Abstract

Background and Objectives: The structural diversity of an urban forest affects ecosystem service provision, and can inform management, planning, as well as policy. Trunk diameter at breast height (DBH) is amongst the most common measures of tree structure due to its ease of measurement and strong relationships with other structural and non-structural urban forest characteristics. Materials and Methods: In this study, the DBH distributions of urban forests are summarised for 38 American cities with a combined population of over 30 million people and a range of geographic, climatic, and demographic conditions. The Anderson–Darling (AD) test was used to test the hypothesis that all DBH distributions came from a common population. Moreover, structural diversity was compared using the Shannon–Wiener index. Results: The AD test results failed to identify any statistically significant differences in DBH distributions. However, qualitatively, the DBH distributions have two primary forms, which have important functional, management, and planning implications. The vast majority of cities have an exponentially inverse-proportional distribution, such that the proportion of trees in each successively larger DBH class decreases exponentially. The Shannon–Wiener index indicates an uneven DBH distribution in the cities with an exponentially inverse-proportional diameter distribution; these cities are dominated by trees in the smallest diameter class. Potential explanations for a large proportion of trees in the smallest diameter classes include a large number of small, naturally regenerating trees; a preference for smaller trees in urban areas; or a recent increase in tree planting efforts. Conclusions: Despite no statistical differences in DBH distributions for the 38 study cities, the functional, management, and planning implications will differ considerably.

Highlights

  • Urban forest managers and policymakers can benefit from urban forest monitoring, including descriptions of structural diversity [1]

  • The null hypothesis, i.e., the diameter distributions of all 38 cities come from the same population, was not rejected by the Anderson–Darling k-Sample test (p = 0.959)

  • The Anderson–Darling test with Bonferroni correction confirmed no differences in the diameter at breast height (DBH) distributions amongst pairs of cities

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Summary

Introduction

Urban forest managers and policymakers can benefit from urban forest monitoring, including descriptions of structural diversity [1]. Structural diversity can be assessed by looking at the diameter distribution of a population of trees [2]. An urban forest’s diameter distribution can inform policy (e.g., tree removal bylaws), planning (e.g., budget, staffing), or management (e.g., planting, maintenance) needs and help project how they will change in the future [3], given population stability. Diameter distributions can be used to characterise various differing levels of ecosystem service provision. The structural diversity of an urban forest affects ecosystem service provision, and can inform management, planning, as well as policy. Materials and Methods: In this study, the DBH distributions of urban forests are summarised for 38 American cities with a combined population of over 30 million people and a range of geographic, climatic, and demographic conditions. The vast majority of cities have an exponentially inverse-proportional distribution, such that the proportion of trees in each successively larger DBH class decreases exponentially

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