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«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 ...»

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It is expected that with some modifications this method can be used to analyse existing tree populations and provide guidance on tree removal and planting management.

Method The likelihood of a tree requiring removal at a certain age may be represented by a notional “Hazard Function” where the probability of removal varies with the age of the tree. In the example (Figure One) the assumption is that when a street tree is in the first few years of life there is an increased probability of removal due to factors such as transplant shock, followed by a period of lower probability of removal while the tree remains vigorous and the final stages where the tree is more likely to require removal due to mortality. The area under this curve will equal a probability of 1.0, i.e. after a period of time (in this example 100 years) all of the trees in this or greater age classes will have been removed. If the removal of trees follows the function in Figure One then the age class distribution is likely to look something like that in Figure Two. It should be noted that it is assumed that removed trees are replaced almost immediately.

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Figure 2. Variable Hazard Function Age Class Distribution The 14th National Street Tree Symposium 2013 It may be possible to conduct further investigation to make this notional hazard function more accurate, however this would involve a much higher level of data capture that may not be justified by an increase in ‘usefulness’ of this model.

The simplest form of hazard function is one where there is an equal probability of a tree requiring removal at any age. Using this function the age class distribution in a tree population will be represented by a negative exponential curve (see Figure Three). Figure Four shows that for practical purposes using either hazard function the result is similar and as the mathematics of a constant hazard function is much easier it is the one used as the starting point for this analysis.

The formula for the negative exponential curve representing the age classes takes the form:

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Maximum Life Span (m) - is the maximum amount of time a tree is likely to exist in a street The average number of individuals in the first age class period (assuming a 1 year age class period) will be equal to the average number of trees removed each year. The average number of individuals in the last age class period will equal × total . By using the number of individuals in the first age class (afirst) and the number of individuals in the final age class (alast) the above formula can be resolved.

Once the formula has been resolved the theoretical number of individuals can be calculated and compared to field data or planting records to identify where certain age classes are appropriately, under or over represented within the population.

Case Study To illustrate this concept data from the Shire of Macedon Ranges were analysed. The data were collected as part of a risk mitigation and works scheduling inventory conducted in 2011. The age of each tree were captured into estimated ranges.

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= 506.088e−0.0121 We can then use a spreadsheet to calculate the theoretical numbers of individuals within a given age class to compare it to the field data. Figure Five shows the comparison from the data collected during the inventory and the calculation of the theoretical numbers of individuals.

The analysis of the data suggests that the Shire of Macedon Ranges is over represented in the Less than 10 years and 10-20 years age classes and underrepresented in the 20-50 years, 50-80 years and 80+ years age classes.

While there is very little that can be done to rectify these figures in the short term as it is not possible to establish 20+ year old trees on a large scale, there may be an issue with tree survival after 20 years that has caused this imbalance and would be worth investigating. For example, a large proportion of the trees in the Less than 10 years age class are natural regeneration in rural roadsides and it may be that after 20 years competition between individuals results in a significantly higher mortality rate.

Discussion The aim of this paper was not to provide a detailed analysis of a particular municipal tree population but rather to explain and demonstrate a particular type of analysis. To that end there are several assumptions that

we must be mindful of while interpreting the results of the analysis. These include:

 The analysis is based on the assumption that all available planting spaces are filled. In most municipal areas this is unlikely to be true, however if the vacant spaces are excluded from the analysis the results are still valid.

 It is difficult to estimate the age of a tree. This can be overcome by using wide age class ranges that decrease the likelihood of miscategorisation. It also highlights the usefulness of recording planting year for all future plantings.

This type of analysis is not intended to drive wholesale tree removals from within age classes to try and even up the theoretical balance, but following the normal routine tree removals in a municipality there may be an opportunity for the removal of an appropriate number of the worst specimens from an over represented age class. In conjunction with other available information this could be a tool that contributes to the sustainability of the tree population.

Provided the assumptions and limitations of any input data are acknowledged and accounted for in the analysis, simple models such as this one can provide great insights into the current state of the tree population and likely future scenarios.

The 14th National Street Tree Symposium 2013 Figure 5. Shire of Macedon Ranges- Theoretical vs. Actual Tree Numbers The 14th National Street Tree Symposium 2013 References Frank, S., Waters, G., Beer, R. and May, P., 2006. An analysis of the street tree population of greater Melbourne at the beginning of the 21st century. Journal of arboriculture, 32(4), pp. 155-163.

Keller, J. and Konijnendijk, C., 2012. Short communication: A comparative analysis of municipal urban tree inventories of selected major cities in North America and Europe. Journal of arboriculture, 38(1), pp. 24-30.

Kirnbauer, M., Kenney, W., Churchill, C. and BAETZ, B., 2009. A prototype decision support system for sustainable urban tree planting programs. Urban forestry & urban greening, 8(1), pp. 3-19.

Ningal, T., Mills, G. and Smithwick, P., 2010. An inventory of trees in Dublin city centre. Irish geography, 43(2), pp. 161-176.

Ordóñez, C. and Duniker, P., 2013. An analysis of urban forest management plans in Canada: Implications for urban forest management. Landscape and Urban Planning, 116, pp. 36-47.

Thaiutsa, B., Puangchit, L., Kjelgren, R. and Arunpraparut, W., 2008. Urban green space, street tree and heritage large tree assessment in Bangkok, Thailand. Urban forestry & urban greening, 7(3), pp. 219-229.

