10 Cowett and Bassett: Street Tree Diversity in Three Northeastern U.S. States ness Zone 7, which has the warmest average mini- mum winter temperatures of all zones associated with New Jersey, New York, and Pennsylvania. Thus, the results suggest insufficient species and genus diversity for street trees at regional, statewide, and municipal levels in three northeastern states. Lack of street tree diversity appears to be more of a pressing concern in New York than in New Jersey or Pennsylvania, owing at least in part to New York’s greater preponderance of Acer spp., less species and genera richness, and colder average minimum win- ter temperatures. However, street tree populations in New Jersey and Pennsylvania would also benefit from greater species and genus diversity. Unfortu- nately, increasing street tree diversity is easier said than done due to a variety of issues discussed in the following, many of which have been addressed by Polakowski et al. (2011) and Lohr (2013). (1) The public street right-of-way, where most street trees are found, is typically an unhospitable environment for tree growth, health, and longevity. Air pollution, urban heat-island effect, drought, flooding, soil compaction, inadequate soil vol- ume, nutrient imbalances and deficiencies, winter street salt, and utility pruning are some of the many stressors associated with public street right- of-way planting locations. These stressors not only negatively impact street tree growth and acceler- ate tree mortality but also limit the number of tree species and genera adaptable to such conditions. (2) Not all tree species adaptable to tough urban conditions make good street trees. Some tree species are poorly suited to be street trees due to their growth and branching habits. For example, silver maples (Acer saccharinum) are a fast-growing species toler- ant of wet and dry soils and easy to transplant. How- ever, they are also “weak-wooded,” prone to rot and decay, and vulnerable to sudden catastrophic branch failure capable of harming persons and property even when in apparently good condition, due to their characteristically narrow, v-shaped branch unions. (3) Planting evenly spaced, even-aged trees of the same species along streets, avenues, and bou- levards to achieve an aesthetically pleasing visual uniformity is a formal planting scheme dating back to sixteenth century Europe (Couch 1992), which continues to be recommended (Gerhold and Porter 2000; Simons and Johnson 2008) and remains operative today. For example, the Monumental ©2017 International Society of Arboriculture Core Framework Plan to preserve Washington, D.C.’s National Mall calls for allées of Dutch-elm- disease-resistant Ulmus americana to be planted along major streets and park roads as a unify- ing landscape element (Sherald 2009), such as 88 ‘Princeton’ American elms planted in 2005 along Pennsylvania Avenue in front of the White House. (4) Recognizing biodiversity as an important ecological concept does not necessarily translate into actions increasing plant diversity. For example, in a survey of plant nursery workers, Polakowski et al. (2011) found general acknowledgement among respondents that diversity of plant species in land- scapes is ecologically important, but also insufficient understanding as to why this is so or about the ways in which diversity relates to landscape practices, such as implementation of Santamour’s 10-20-30 rule. (5) Less prevalent street tree species and genera are oſten unavailable from local suppliers or are unavailable in large enough numbers to significantly increase diversity. For example, Iles and Vold (2003) found that Iowa, U.S., nurseries overproduce, and Iowa landscape professionals specify a dispropor- tionately small number of species and cultivars; Ries (2009) interviewed numerous municipal foresters who were either unable to obtain from local suppli- ers less prevalent tree species for new plantings or were forced to obtain these tree species from more distant, non-local suppliers. Sydnor et al. (2010) found that only 3% to 5% of the trees desired by Ohio urban foresters were available from Ohio nurseries. Many of these issues can be mitigated. Planting conditions for street tree species and genera can be improved by practices such as selecting the correct tree for a planting location based on above- and belowground conditions, watering newly planted trees during establishment and during extreme heat and/or drought, converting tree pits to continuous soil trenches, and using structural soil to facilitate root growth under paving into adjacent lawn areas. Visual uniformity can be accomplished while also satisfying the need for species diversity by group- ing trees that are visually compatible based on size, shape, branching density, and foliage texture (Bas- suk et al. undated). Better understanding of diversity importance can be achieved through increased edu- cation (Polakowski et al. 2011) and perhaps through a change in terminology by stressing the reduction of species and genera overuse (Ball et al. 2007).
January 2017
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