Arboriculture & Urban Forestry 41(6): November 2015 are interested in, but the difference between more maintenance and less maintenance. In this vein, future experimental research should track the impacts of different levels of maintenance or maintenance regimes (Table 3). Borrowing lan- guage ecologists use to talk about disturbance (e.g., Walker and Willig 1999), the level of urban forest maintenance can be described as occurring at a particular intensity (how much), frequency (how often), duration (for how long), and extent (what part of a tree or which trees). Who per- forms the maintenance (e.g., an adjacent home- owner, municipal forestry crews, contractor) and what type of maintenance they perform (e.g., watering, pruning) also matters. The combina- tion of the types of maintenance activities per- formed with any given frequency, intensity, and duration across a particular area of the urban forest (extent) can be called a maintenance re- gime. For example, city forestry crews will water the trees on 3rd Street with 56.78 L of water each week that it does not rain at least 2.54 cm, for three summers after transplanting. It would be helpful to be able to compare the economic costs (e.g., dollar expenditure) of different main- tenance regimes with the tree outcomes (e.g., structure, function) and benefits produced. This would allow urban forest managers to de- termine ideal maintenance regimes, given the available budget resources and desired goals (e.g., benefits). [Note that this concept of main- tenance regime is similar to the “maintenance intensity” idea used by Escobedo et al. (2011).] Long-Term Data on Maintenance A final piece of information that is lacking in the current literature is long-term data about Table 3. Key elements of an urban forest maintenance regime. Element Explanation Type Who Intensity Frequency Duration Extent The particular maintenance activity Party physically performing maintenance activity How much How oſten How long What part of a tree or which trees 315 the impacts of levels of maintenance on tree longevity or lifetime-benefits produced. Al- though municipal or other tree inventories commonly include maintenance recommen- dations of some sort, these inventories are rarely updated to capture performed main- tenance activities or the costs of these activi- ties. Incorporating procedures for captur- ing costs into repeated inventories would help provide long-term data on maintenance regimes and their impact on tree populations. CONCLUSION A wealth of approaches and studies document the benefits of urban trees, forests, and greenspaces; research on costs has yet to catch up. Several take home messages emerged through this literature re- view. First, the costs of maintaining trees are clearer than are the costs of not maintaining trees in the urban forest. Municipal budget expenditures are the primary source of dollar figures used in benefit- cost analyses of tree populations, and so the mon- etary costs of maintenance commonly tracked by municipalities (planting, pruning, removal, pest management, and sometimes infrastructure repair) are fairly well understood. It is rare to find figures reported in the literature for types of maintenance not featured as line items in municipal budgets. Second, the costs of not maintaining trees does not only equate to deferring maintenance to a later date; rather, urban forest managers at the municipal level must oſten make the deci- sion (or non-decision) to not care for trees, and there can be a resultant cost much later in a tree’s life span that was not anticipated. These costs are then leſt for the next generation of urban foresters and city residents to deal with. Examples Prune, mulch, stake, etc. City tree crews, contracted certified arborist, nonprofit, adjacent homeowner, etc. 20% of the crown, 18.9 L or 57.8 L of water, etc. A four- or six-year pruning cycle, once per week, etc. For just the first growing season aſter transplanting, throughout a tree’s life, etc. Branches below 4.3 m clearance level, all trees in a city, trees on heavily traveled road corridors, trees in a downtown area, etc. ©2015 International Society of Arboriculture
November 2015
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