Arboriculture & Urban Forestry 35(4): July 2009 sons with subsequent plantings are not appropriate since, for example, quality of the palms, handling, and weather may all have been different. A former landscape contractor in Florida reports routinely planting cabbage palms (S. palmetto) 100 to 130 cm (3 to 4 ft) deep in sandy soils. In one instance they ob- served roots growing out of the entire planted portion of the trunk when a deep planted tree was later moved (pers. comm., from coauthor, J. R. Harris), thus indicating that it is possible for the root initiation zone to extend more than a meter up the trunk. Adventitious Roots on Dicot Trees Many dicots, such as London planetree (Platanus × occidentalis) and red maple (Acer rubrum) can form adventitious roots when moist soil is placed against the trunk. Propagation by stem cut- tings is a common means of nursery production for these and other species. It is possible that such an adventitious root sys- tem would form on these deeply planted species while the original root system serves as an establishing or “nurse” root system. No scientific studies of such phenomena have been re- ported. In addition, the response of palms to such practices suggests that species tolerance to the environmental condi- tions of lower soil regions must always be taken into account. Mature trees that have been buried by changes in soil grade sometimes form new adventitious root systems (e.g., Plata- nus × occidentalis, personal observation of the authors). This also occurs in natural ecosystems subject to alluvial or wind- blown soil deposition (Stone and Vasey 1968; Filion and Marin 1998). In urban landscapes, however, cases have been observed where such adventitious root systems support the tree physi- ologically but not structurally. In one such case, described by Harris et al. (2004), when the original roots decayed, the tree was taken down. A systematic analysis of this phenom- enon has apparently not been made. However, deep-planted trees have also been reported to be less stable if adventitious roots do not form on the buried stem (Lyons and Yoder 1981). Root Structure Deep planting creates the potential for roots to grow upwards through the soil and come in contact with the trunk. When roots are in tight tangential contact with the trunk, they are generally termed girdling roots even when the trunk is not fully encircled. Deep structural roots have been observed to increase the incidence of girdling roots, but this appears to be at least partially species dependent (Wells et al. 2006) and possibly time dependent (Wat- son 1990; Day and Harris 2008). Although poor root structure may be detrimental to trees in the long term, it is not believed to be a factor influencing the initial establishment of landscape trees. Trunk-Soil Contact Unless soil is held back by a tree well or retaining wall, deeply planted trees invariably will have soil in direct contact with the trunk. Mulch piled against the trunk creates an analogous situa- tion. Does this trunk/substrate contact increase infection by soil- borne pathogens such as Phytophthora spp.? Some diseases, such as Sphaeropsis tip blight, can be harbored in cone-bearing mulch resulting in increased disease incidence in pines (Jacobs 2005). Wells et al. (2006) observed sporadic Phytopthora infection on deeply planted Prunus × yedoensis; but whether the infection was 187 related to trunk/soil contact or to the increased susceptibility of stressed trees could not be determined. Mortality was high among deeply planted trees and infection was also present on trees plant- ed at grade. Day et al. (2005) used bark biopsies to assess fungi present on white oak (Quercus alba) and sweetgum (Liquidambar styraciflua) trunks nine years after burial by construction fill. Oak bark was observed to be decaying, but only saprophytic fungi (Penicillium spp., Trichoderma spp., and Pestalotia spp.) were recovered. In contrast, no fungi were recovered from sweetgum bark, which was visually unchanged from unburied trees. Drilias et al (1982) observed urban sugar maples (Acer saccharum) in Wisconsin, U.S. infected with Fusarium and Phytopthora species that causes “basal rot” and “collar rot.” They found a high inci- dence of disease in urban maples, many of which had buried root collars, and no infection in comparable woodland trees. The au- thors concluded that an unidentified factor associated with urban trees increased incidence of these diseases and suggested nursery production and deep planting as possible topics of future research. Indeed, for a given site, some of the environmental factors that can contribute to infection by a particular disease, for example flood- ing and Phytopthora in apples (Browne and Mircetich 1988), may be present in deeper soil regions but not in shallower regions. Optimal Depth of Structural Roots “How deep is too deep?” still remains a question of some contro- versy. Roots must be covered by some soil or other media if they are to take up water and nutrients, generate secondary roots, be protected from temperature and moisture extremes, and provide stability. On the other hand, deep structural roots can adversely affect tree development. In the studies analyzed in this review, effects were typically seen only when root collars were 15 cm or more deep. The shallowest depth having any apparent impact in any situation was 7.5 cm (Table 1). It must be emphasized, however, that few studies examined a range of depths within 0–15 cm (0–6 in). Effects also vary by species, soil type, and climate. The BMP for identifying deep root systems in nursery stock is based on the depth to the first two structural roots (less than 7.5 cm deep, measured 7.5–10 cm out from the trunk), while the establishment studies cited here refer to the depth of the root collar. Thus a tree planted 7.5 cm too deep, may have the first structural root position several centimeters below that. A Model for Understanding Deep Structural Roots Two principle changes occur when trees are planted too deep: 1) planting exposes tree roots to a different environment than that near the soil surface and, 2) trunk tissue is in contact with soil. These two principle components create a framework for understanding the effects of deep planting on tree establishment and for forming field prognoses for short-term survival (Figure 2). This framework helps explain the often conflicting results from research studies and aids the practitioner in assessing the consequences of deep structural roots. To use this model, prac- titioners must know the soil conditions at the depth where the structural roots have been placed, and also understand the site tolerances of the particular tree species. This model can be re- fined and expanded as more research becomes available assess- ing the long-term effects of girdling roots, remediation practices, and the influence of species’ ability to form adventitious roots. ©2009 International Society of Arboriculture
July 2009
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