Urban

Increases in impervious cover can dramatically increase the size and frequency of localized flooding. Typically, urban floods are short-lived, but extended flooding can stress trees, leading to leaf yellowing, defoliation, and crown dieback. If damage is severe, mortality can occur. In addition, flooding can lead to secondary attacks by insect pests and diseases. Some species are more tolerant of flooding than others. Flood-intolerant species include upland species such as bitternut and shagbark hickory, Kentucky coffeetree, and white oak.

Urban areas with one million or more people can be 2 to 13° F warmer than their surrounding rural areas due to the “urban heat island effect” from heat-absorbing infrastructure such as pavement and buildings as well as waste heat generated from manufacturing and automobiles. The urban heat island is often more pronounced in historically redlined areas with lower tree cover.

Changes in climate may allow some invasive plant species to survive further north than they had previously. Warmer temperatures could aid the spread of kudzu and privet across Illinois by the end of the century. Milder winters could be beneficial for the emerald ash borer, which is already causing extensive damage to ash trees across the area. