«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 ...»
The ratio of the amount of phloem tissue to xylem tissue may be as high as 1 to 4, but is more usually about 1 to 6 and in many tree species is closer to 1 to 10 (Fahn, 1975). The velocity of the movement of solutes through the phloem over long distances can be quite rapid, varying from about 100 – 1000 mm per hour (Fahn 1975, Salisbury and Ross 1992; Atwell et al, 1999) with sieve tube cells emptying between 3 to 10 times per second (Fahn, 1975; Salisbury and Ross 1992; Atwell et al, 1999). Interruption to phloem transport by ringbarking and girdling would lead to rapid depletion of carbohydrates.
The 14th National Street Tree Symposium 2013 Table 1. Some natural and human causes of ring-barking and/or girdling Human caused Ring-barking/Girdling Naturally caused Ring-barking/Girdling Agricultural killing of trees to clear paddocks Grazing by animals, particularly horses Foresters killing selected trees to thin stands Stripping of bark by birds, such as cockatoos Orchardists killing branches and controlling Tunneling insects under the bark grazing on bark vegetative growth and cambial tissues to excess Orchardists controlling fruit, yield, size and sugar Fungal diseases, such as collar rot content Placement of wires and nylon ropes around tree Circling or girdling roots which can occur naturally, trunks and branches as well as from poor nursery propagation Unintended damage from use of poor staking Trunk damage from rocks, such as in trees germinating in crevices Unintended damage from mowers and Bark eating rodents whippersnippers Unintended root girdling of the stem by roots due to poor propagation and/or planting techniques Accidental damage from motor vehicle impact Unintended damage from construction works Deliberate vandalism to trunk and branches Unintended trunk damage from pavements and hard surfaces The most immediate effect of these changes in transport is that hormones synthesised in the roots no longer travel above the zone of ring-barking and those produced by the foliage no longer reach the roots below.
Often it is the interaction of different hormones at appropriate concentrations that affect the physiological responses and so root and shoot growth and development can be impacted. Over the longer term, however, it is the failure of photosynthate to reach the root system that has significant consequences that can kill the tree.
For some time after damage, growth and both branch and trunk incremental increases above the zone of ringbarking continue. Indeed foliage condition may improve and incremental growth rates increase as all of the carbohydrate produced by the foliage remains in the region of the trunk and canopy, as none is able to reach the root system. So the trunk above the ring-barked zone increases in girth and there is often a noticeable swelling above the ring-barking cut. Growth below the cut slows and eventually ceases and so an obvious difference develops in the trunk diameters above and below the ring-barking zone (Figure 1).
Immediately after ring-barking, most trees have sufficient carbohydrate reserves in the root cells to maintain an active cell metabolism and root growth. However, as time passes these reserves are gradually consumed, at which point root growth ceases and root cells begin to starve from lack of carbohydrate (Salisbury& Ross, 1992; Taiz and Zeiger, 2002). Water and nutrient uptake is then affected and the tree starts to shed foliage, foliage becomes chlorotic and finally, and often quite suddenly, the tree wilts and the plant above the zone of ring-barking dies, which may result in the death of the whole plant.
The 14th National Street Tree Symposium 2013 Figure 1. Swelling above the cut on a ring-barked stem (Taiz and Zeiger, 2002) For a large tree with substantial carbohydrate reserves and a good root system, this process may take place over a period of between 2 to 5 years. However, if there are additional environmental stresses such as drought, flooding or waterlogging the decline of the tree will be accelerated.
Girdling affecting xylem and phloem tissues and transport When girdling occurs, both translocation through phloem and transpiration through the xylem tissues are affected. However, the effect on transpiration is immediate as water supply to the trunk and canopy above the zone of girdling is cut and so on a warm windy day, wilting can begin almost immediately (McLuckie and McKee, 1954; Kramer & Kozlowski, 1960; Taiz and Zeiger, 2002). For most of the canopy and trunk above the girdling cut, permanent wilting will be reached within 24-48 hours depending on the size of the tree and environmental conditions. This girdling is a very effective method of killing plant tissues above the cut and the effects are almost immediate.
