«Proceedings of the th 14 National Street Tree Symposium 2013 ISBN: 978-0-9806814-1-3 TREENET Proceedings of the 14th National Street Tree Symposium ...»
When bark is removed from a tree accidently or by vandalism, the bark should be replaced in position immediately as natural grafting and callus growth can take place so that growing over occurs very quickly. This process can be described as bark patch grafting. The key to success is speed, as the bark that has been detached cannot be allowed to dry out nor can the damaged edges of the bark remaining on the tree dry. The work of Chandler (2009) showed that keeping eucalypt woody tissues moist facilitated successful callus growth and grafting success in Eucalyptus leucoxylon. Care must also be taken to replace the bark at the right orientation so that, for example, the part facing upward remains in that orientation and that there is as much contact as possible between the replaced patches of bark and the bark on the tree (McGarry, 2001). The bark can be held in place by any biodegradable material, but any fastening will suffice given the size and seriousness of the wound. Success may also be affected for some species by season, with better rates of patch grafts occurring in spring and autumn for eucalypts than in winter or mid-summer (McGarry, 2001).
Bridge grafting is a well-known horticultural technique that has a long history of use in repairing damaged orchard trees (Hartmann and Kester, 1975; Harris et al., 2004) but it has also been used to repair damaged ornamental trees of historic, heritage, cultural, landscape and horticulture significance which warrant the expenditure. The technique uses bark tissue from the same specimen, a clone, or the same species, which is inserted into the remaining healthy bark of a ringbarked or girdled tree. The objective of bridge grafting, as the name suggests, is to provide channels of connection of both xylem and phloem tissue that allow transport basipetally and acropetally once more (Figure 4). Success relies on healthy cambium producing callus at both ends of the grafted bark and the rate of success can be influences by species and seasonal factors. Bridge grafting requires skill and is quite expensive to undertake and so it is usually only contemplated for outstanding and significant trees. On a large tree, a number of grafts, up to 10 or more may be inserted, and the aesthetics of the outcome are sometimes questioned by arborists and the general public.
Figure 4. Bridge grafting of a ring-barked trunk (modified from Hartmann and Kester, 1975) Approach grafting and inarching are other well-known horticultural techniques used in repairing damaged orchard and valuable ornamental trees (Hartmann and Kester, 1975; Harris et al.
They differ in that for inarching, the top of the new rootstock plant does not extend above the point of the graft union. Inarching is considered to be a form of ‘repair grafting’. Both techniques involve growing young seedlings that are progeny of the damaged plant, clones or at least of the same species as the damaged plant around the base of the damaged tree. The young trees should be of a reasonable size (1-2m in height with a stem diameter of 20-25mm if possible) and the trunk or one of the larger branches is then inserted into the healthy cambium of the damaged tree above the upper cut of the ring-barked or girdled region. The objective of approach grafting is to provide water and nutrients to the part of the damage tree above the zone of ring-barking or girdling, but it does not provide for transport downwards to the original root system. However, if successful and given enough time the young tree root systems develop as the original system declines and in some cases natural root grafting between tree and seedling may occur (Tarroux and DesRochers, 2011). This technique is relevant when water is likely to be a limiting factor in the survival of a damaged tree and there is a significant risk of imminent wilting. Once more, approach grafting requires skill and is quite expensive to undertake. On a large tree a number of grafts, up to 6-8, or more grafts may be inserted, and the aesthetics of the outcome are sometimes questioned as there are a number of smaller trees growing around the trunk of the specimen.
Another aspect of post-damage management that an arborist can undertake is to minimize the risks from environmental stresses. For the most part this will involve making sure that water and nutrients are not limiting and that there is no risk of waterlogging to the already stressed root system. Good subsurface irrigation and drainage and proper mulching around the drip line would be useful practices. Post-damage control of pests and diseases is also wise (Priestley, 2004). Even partial ring-barking and girdling of trunks or larger branches exposes plants to significant stress which may leave them vulnerable to pest and disease attack. For example, the attack on E camauldensis by the psyllid, white lace lerp (Cardiaspina albitextura) was confined to trees that had been ring-barked or girdled and not to undamaged control trees that were largely unaffected (Priestley, 2004). Psyllids are attracted to high nutrient levels in foliage and population numbers increase rapidly in these conditions (Collett, 2001) which is consistent with ring-barking and girdling causing an increase in sugar and carbohydrate accumulation above the zone of damage (Kramer & Kozlowski, 1960). It is possible that these conditions might also suit some fungal pathogens.
Injections of sucrose into the soil have been reported to significantly improve fine root growth of established trees with responses dependent on species and the sugar concentrations applied (Percival et al, 2004). It is unclear whether the response is due to the direct uptake of the sugar by The 14th National Street Tree Symposium 2013 the roots or to enhanced mycorrhizal growth, which would also benefit the tree. The timing of such applications is also critical. It should not be too early after damage as the roots, under normal circumstances, should have sufficient carbohydrate reserves, but could be applied when carbohydrate resources are in danger of depletion. Measurement of carbohydrate concentration in root tissue could inform the timing of application. Care must also be undertaken to ensure that the injected sugar does not benefit non-target organisms.
Conclusion Depending on the tree and the conditions that it is growing under, ring-barking may not mean the death of a tree, but little can be done if a tree is effectively girdled severing the active xylem tissue. Arboricultural treatments that respond rapidly (within hours) to the removal of bark and which provide ideal growing conditions for the tree enhance the chances of recovery from ring-barking. Treatments may involve irrigation, mulching, prevention of compaction and waterlogging and effective pest and disease control.
Depending on the species, environmental conditions and the time of year, re-affixing displaced bark (bark or patch grafting) can be successful if it is done within hours of removal and the tissues, both intact and displaced, have not dried out. If successful, callus production can be very rapid and growing over can occur within months. If the tissues dry or cannot be replaced other interventions such as bridge or approach grafting may be contemplated, but they can affect the aesthetic value of the specimen.
It should also be understood that healthy vigorous trees that appear to have been fully ring-barked or girdled, on closer and detailed inspection may prove to have only been partially girdled or ring-barked. Such trees may survive with as little as 10-20% vascular connection or less if they are young and healthy. Under these circumstances, the “do nothing to the tree” option may be an appropriate response provided that good arboricultural management practices are implemented subsequent to the injury.
Acknowledgements The work of Sarah Priestly for her Industry Project Report at the Burnley Campus, University of Melbourne, is acknowledged. I also acknowledge the contribution of my students in the 2013 Graduate Certificate in Arboriculture, Urban Tree Growth and Function, particularly Sian Bloom, whose questions and essays on a related topic assisted in the writing of this paper. Ms E Moore, linguist, is thanked for her reading of and helpful suggestions for improving the manuscript. I also thank Dr Sue Hughes for her excellent and thought provoking reading of the manuscript which enhanced the work.
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