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Abstract Girdling and ring-barking of trees occurs for many reasons - vehicle impact, grazing by animals, insect and fungal attack and human vandalism. Ring-barking that removes only phloem and cambial tissue has a vastly different impact on tree physiology than girdling which removes phloem, cambial and xylem tissue. Girdling has an almost immediate effect on transpiration and so plants wilt quickly and tissues can die within days.

Ring-barking however effects translocation and so causes a slow starvation of the root system and can take many months or even years before root tissues start to die from starvation and the tree wilts and dies.

Many trees survive partial ring-barking, but how much of the vascular tissue needs to be intact for plants to survive and what effective treatments are available, if any, to arborists for improving the chances of tree survival? As little as 10% of vascular tissue may be all that is required for an otherwise healthy and vigorous young tree to survive and recover. Furthermore, while ring-barking and girdling may not kill a tree they may leave it vulnerable to attack from insect pests and fungal diseases.

Tree management techniques such as bridge, approach or patch grafting may be considered as remedial treatments for ring-barked or girdled trees. Soil injection of sugar solutions may also be of benefit. However, making sure that the tree is free from environmental stresses and pest and diseases are important to recovery and allowing time for the tree to produce callus and wound wood.

Introduction It is unfortunate the terms girdling, ring barking, ring-barking, ringbarking (ring-barking with the hyphen seems to about twice as common in use as ringbarking) and even ringing are used as synonyms to refer to the removal of a band or strip of bark which contains the cork and cork cambium, phloem and usually the cambium around the entire circumference of a tree (Salisbury and Ross, 1992; Raven et al, 2005). It is unfortunate that so many terms are used for the same imprecisely specified action, as the depth to which the band of tissue is cut can markedly affect the impact on the tree’s vascular system and the subsequent effect on the tree of the action.

In this paper, ring-barking will be defined as a circumferential cut made around the trunk of a tree which removes a band of tissue to the depth of an including the cambium. Such a cut removes a band which contains cork and cork cambium, phloem tissues and the cambium and so has an immediate impact on the translocation of materials in the phloem tissues. Girdling will be defined as a circumferential cut made around the trunk of a tree which removes a band of tissue to the depth of the active or functional xylem tissues. Such a cut removes a band which contains cork and cork cambium, phloem tissues, the cambium and the current season’s active or functional xylem tissue growth ring and so has an immediate impact on both the translocational and transpirational processes. The only way of telling whether a tree has been ring-barked or girdled is to examine the tissues which have been severed.

Because the effects and consequences of ring-barking and girdling on trees are so different and impact on the tree over such different time scales, it would probably be wise if different terms were adopted for the different actions. Such a distinction would bring clarity of meaning to the terms, aid in diagnosis of injury to trees, avoid ambiguity, aid in defining remedial treatment and benefit the legal system in matters related to litigation involving either activity.

Causes of Ring-barking and Girdling There are many different causes of ring-barking and girdling from both natural and human interventions (Table 1). Both ring-barking and girdling have a long history of being used as management tools in forestry and agriculture for clearing land and removing trees from paddocks (Stubbs, 1998). It is cost effective for The 14th National Street Tree Symposium 2013 selectively thinning forests and plantations and for the control of invasive woody species (Kilroy and Windell, 1999). In more subtle ways, orchardists and horticulturists have used ring-barking and girdling to manipulate plant growth form, soluble sugar content and fruit yield and production, but they are careful not to completely ring-bark the whole stem or selected branch (Hartmann et al, 1981; Goren et al, 2004).

In urban arboriculture, the most common causes of ring-barking and girdling arise from accidents, poor landscape management practices and attempts to kill trees. Accidental occurrences include motor vehicle accidents and wire and other non-degradable materials tied around tree trunks. Poor landscape management practices such as girdling roots from poor propagation technique, mower and whippersnipper damage, poor staking and tree guards, and pavement and concrete surrounding trunks in paved areas can all cause serious damage. Finally, there are attempts at killing trees in disputes between neighbours and acts of deliberate vandalism (Harris et al, 2004).

Not all ring-barking and girdling damage, however, is caused by human action as animal grazing, fungal and insect attack and poor root growth habit can occur naturally. While these cases are not all that common, damage by sulphur-crested cockatoos (Cacatua galerita) can be extensive and cause significant structural damage to large trees. Horses have also been known to ring-bark large trees in their paddocks by grazing the bark in great strips, apparently to meet a nutrient deficiency. Insect and fungal damage to trunks and large limbs is not uncommon, but usually affects trees that are already stressed.

The physiology of Ring-barking and Girdling

Ring-barking affecting phloem tissues and transport:

The effects of ring-barking, as defined earlier, on the physiology of a tree are dramatically different from the effects of girdling. The removal of the bark and cambium only has an impact on translocation via the phloem tissues, but water and nutrient transport continues as xylem tissues are undamaged (Weier et al, 1982;

Salisbury and Ross 1992). The removal of phloem tissues affects transport of complex organic molecules such as sugars, amino acids and hormones, as well as other simpler substances dissolved in the phloem sap (Holmes, 1984). Transport of these substances from roots to foliage and stem above the region ring-barked is halted but so too is transport from the foliage to the root system, especially of photosynthates and hormones.

The direction of transport through phloem tissues and its impact on tree physiology can also vary according to the seasons. During periods of active growth when photosynthetic activity is high, transport is often predominantly basipetal from foliage to roots. However, in deciduous species coming out of dormancy in early spring, transport may be predominantly acropetal as carbohydrate stored in the roots and trunks is mobilized to facilitate bud burst and leaf production. Translocation and phloem transport is symplastic movement of substances through the interconnected cytoplasm of interconnected living cells (Salisbury and Ross, 1992).

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