22 size range estimate is practical and effective for the use of seca- teurs. Below this size the branches are inconsequential and take little time to prune, and above this size the difficulty of pruning increases. The use of a handsaw over this size may also reduce the risk of musculoskeletal disorders developing in arborists. Pruning using a handsaw also showed a clear linear rela- tionship (R2 = 0.88) between branch size and pruning time (Table 7). The data obtained were mostly for branches in the 20 mm–40 mm range with an average time of 25 seconds tak- en to prune an average branch diameter of 36 mm. In broad scale tree management, knowledge of branch sizes would help in preparing tenders and contracts. Many larger branch removals would significantly increase the cost of pruning. Unlike secateurs and handsaws, the use of a pole prun- er did not return a linear regression (R2 = 0.08), due the con- founding factor of height (Table 7). The higher the branch to be pruned, the longer it took to remove, regardless of whether secateurs or handsaws were used. With trees up to 6.5 m, however, the number of higher large branches was limited. The total time required to prune trees included an allo- cated inspection time averaging 30 seconds per tree. While this seemed reasonable, small trees took significantly less time while larger trees took longer. The time taken to walk from tree to tree (6 seconds) was included in the total time. The time taken to formatively prune varied across spe- cies with Corymbia citriodora taking only 52 seconds on average, while Platanus × acerifolia averaged 218 seconds (Table 8). To convert these times to a current market val- ue, the time was multiplied by the cost of labor (Table 10). In order to compare the cost of formative pruning with structural pruning in a mature tree, structural pruning costs need to be known. A group of 37 trees in a car park were pruned for the first time at an age of approximately 20 years. The average cost per tree was $44.59. Assuming an inflation rate of between 3%–5%, the cost of this structural pruning of mature trees is projected to be approximately $80–$110 in 20 years (Table 12). Delaying pruning makes it more dif- ficult to correct structural defects by pruning, and so the cost increases accordingly. Furthermore, the potential for developing good structure later in the tree’s life is reduced. Formative pruning is designed to remove or prevent many of the structural defects that structural pruning tries to fix. An average cost of $2.79 to prune a tree is substantial- ly less than pruning a tree at age 20. This is not to say that formative pruning will completely erase the need for struc- tural pruning in the future. It should however reduce the need substantially. Formative pruning should be seen as a sound economic investment, improving trees into maturity. Corymbia citriodora had the lowest number of required cuts and time for all species. An average cost of $1 suggested that they are very economical to maintain. While there was a high incidence of codominant stems with included bark, their early removal would rectify this fault. Alternatively, failure to prune could result in the tree needing to be replaced. A cost benefit analysis could be performed to decide which management approach to take. Ryder and Moore: Economic Benefits of Formative Pruning Platanus × acerifolia had the highest number of cuts and longest time for all species. More than three and a half minutes were required for each specimen, totaling more than $4 per tree. They are a tree commonly planted in streets for their tolerance to adverse conditions, but there is quite a lot of work maintaining them to maturity. These trees were very healthy and vigorous and their rapid growth may have led to a loss of apical dominance and hence the need for greater levels of pruning. Cultivar selection may improve the requirement for formative pruning in the spe- cies; however, it is clear that some pruning to establish a strong framework is required. This highlights the need for formative pruning within nurseries before trees are consigned for planting. Pyrus calleryana had a high time value, despite its smaller size. Many of the trees were prone to codominant stems and included bark. Approximately two and a half minutes was required per tree, costing $2.76. There is variation in this species with different cultivars developing different canopy structures. The more fasti- giate form, Pyrus calleryana cv chanticleer, demonstrated higher numbers of structural defects. The decision to use this tree should be seriously considered as it provided the lowest cost benefit ratio of ten street tree species in the United States (McPherson 2003). Quercus palustris was in general a well structured and healthy species. Although it was on average the tallest species sam- pled, work required on this tree was the second lowest, with an average cost of $1.62. Little pruning was required to establish a strong trunk as Q. palustris naturally formed an excurrent shape. Although the average height of Ulmus parvifolia was the second lowest for all species, the recorded works and time required were the second highest at nearly three minutes per tree, at a cost of $3.25. This tree did not naturally form a straight trunk and a lot of work was required to develop one. The tree had a tendency to produce many large lower limbs, often with included bark, which would have to be removed. These trees required a lot of work with handsaws to remove lower limbs. Creating a well-structured and mature U. parvifolia appears from this study to require a substantial input of resources. A proper formative pruning regime may require at least two cycles. Assuming good quality planting stock has been used, pruning at planting shouldn’t be required. If the trees were then pruned after three years and again at seven years, the total cost should be approximately $6.40–$7.00 per tree (Table 13). This cost makes formative pruning a cost effec- tive exercise that could potentially improve the structure of the trees in urban environments, and represent a significant saving for large tree populations. Pruning in cycles would see formative pruning crews established for this explicit purpose. Table 13. Total cost for two formative pruning cycles for trees that have been formatively pruned prior to delivery from the nursery. Currency is represented in Australian dollars. Year 1 4 7 Total 3% - $3.05 $3.33 $6.38 4% - $3.14 $3.53 $6.67 5% - $3.23 $3.74 $6.97 ©2013 International Society of Arboriculture
January 2013
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