Constraints and challenges with urban forest health and mortality in the Oklahoma City metro region
With increasing recognition of the potential benefits of urban forests, many cities wish to increase tree cover and/or improve the quality of the existing forest by promoting large, healthy trees which provide the highest levels of services. Although urban trees may benefit from reduced competition and increased resource availability, urban trees are generally assumed to face high stress conditions, be more vulnerable to storm damage, and have reduced lifespans compared to natural trees. However, the role of natural stresses or disturbances, such as droughts and storms, compared to management-related damage or removal is uncertain. Using ground- and aerial image-based surveys, as well as resident surveys of landscaping practices, we investigated urban tree health and mortality in the Oklahoma City, Oklahoma metro area. As one of the geographically largest cities in the US, Oklahoma City has a range of land cover and microclimates. Our objectives were to determine 1) What factors are associated with early mortality and poor urban tree health? and 2) What is the relative prevalence of natural versus human-induced stresses on urban trees? A better knowledge of urban forest dynamics is critical to understanding patterns in urban forest structure, function, and service provisioning.
Assessments of recently planted trees reveal a mortality rate (within 2 years of planting) of up to 50%. Early mortality rates were similar for native and non-native trees species, and were higher during drought periods. On public lands, trees were seldom removed for reasons other than death (or near death), whereas reasons were more diverse for private land. However, choices relating to the removal or replacement of trees were seldom related to concerns over water use. Only ~30% of sampled public trees were classified as being in “good” condition, and human-induced damage – such as root girdling, trunk wounds, and improper pruning – were more commonly observed than storm damage or canopy dieback. Further investigation is needed to fully resolve spatial patterns in health and mortality. High mortality rates and poor tree health result in diminished return on financial investments, the lack of realization of the full extent of services that can be provided by urban forests, and challenges to human management efforts to mitigate urban heat island effects, CO2 emissions, and other climate change-related stressors impacting cities.