In contrast to transport through phloem tissue, transport of water and nutrients can be both symplastic and apoplastic (Figure 2). The latter is the movement of water and dissolved substances through the non-living cell walls and intercellular spaces of the plant. It is often forgotten that movement through the cell walls and intercellular spaces on a large tree can be quite significant and it is this movement and the properties of water, that go a long way to explaining why tissues immediately above cuts made in the trunk may not dry out or die.
This may also explain why trees with major cuts though their trunks remain hydrated, healthy and growing.
It should also be noted that some species have anomalous secondary growth (Esau 1965). Such growth may result in some trees having alternating rings of cambia, xylem and phloem while others have lobes of xylem alternating with phloem. For some species from some dicotyledonous plant families, including Myrtaceae, phloem may occur inside as well as outside the xylem (Fahn, 1974). This intraxylary phloem may make it difficult to effectively ring-bark or girdle trees that exhibit this unusual structure and may explain why some juvenile trees which appear to be ring-barked or girdled remain unaffected.
Figure 2. Symplastic and apoplastic pathways of plant transport (Salisbury and Ross, 1992)
The importance of the ring-barking and girdling cut and tree responses:
The physiological response of a tree is also influenced by the depth, width and location of the cuts made to affect the ring-barking and girdling (Figure 3). If the width of the cut made is quite narrow then the tree may be able to grow over the cut by producing callus, which can differentiate into woundwood within weeks to a few months (Neely, 1988; Goren et al, 2004). Trees are well-known to have simply grown over wire and other narrow obstructions, and ring-barking bands narrower than 100-150mm have been known to be grown over by large mature trees with substantial girths and carbohydrate reserves. Deliberate attempts to kill trees by ignorant or lazy vandals have also been thwarted when the cut narrow band (as narrow as 20-25mm) was simply grown over within a few months and the tree remained healthy and vigorous.
Figure 3. The width and depth of the cut affects the tree’s response to ring-barking Ring-barking and girdling are large wounds and the usual tree response is to produce callus from the cambium at the margins surrounding the damage.
Callus production is greatest in vigorous trees but is affected by tree size, species and season (Neely, 1988). Spring, particularly early in the growing season, is typified by very fast responses to wounding and very rapid callus production, which can cover the damaged surface. Callus and The 14th National Street Tree Symposium 2013 woundwood that predominantly develops from xylem ray cells grow best when xylem tissue growth is most active (Harris et al, 2004).
If the tree has dormant buds, such as axillary or epicormic buds, below the cut made to ring-bark or girdle a tree, these may be stimulated to develop by the cessation of basipetal transport of auxins from the canopy.
The auxins will be the primary hormone involved in the inhibition of these dormant buds. If these buds develop with sufficient speed and grow to be large enough, they may send photosynthate down to the root system which will continue to absorb and supply water and nutrients to the canopy. Similarly trees that have adventitious buds or roots may provide a system for circumventing the damage from ring-barking and girdling.
In these situations it is possible that the parts of the tree above and below the ring-barking cut may survive for very long periods of time and even many decades. Species that can produce adventitious roots, such as species propagated by layering, for example Ficus species or some river red gums, E camaldulensis, are capable of surviving for decades, and possibly centuries under such circumstances. However, the part above the cut usually eventually becomes stressed from environmental factors, such a drought or waterlogging, or the impact of insect grazing.
Another important aspect of ring-barking and girdling is the extent to which it occurs. There may be full or partial ring-barking and girdling of the trunk or major branches and stems. The effects of full ring-barking and girdling are clear, but a question arises as to how much of the vascular tissue needs to be intact for a tree to survive and recover over the longer term. Unfortunately there is little published information on this matter (Priestley, 2004) but it is known that there is variability in response for different species of trees, which is also influenced by season and environmental conditions (Neely, 1988).
