Arboriculture & Urban Forestry 37(3): May 2011 able results, it is hoped that this assessment framework may once again become a leading model to be taken up by urban forest stew- ards interested in building livable and sustainable communities. CANOPY COVER All too often, urban forest management programs are driven by the need to increase urban forest canopy cover. Setting canopy cover goals has many implications, including the associated need for increased tree planting and the long-term resources re- quired to manage the expanding urban forest. The following dis- cussion on targets for canopy cover provides some context for policy makers and managers, and further justifies the need for a broader approach to urban forest assessment and management. The advantage of measuring canopy cover is that it is a sim- ple, intuitive indicator of the extent of the urban forest. However, measuring only canopy cover does not provide information about other essential parameters required to effectively manage and sustain a community’s urban forest. For example, canopy cover provides no indication of the species diversity of the forest, no measure of the condition of forest resources, and no indication of the age or size class distribution of the trees making up the urban forest. A popular target for urban forest canopy cover rec- ommended by American Forests is 40% (30% in arid regions) (American Forests 2009). While ambitious and desirable, for a variety of reasons this figure may be unattainable in many urban centers, and difficult to attain in others. Setting overly ambitious canopy cover targets can unduly focus urban forest management activities on tree planting. This could be to the detriment of other strategic and more comprehensive approaches to management. Also, canopy cover measurements alone are unable to provide an estimate of the carrying capacity of any particular part of a com- munity. For example, a commercial area may have a canopy cover of 10%, and this may be all that the area can support due to a high proportion of hard surface cover. Another part of the community may be dominated by light industry and also exhibit 10% canopy cover, but with the potential to support significantly more. Mea- suring canopy cover alone tells us little of this possible variation and does not reflect potential regional differences (Sanders 1984). Without a clear understanding of several factors that ultimately determine canopy cover, setting meaning- ful targets is a significant challenge. There are some fac- tors beyond the control of urban forest managers that may render canopy cover estimates unreliable, including: • Mortality rates: Little information is available about base- line tree mortality rates in urban forests (Nowak et al. 2004). • Climate change: The potential impacts of climate change in general, and extreme weather events more specifically, are difficult to predict. • Invasive insects: The potential impacts of invasive alien insects are difficult to predict but could have significant impacts. For example, Humble and Allen (2004) note that many invasive insects have been detected adjacent to the port of Vancouver, British Columbia, Canada, illustrating that continued vigilance is imperative and that tree mortal- ity rates must be considered with caution. • In-fill development/intensification: In established residen- tial neighborhoods, in-fill development can be expected to contribute to further losses of mature tree canopy. 109 • Tree habitat: Uncertainty about the availability and quality of growing space for new trees. • Land and tree tenure: A high proportion of the urban forest is under private ownership, placing a disproportionate reli- ance upon landowners to maintain and expand the urban forest. • Financial considerations: Budgetary and resource implica- tions for the long-term maintenance required to support ag- gressive tree establishment goals. Ideally, an assessment of a community’s potential canopy cover capacity, such as the USDA Forest Service’s Urban Tree Canopy (USDA Forest Service 2010) and Forest Opportunity Spectrum assessment, should be conducted before any mean- ingful targets are set. Similar methods have also been developed by Kenney (2008), Wua et al. (2008), Kirnbauer et al. (2009), and Monear and Hanou (2010). A high-quality potential canopy cover assessment should not only provide an indication of avail- able plantable spaces, but also take into consideration aboveg- round growing space for future canopy expansion, current and future land uses, regional climate and soils, and other key vari- ables that may affect tree growth and longevity. While effective tree establishment is important, it is only part of a strategy for sustainable urban forest management. The protection and main- tenance of the existing trees that form the community’s urban forest canopy is critical. Additionally, the importance of plan- ning for, and adequate funding of, tree maintenance and pro- tection throughout the life of the trees cannot be overstated. In recognition of the significant limitations of using tree canopy cover as a strategic objective for urban forest manage- ment, the study authors suggest an alternative means for setting and achieving management targets through the use of twenty-five criteria and indicators for urban forest sustainability. The con- cept of canopy cover is included as only one of these criteria. CRITERIA AND INDICATORS OF URBAN FOREST MANAGEMENT SUCCESS The concept of using criteria and indicators as sustainable for- est management tools originates from the 1994 meeting of the Working Group on Criteria and Indicators for the Conservation and Sustainable Management of Temperate and Boreal Forests in Geneva, Switzerland, as part of the Montréal Process. Since then, many sets of criteria and indicators of sustainable forest man- agement have been developed around the world. For example, the Canadian Council of Forest Ministers has published Criteria and Indicators of Sustainable Forest Management in Canada: National Status 2005 (Canadian Council of Forest Ministers 2006) to underscore the applicability of the C&I approach to more traditional production-based forest management planning. Urban forest managers must be able to clearly identify where specific goals or targets have been met and when adaptations to management approaches appear to be necessary. Assessing suc- cessful urban forest management therefore also requires clearly defined targets, or criteria, and specific performance indicators of success. The performance indicators enable measurement of progress towards the achievement of the key objectives for each criterion, which in turn permits the ongoing evaluation of suc- cess in implementing the community’s urban forest strategy. ©2011 International Society of Arboriculture
May 2011
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