Arboriculture & Urban Forestry 32(6): November 2006 309 position on cherry survival was not statistically significant, there was a trend toward greater mortality on the lower por- tions of the site (data not shown). It is also suggestive that the majority of cherry deaths occurred in 1998 after a particularly wet early Spring. In flooded or waterlogged soils, aerobic conditions may extend only a few centimeters beneath the soil surface (Taiz and Zeiger 2002). Under such conditions, trees with the ma- jority of their root system below 15 cm (6 in) would be subjected to severe anoxia. Yoshino cherry, in particular, is known to be intolerant of flooding (Rowe and Beardsell 1973; Jacobs and Johnson 1996). Nonetheless, we cannot rule out other mechanisms of deep planting stress such as reduced water infiltration to the root ball or insufficient access to shallow mineral nutrient pools. Infection by pathogenic fungi such as Phytophthora is en- couraged under flooded or poorly drained conditions (Wilcox 1993). However, tests at our site showed no relationship be- tween treatment or slope position and the sporadic incidence of Phytophthora infection in the planting (data not shown). It is therefore unlikely that enhanced Phytophthora infection was the proximate cause of deep-planting stress. An interesting comparison can be made between our re- sults and those of MacDonald et al. (2004), also working with Yoshino cherry. In their study, the addition of 30 cm (12 in) of fill soil over the root zones of established cherries had no effect on soil aeration or tree performance. This result sug- gests that root collar burial, in and of itself, may not cause stress under all conditions. A potentially important difference between our study and that of MacDonald et al. (2004) concerns the initial condition of the plant material: trees in the MacDonald study had been growing in situ for 3 years, whereas those in our study had recently been dug for transplant. Balled-and-burlapped trees lose a significant fraction of their fine root system at trans- plant, and the trees in our study therefore experienced root loss and root system burial simultaneously. The combination of these stresses was likely more stressful than root system burial alone, as suggested by the differences in root growth between our study and that of MacDonald et al. At our site, deep-planted trees showed extremely limited root regrowth into the upper soil layers 1 year after transplant, whereas in the MacDonald study, the trees produced a substantial num- ber of new roots in the upper fill soil. Maples were better able to tolerate deep planting than cher- ries in the short term. Surprisingly, this greater tolerance was not related to more rapid reestablishment of a normal root depth distribution. One full year after transplant, the root distribution of the deep-planted maples was not different from that of deep-planted cherries and was still strongly skewed toward the lower soil profile. However, two other factors may have enhanced maple deep-planting tolerance. First, the maples produced significantly more roots than the cherries: their root mass densities were nearly twice as large. Therefore, although they did no better than the cherries in reestablishing a normal root depth distribution, they did have a larger absolute mass of roots in the upper soil layers. This additional shallow root mass may have allowed them better access to critical, shallow distributed soil resources. It is also possible that red maples had more effective physiological mechanisms of anoxia tolerance such as aerenchyma forma- tion, efficient anaerobic metabolism, and antioxidant de- fenses after reaeration. Potential Long-Term Effects Although the maples appeared to tolerate deep planting in the short term, air spade excavations indicated that deep planting may have set the stage for future problems with stem-girdling roots. It is unclear why deep planting should predispose trees to girdling root formation. Stem-girdling roots are known to form at transplant when lateral root growth is stimulated by the severance of major structural roots radiating from the stem base (Watson et al. 1990). This effect is particularly pronounced in maples whose laterals tend to emerge at right angles to their parent roots. Roots that grow vertically toward the soil surface after deep planting may be more likely to assume a girdling orientation, particularly if they tend to grow along the oxygen-rich interface between the deeply planted trunk and the bulk soil. Conflicting observations have been published on the po- tential link between deep planting and girdling root forma- tion. In a series of root collar excavations on maples, Watson et al. (1990) found no relationship between planting depth and girdling root formation. However, recent work by Johnson and Hauer (2000) suggests that such a relationship exists in some species. Our work represents the first time that the effects of deep planting on girdling root formation have been tested experimentally. Although deep-planted maples showed substantial girdling root development, it is not yet clear whether these girdling roots will persist and cause long-term damage to the trees. Serious symptoms of girdling root injury do not appear in Norway maple until the trees approach 8 to 10 in (20 to 25 cm) caliper (Gouin 1984), and it therefore may be many years before the girdled red maples at our site exhibit reductions in growth or vigor. Furthermore, there is evidence to suggest that girdling roots of red maple, although numerous after transplantation, may not persist long enough to cause serious damage (Watson et al. 1990). Follow-up measurements at our site will shed light on the degree to which early girdling root formation causes long-term injury in red maple. In conclusion, our results support the hypothesis that deep planting can predispose trees to transplant failure and girdling root formation. Although the effects of deep planting will certainly vary with species and site conditions, the few extra minutes needed to identify a tree’s root collar and place it at ©2006 International Society of Arboriculture
November 2006
| 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
