298 Vogt et al.: The Costs of Maintaining and Not Maintaining the Urban Forest Direct costs associated with provisioning the urban forest Maintenance-related costs begin at the time of planting (also called installation costs) and con- tinue throughout a tree’s useful life through the time of removal. Maintenance costs vary through- out a tree’s lifetime and by species and location (Schwarz and Wagar 1987; McPherson 2003; Leal et al. 2008). Planting and establishment costs can include the cost of purchasing the tree, any costs associated with reworking infrastructure around the tree (e.g., removing sidewalk or paving stones, modifying or amending soil in the plant- ing space), the cost of labor to install the tree, and any subsequent at-planting maintenance, such as staking, pruning, mulching, or watering. Tree maintenance activities such as pruning, mulch- ing, watering, and pest and disease management also occur throughout the lifetime of the tree and have costs during this time as well. Finally, dur- ing senescence, tree maintenance costs related to the removal of dead branches to reduce liability and risk and eventually removal of the entire tree could be substantial. This pattern of maintenance is what drives the theoretical costs throughout a tree’s life cycle that are seen in Figure 2, and dif- ferent levels of maintenance at different points in time may affect subsequent maintenance needs. Costs associated with tree interference with infrastructure Costs that result from infrastructure interference, damage, and repair are usually incurred when an improperly selected tree is used, the tree is not planted correctly, or is planted in an inappropriate location. Vegetation improperly located may block road signs, leading to vehicle accidents, or block business signs, decreasing visibility of a store. Side- walk or street repairs are commonly observed for trees that are planted in too small of planting areas and thus their root systems damage the pavement (see section "Infrastructure repair"). Additionally, costs can be incurred due to lack of service result- ing from tree-initiated power outages (e.g., a tree falling on a power line). Fire may result from tree contacts with electrical lines. Tree-caused power outages result in the costs of repairing the line and restoring service as well as cause lost electric- ity revenue during the outage (see section "Util- ity pruning costs"). Tree or branch failures also ©2015 International Society of Arboriculture have costs in terms of roads blocked either due to the failed tree itself or to the tree crew cleaning- up the failed tree or branch (Randrup et al. 2001). Negative externalities as costs Externalities are outcomes (benefits or costs) of a good or service that are not accounted for in the market price of that good or service. Most of the benefits of urban trees (e.g., stormwater manage- ment, aesthetic beauty) can be considered posi- tive externalities. However, there are also negative externalities of trees. Costs related to the negative environmental externalities (occasionally called “ecosystem disservices,” per Escobedo et al. 2011) of trees include net emission of biogenic volatile or- ganic compounds (BVOCs) during the life of some tree species, leachate from foliar nutrients into sur- face water, as well as release of carbon dioxide and other greenhouse gases (e.g., methane) during de- composition at the end of a tree’s life. Environmen- tal costs can be related to the maintenance regime used by urban foresters taking care of an urban tree population. For instance, tree maintenance requir- ing the use of equipment (e.g., a front end loader, chip truck, aerial liſt truck, wood chipper, or a chain saw) that burn fossil fuels releases of car- bon dioxide into the atmosphere [see, for instance, Nowak et al. (2002), described in next section]. Life cycle assessment of costs Life cycle assessment is one means of assessing the externality-related costs of urban trees that takes into account the entire life cycle of a tree from pro- duction (i.e., nursery) to removal and disposal. One common cost examined via life cycle assessment is greenhouse gas or carbon emissions resulting from urban forest activities. For instance, Nowak et al. (2002) examine the impact of minimal (low- carbon), “conservative” (i.e., deferred), and “inten- sive” maintenance scenarios on the life cycle net carbon balance of planted urban trees: Minimal (low-carbon) maintenance involved no return visits aſter tree planting, while conservative and intensive maintenance scenarios involved pruning visits with chain saws, an aerial-liſt truck, and a wood chipper every 15 and 7 years, respectively. Their analysis revealed that conservative and intensive main- tenance had a negative impact on a tree’s carbon budget, resulting in a decrease in the length of time
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