ARBORICULTURE & CONTENTS URBAN FORESTRY Volume 47, No. 5, September 2021 Formerly the Journal of Arboriculture, 1975 – 2005 (Volumes 1 – 31) Bimal Aryal, MES, James W.N. Steenberg, PhD, and Peter N. Duinker, PhD The Effects of Residential Street Tree Spacing and Crown Interactions on Crown Dimensions and Canopy Cover .................................................................................................183 Abstract. Urban trees provide people with a range of ecosystem services. Trees planted along streets have been a large focus of urban forest research and practice, and municipalities invest significant resources in their survival. However, the optimal spacing of street trees is not addressed in the scientific literature, and existing municipal street tree spacing standards are highly variable and poorly enforced. In this study, we examine variability in crown shape and size for street trees to test for possible interaction effects at closer spacings. We measured variabil- ity in crown diameters both parallel and perpendicular to street tree rows to test whether changes in crown dimensions can be explained by inter- action effects with neighbouring trees, and whether crown interactions lead to a reduction in total crown projection area (i.e., canopy cover). We measured the crown dimensions and diameter at breast height of 1,338 street trees in Halifax, Canada. We used two-way analysis of vari- ance to test whether crown shape and crown projection area were affected by crown interactions and spacing. We found that the effect of nar- rower spacing and interactions (i.e., crowns touching/overlapping) among trees translated to crowns extending away from the direction of interaction. We also found that these changing crown dimensions were associated with increases in canopy cover. Urban forest ecosystems are a vital resource for the increasingly urban population. There is a need for empirical research on spacing standards and practices that investigate their influence on the supply of ecosystem services, such as stormwater retention, air pollution removal, and cooling. Keywords. Canopy Cover; Competition; Ecosystem Services; Spacing; Street Trees; Tree Planting. F.D. Cowett and N.L. Bassuk Is Street Tree Diversity Increasing in New York State, USA? ................................................196 Abstract. Diversity in tree populations is viewed as essential for protecting the public investment in urban trees and for preserving the environ- mental, social, and economic benefits that these trees provide. It is therefore crucial for officials responsible for the management of municipal trees to know the diversity of their municipal tree populations and whether their efforts to increase diversity have been effective or should be modified. We assessed street tree diversity in New York State, USA by analyzing municipal street tree inventory data from two data sets, the first comprised of 75 inventories collated from municipalities, and the second comprised of 32 sets of inventories conducted at multiple points in time. This analysis builds on two previous papers containing similar assessments by analyzing more current data and by calculating diversity index statistics and relative abundance percentages for prevalent street tree species and genera. Findings indicate that there has been substantial progress to increase street tree diversity in New York State. This progress is correlated with reductions in the dominance of Norway maple (Acer platanoides), the state’s most prevalent street tree species (17% of street trees statewide), and in the dominance of maple (Acer), the state’s most prevalent street tree genus (35% of street trees statewide). Work remains to be done to further increase species and genus diversity so as to meet the challenges posed to municipal street tree populations by invasive pests and climate change. Strategies are proposed for accomplishing this. Keywords. Climate Change; Diversity Index; Invasive Pests; Municipal Trees; Norway Maple. Kevin L. Griffin, PhD, Thomas G. Harris, Sarah Bruner, MA, Patrick McKenzie, MA, and Jeremy Hise, BA Is the Radial Growth of Irrigated Urban Trees More Strongly Correlated to Light and Temperature than Water? ..................................................................................................214 Abstract. Background: Real-time monitoring of tree growth can provide novel information about trees in urban/suburban areas and the myriad ecosystem services they provide. By monitoring irrigated specimen trees, we tested the hypothesis that in trees with sufficient water, growth is governed by environmental factors regulating energy gain rather than by factors related to water use. Methods: Internet-enabled, high-resolution dendrometers were installed on 3 trees in Southampton, NY, USA. The instruments, along with a weather station, streamed data to a project ©2021 International Society of Arboriculture
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