Site and Soil Characteristics Driving White Oak Decline in the Chicago Region

Abstract

The mature oak trees of the Chicago region urban forest provide a wide range of financial, ecological, and health benefits to residents. However, a widespread trend of declining and dying mature members of the white oak group (Quercus alba, Q. macrocarpa, and Q. bicolor) has been observed in the Chicago region since the late 2010s. This decline is widely referred to as oak decline. Symptomatic trees are often treated for contributing factors, including root rot caused by three oomycete genera (Phytophthora, Pythium, and Phytopythium) species, damage from the two-lined chestnut borer (Agrilus bilineatus), and symptoms from other opportunistic pests and diseases. Although the stress-physiology of white oaks and the epidemiology of decline diseases have been studied, none have focused on the relationship between predisposing abiotic site and soil characteristics and white oak decline in the Chicago metro area. The objective of this study was to determine if certain site- and soil-characteristics are significantly related to white oak decline in the Chicago region. Soil- and site-characteristics were compared between healthy and declining trees. Trees on thirty-two sites were visited throughout the summer of 2023 in each of the seven counties surrounding the Chicago area. On each site, one healthy and one declining tree were selected, each in its own distinct grove. During site visits, deep pedon samples, composite soil samples, fine surface roots, site characteristics, tree health assessments, and site management information were collected. Composite surface samples were tested for a variety of potentially important soil physical, chemical, and biological characteristics. Fine surface roots were tested with DAS-ELISA tests for oomycete detection. Pedon and composite samples were used to correlate each soil to a USDA NRCS soil series. Our analyses revealed factors associated with white oak decline. 88% of all the study’s trees’ roots tested positive with DAS-ELISA tests, indicating the presence of antigens of Phytophthora, Pythium, or Phytopythium. 77% of the healthy trees and 97% of the declining trees were positive. ANOVAs and linear regressions with the Urban Tree Health index found significant relationships between tree condition and soil zinc, sodium, depths to redoximorphic features, and depths to depletions. Low zinc levels, compaction, elevated soil sodium, increased labile organic carbon availability, and poor drainage were identified as the most relevant predisposing abiotic site characteristics for white oak decline through structural equation modeling and generalized linear mixed models. Results from this study can help arborists and urban foresters improve management and treatment design and implementation for white oaks, our understanding of the predisposing factors that influence the health of managed trees, and future planting guidelines. Based on these results, planting efforts to restore urban white oaks and bur oaks should prioritize areas with reduced urban stressors, such as deicing salts, compaction, paved surfaces, and disrupted nutrient cycling. Planting these trees should also be avoided in poorly drained areas with finer textured soil. Soil quality on existing trees should be maintained, monitored, and improved following BMPs to promote their survival with changing climate conditions. Appropriate irrigation and watering of these trees during drought conditions may help reduce climatic stressors. These management applications can help improve planting outcomes, long-term benefits provided by mature oaks and decrease intensity or occurrence of decline symptoms.