296 Vogt et al.: The Costs of Maintaining and Not Maintaining the Urban Forest Figure 2. Hypothetical cost and benefit profiles over the life- time of an individual tree (street tree), with (solid lines) and without (dashed lines) adequate maintenance. Benefits are maximized during the mature phase of a tree, and decline rapidly through senescence, while costs show an inverse pattern. Compare the benefits and costs profiles over the course of a tree’s life cycle in this figure to the profiles over the tree size classes in Figure 5. Note that the benefits and costs profiles for an individual tree will vary depending on the tree’s location, the party benefiting from and incurring costs of the tree, and other factors (weather, etc.). Figure modified from Vogt et al. (2014). Maintenance throughout a tree’s life Maintenance can be linked to tree success both at the beginning and end of its life span. Early in a tree’s life, during the establishment and immature (i.e., juvenile) phases, maintenance must be ade- quate to ensure early survival and establishment in the urban landscape. Presumably, any post-planting maintenance performed on a tree that improves its chances of survival to maturity or lengthens the time that tree spends in its mature phase (where benefits are produced in the greatest amount) in- creases the monetary value of that tree (Figure 2). The cost of not maintaining trees early in life may translate to greater maintenance costs down the road; this is deferring maintenance (and its costs) to the future in order to save on maintenance costs today. Later in a tree’s life, maintenance may aim to extend the tree’s life span or prevent tree fail- ure. In this way, late-stage maintenance can defer removal costs. For instance, structural pruning or crown thinning may aim to improve tree stability and reduce canopy weight and thereby reduce the likelihood of tree failure (although the research supporting this connection is not well document- ed; Clark and Matheny 2010). If maintenance does ©2015 International Society of Arboriculture prolong a tree’s useful life (i.e., delays the onset of senescence and a tree’s removal), it increases the amount of benefits it produces over its life span. Alternatively, removing the dangling limbs on an aging tree can prevent these limbs from failing (during a wind storm or otherwise) and damaging people or property, and thereby avoiding subse- quent repair- or liability-related costs (e.g., Bak- ken 1995). Tree pruning to remove high-risk limbs and removal of the entire tree can be considered a type of maintenance that purportedly can save money due to avoided litigation costs (depending on frequency, likelihood, and costs of litigation). The stylized benefits and costs curves presented in Figure 2 are in reality influenced by tree location, weather, pests/diseases, and many other factors both controllable and not controllable by people. Useful concepts from economics Translating tree maintenance into urban forest benefits can be informed by some key concepts from economics. Accounting for the costs and benefits of any activity—from a factory producing widgets to the ecosystem services produced by the urban forest—involves principles of effective- ness and efficiency. Efficiency is the optimal use of resources (inputs) to produce a given output, and can be qualitatively or quantitatively expressed, such as the number of dollars spent to prune trees per one-diameter-inch of trunk size. Effectiveness is less measurable, aiming considering whether what is done actually works and achieves an ob- jective or outcome. Evaluation of efficient and effective use of resources also considers the ele- ment of time (when the maintenance occurred during a tree’s life cycle), the changing value of money (e.g., due to inflation; see also the “dollar values” in METHODS), as well as risk and un- certainty in the use of resources (not to mention human willingness to accept risk and uncertain- ty). Researchers consider these concepts implic- itly in the discussion of the literature that follows. RESULTS & DISCUSSION Summary of Publications Reviewed Over 300 articles were compiled, of which 163 were deemed useful for the literature review and includ- ed in the annotated bibliography and summaries
November 2015
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