the Florida Grades and Standards for Nursery Stock (Florida Department of Agriculture and Consumer Services 1998). This effect was most pronounced in the most deeply planted trees. DEEP STRUCTURAL ROOTS: LANDSCAPE ESTABLISHMENT ISSUES Deep planting can improve establishment (Slocum and Maki 1956; Harms 1969), have no effect on establishment (Day and Harris 2008) or be detrimental to establishment (Arnold et al. 2007). Studies demonstrating improved establishment from deep planting are typically limited to small reforestation seed- lings where “deep” planting is a matter of 5–10 cm (2–4 in) be- cause of the small stature of the plants (Harms 1969; Macadam and Bedford 1998). Recently, however, Dreesen and Fenchel (2008) reported that the USDA Natural Resources Conserva- tion Service has developed a system for revegetating riparian zones in arid climates that relies on tall nursery stock [2.6 m (8.5 ft)] being planted in 2 m (6.6 ft) holes, i.e. approximately 1.6 m (5.25 ft) too deep, in order to access deep water tables in sandy soils. This technique resulted in successful establish- ment of hackberry (Celtis laevigata) and box-elder (Acer negun- do), but no comparisons were made with trees planted at grade. Landscape-sized trees in landscape sites, the subject of this re- view, generally do not respond as favorably as these examples. These trees have already developed an extensive root system and are subject to establishment periods of several years once transplanted into the landscape. In addition, the soil conditions around a deeply-planted rootball are often unfavorable for root 185 growth in many sites (Harris 2007), especially in urbanized ar- eas where extensive grading and topsoil removal are common. When conditions lower in the soil profile are less favorable than those near the surface, deep planting can inhibit estab- lishment. Arnold et al. (2007) found that planting small [9.3 l (3 gal)], container-grown trees as little as 7.5 cm below grade decreased survival and growth of all but one of five species af- ter three years in a sandy loam underlain at 15–30 cm (6–12 in) with a hard clay pan in Texas. The clay pan in this layered soil was punctured during planting hole excavation, suggesting that the root systems of the deeply planted trees were partly sur- rounded by clay. Growth increased in some instances for trees planted above grade, suggesting that minimizing exposure to the clay hardpan may have been beneficial. An earlier study pro- duced similar results, but also found that increased mulch depth reduced growth, apparently because rainfall was unable to ad- equately penetrate the soil in order to reach the deep-planted roots (Arnold et al. 2005). In a seven-month establishment study with large, ~7.6 cm (3 in) trunk diameter, field-grown live oak, tree growth was unaffected by planting as much as 18 cm (7 in) below grade in a fine sand soil. However, deep-planted trees experienced greater water deficits than trees planted at grade when lightly irrigated after an extended dry period. This irriga- tion event was apparently unable to penetrate down to the deep root balls, even in sandy field soils (Gilman and Grabosky 2004). These results suggest that the interaction of climate and soil properties at differing depths are primary factors influencing survival and growth of deep-planted trees during the establish- ment period. For example, if lower soil regions were highly compacted, very wet, or very dry, establishment would likely be impaired, especially in species sensitive to the particular con- ditions present. With the exception of girdling roots, all effects shown to arise from deep planting to date (see Table 1) can be understood in the context of species response to the immediate conditions encountered by the transplanted root system. On the other hand, when no exacerbating conditions are present, trees may grow normally for many years. For example, in the first five years of a study with Turkish hazel (Corylus colurna), growth and establishment of trees planted both 15 and 30 cm deep were essentially identical to trees planted at grade. But after two se- vere flooding events, 40% of the most deeply planted trees died compared with no deaths among trees planted at grade or only 15 cm (6 in) deep (Day and Harris 2008). Tree establishment may also be unaffected if the species in question can tolerate the adverse soil conditions present in the lower regions of the site. For example, red maple (Acer rubrum) is a very flood-tolerant tree (Whitlow and Harris 1979), while Yoshino cherry (Prunus × yedoensis) is not (Ranney 1994). When these two species were planted 30 cm (12 in) below grade on a slope where drainage was poorer on the lower end of the slope, all red maples sur- vived, while deep-planted cherries died in much greater num- bers during establishment than at-grade trees (50% versus none), especially on the lower end of the slope (Wells et al. 2006). The conditions encountered in lower soil regions may vary considerably and arguably play a much more important role in tree establishment and survival than previously thought. Ur- ban soils disturbed by cut-and-fill practices may have low- er soil layers that are very wet or very dry (Day et al. 2001). Even undisturbed soil profiles can have denser horizons with lower organic matter (B and C horizons) within about 30 cm ©2009 International Society of Arboriculture
July 2009
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
