Arboriculture & Urban Forestry 43(5): September 2017 and under certain circumstances, such cells can be described as the equivalent of stem cells in animal tissues (Vernoux and Benfey 2005; Din- neny and Benfey 2008). The production of callus is largely dependent on the interaction of phyto- hormones and particularly auxins, gibberellins, and cytokinins (Aloni et al. 2010; Aloni 2015). Callus formation is the basis of the formation of graft unions utilized in horticulture (Neely 1970; Shigo and Marx 1977; Hudler 1984; Perry and Hickman 1987; Mattheck and Korseska 1989; McDougall and Blanchette 1996; McGarry 2001). Callus formation not only serves to reconnect vascular and other tissues across a wound (David- Schwartz and Sinha 2007), but also has an impor- tant role in forming a cap on the external edges of the wound, closing them (Shigo 1986). The shape of a wound has little effect on the rate of wound clo- sure, but its width does, with wounds less than 12 mm in diameter likely to close within a year (Neely 1970; Neely 1983). Closure occurs aſter the same length of time for similar-sized wounds (Neely 1970; Kramer and Kozlowski 1979; Harris et al. 2004), and wound closure rates correlate positively with rates of trunk growth but vary with species and years aſter wounding (Neely 1988b). More cal- lus tissue was produced around large wounds than small and callus growth at the edge of the wound was shown to be greater than the radial growth of the stem. Wound size and location have an impact on the rates of wound closure (Neely 1988c). Once a mass of callus cells becomes lignified it is termed woundwood (Harris et al. 2004), and as it grows it eventually seals the wound. In small wounds, the callus tissue normally closes the wound quickly before woundwood can be formed. The formation of callus tissue, and subsequent woundwood, is part of the tree’s defensive response to wounding. The differentiation of callus into xylem or phloem tissues is largely controlled by the concen- trations and interactions of the hormones auxin, gibberellins, cytokinins, and ethylene (Aloni 2015). However, the importance of the auxin, indole- 3-acetic acid (IAA), is of particular significance in vascular tissue differentiation and involves the basipetal movement of IAA (Sawchuk and Scar- pella 2013). The auxin canalization hypothesis is based on mature tissue responses to applied auxin application, but is also consistent with cel- 187 lular development in embryonic tissues and shoot organ development (Sawchuk and Scarpella 2013). However, if the bark has been damaged, lost, or cannot be replaced, techniques such as bridge graſting, approach graſting, and inarching can be used for repairing damaged orchard and valu- able ornamental trees (McGarry 2001; Goren et al. 2004; Harris et al. 2004; Hartmann et al. 2010). These techniques involve growing young seedlings that are progeny of the damaged plant, clones, or at least of the same species as the dam- aged plant around the base of the damaged tree. (Harris et al. 2004; Hartmann et al. 2010). These techniques require considerable skill and are expensive to undertake, but can be used to repair damaged ornamental trees of historic, heritage, cultural, landscape, environmental, and horticul- tural significance that warrant the expenditure. The Effect of Season on Tree Responses to Damage Tree wound healing rates in three species of de- ciduous trees and Eucalyptus camaldulensis were fastest during the active growth phase from mid- spring through to early summer (Neely 1970; Perry and Hickman 1987). Mercer (1979) described late- winter and spring as the seasons when wounds gen- erally callus-over best. The slowest rates of wound closure occur in wounds made during late-autumn and early-winter (Neely 1970; Perry and Hick- man 1987), and wounds inflicted in autumn may be detrimental to tree health because of the in- creased risk of infection, as many fungi sporulate at this time (Felix and Shigo 1977; Mercer 1979). Callus cell division, enlargement and differ- entiation occur best from mid-winter through to summer, when sap flow is heavy. In regions with definite seasonal climates, the cambium may cease activity and become dormant for peri- ods as long as from late-summer to early-spring (Fahn 1985). The cambium is often responsible for generating callus tissue, which means that a tree is best able to react to damage and close wounds at times of the year when the cambium is most active. The rate of wound closure in Prunus persica depended most on the average temperature after wounding (Biggs 1990); the warmer the temperature, the faster the heal- ing (Kozlowski et al. 1991). Moisture stress ©2017 International Society of Arboriculture
September 2017
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