Trees have certainly survived ring-barking and girdling to 50% of their trunk vascular tissues (Homes, 1984) and young trees of Eucalyptus camaldulensis, Platanus orientalis and Acacia melanoxylon survived and recovered from 60, 75, 90 and even 100% damage (Priestley 2004). Furthermore, foresters trying to kill weedy woody species, such as beech, poplar and some maple species by girdling have reported how difficult it can be (Glass, 2011; Kilroy and Windell, 1999). For the white poplar, Populus alba, which has the capacity to prolifically sucker, it has been reported that new bark can develop over the cuts in a matter of weeks (Glass, 2011).
The author’s observations have been that as little as 10% vascular connection can be enough for trees to remain healthy, if the tree is growing in ideal situations and is kept free of pests and diseases (Moore, 2011).
Deliberate attempts to kill the historic Separation Tree in the Royal Botanic Gardens Melbourne in 2010 by ring-barking or girdling reported that a band of bark between 400 and 900mm wide was removed from 80% of the circumference (Fagg, 2012; Moore, 2011). With 20% vascular connection, the tree remains in full foliage, healthy and both callus and woundwood have been produced expanding the vascular connection. The woundwood differentiates into xylem and phloem tissues and new vascular cambium is also developed (Harris et al, 2004).
Other effects of Ring-barking and Girdling One of the reported consequences of ring-barking has been an increase in fruiting and flowering, which is often attributed to the retention of and higher levels of carbohydrate in the canopy of the tree (Kramer and Kozlowski, 1960), as well as a survival response at times of extreme stress (Taiz and Zeiger, 2002). This response is the basis of the use of horticultural girdling and ring-barking, which usually leaves between 10-20% of the vascular connection intact (Goren et al, 2004). However, there are little, if any data published on plant longevity after ring-barking or girdling.
In research on the effects of ring-barking and girdling young trees of Eucalyptus camaldulensis andPlatanus orientalis trees were girdled and Acacia melanoxylon, trees were ring-barked for 60, 75, 90 and 100% of their girth (Priestley, 2004) using the definitions of ring-barking and girdling presented earlier. While the depths of cut were different, the results were not as there were no apparent differences between trees in their responses regardless of whether they had been ring-barked or girdled.
Interestingly, whole tree or above cut deaths only occurred in the 100% treatment. All specimens survived even 90% ring-barking or girdling, probably because the experiment was conducted over a 13 week period which was not long enough for plants to die and because the trees were juvenile and vigorous, they simply grew over the cuts that were made to the trunks. Callus tissue is produced by repeated divisions of the most recent derivatives from the cambium with the majority of callus (parenchyma) cells originating from cells The 14th National Street Tree Symposium 2013 destined to form xylem rays (Neely, 1988). Young trees would contain a lot of such tissue. However, a number
of interesting other effects emerged (Priestley, 2004):
For P. orientalis, the more severe the treatment the slower the bud burst in spring and the less dense the canopy that subsequently developed (a greater response as you go from 60-100%). Later in the season the numbers of fruits produced by the 90 and 100% treatments were significantly lower averaging 6.25 and 4.00 per tree respectively compared to 14.25 for untreated controls.
For P. orientalis, the more severe the treatment the greater the number of branches that were shed from these young tree (again, a greater response as you go from 60-100%).
For E camaldulensis, the undamaged controls showed an average increase in height of about 62mm, while none of the girdled treatments average over 30mm and most were considerably less.
For E camaldulensis, the level of Psyllid infection at the end of the experiment was between 60 and 90% for ring-barked specimens compared with an infection rate of 12.5% in undamaged controls.
For A melanoxylon, there was an effect for infection with leaf blight but in the reverse direction. The blight affected control plants but was much reduced for the most severely ring-barked treatments.
What these data reveal is that even incomplete ring-barking or girdling can affect the growth and development of injured trees as well as their responses to pest and diseases.
Arboricultural treatments for Ring-barking and Girdling
A number of arboricultural treatments for ring-barking and girdling have been suggested, including: