Arboriculture & Urban Forestry 43(1): January 2017 A more difficult issue to mitigate may be the avail- ability of less prevalent street tree species and gen- era. One possible strategy for improving availability is to implement forward contracting, in which the municipality enters into a formal agreement with a tree supplier to grow a stipulated number of trees and tree species at a predetermined price for delivery at a future date. For example, to meet the demands of its MillionTreesNYC tree planting cam- paign and increase street tree diversity, New York City contracted with a number of nurseries to grow less prevalent tree species and genera (Ries 2009). Another possible strategy is for a municipality to grow its own street trees. For example, in the 1990s, unable to obtain a desired mix of new plantings, Columbus, Ohio, decided to expand its municipal nursery to produce the trees required (Sydnor et al. 2010). However, while forward contracting and municipal tree production may be successful strate- gies for improving the availability of less prevalent street tree species and genera, as well as increasing street tree diversity, they may not be feasible for many if not most municipalities, especially munici- palities of smaller size who must buy from the exist- ing stock of wholesale tree nurseries. This stock was depleted by the “great recession” of 2008–2009, when many tree growers either went out of business or downsized. As a result, there is currently a short- age in the United States of 7.6 cm caliper trees for new plantings until at least 2017 (McClellan 2014; KAT 2015). Nevertheless, the demand for less prev- alent street tree species and genera exists and may in fact be increasing, due at least in part to shortages of previously overproduced species, such as red maple, zelkova, and pin oak (Rodda 2014), and also to the need for alternatives to ash in response to emer- ald ash borer (Zawislak 2015). Therefore, there is a need to better understand the factors influencing nursery growing decisions and how to best encour- age nurseries to make diverse species and genera available for sale (Conway and Vander Vecht 2015). Implicit in this discussion are issues of time and scale. Just as it takes four to five years for growers to get trees to selling size (KAT 2015), increases in municipal street tree diversity cannot be achieved overnight. Apart from preemptive removals to deal with an invasive pest or disease, municipalities are not going to remove healthy, well-performing street trees simply to increase diversity. Therefore, although 11 most municipalities have no shortage of vacant sites where new street trees can be planted, significant structural change in municipal street tree species and genus composition can likely only take place through a consistent, long-term strategy of replacing more prevalent species and genera that have reached the end of their life cycles with less prevalent spe- cies and genera appropriate to planting conditions. For municipalities where the street tree population is more aged, the transition to greater diversity may be accomplished more quickly than in municipali- ties where the street tree population is younger. Scale will also play a role, since, strictly in terms of the number of trees required, it is an easier task to increase species and genus diversity in a smaller sized municipality that has fewer street trees overall than in a larger sized municipality with more trees. For example, the percentages of Acer spp. in the street tree populations of Syracuse, New York, and Hastings-on-Hudson, New York, based on 2014 and 2013 street tree inventories, respectively, are relatively similar, with 35% in Syracuse and 39% in Hastings-on-Hudson. Syracuse is a much larger municipality with a much larger street tree popu- lation. Lowering the percentage of maples in Syra- cuse to 20% of the overall street tree population would entail replacing 4,800 street trees with other street tree genera, whereas to accomplish the same goal in Hastings-on-Hudson would entail replac- ing only 200 street trees. Lowering the percentage of maples in Syracuse to 10% of the overall street tree population would entail replacing 8,500 street trees with other street tree genera, whereas to accomplish the same goal in Hastings-on-Hudson would entail replacing only 300 street trees. CONCLUSION Increasing street tree diversity is not a panacea for maintaining municipal street tree populations at their existing levels and thereby preserving the eco- system services and social benefits they provide. Many other factors have impacted or will impact street tree populations, including urban develop- ment, state and municipal budgets, and climate change. Even so, increasing street tree diversity by making the most common species a little less preva- lent is a big step in the right direction toward sustain- able street tree management. Based on the results presented here, indicating a current concentration ©2017 International Society of Arboriculture
January 2017
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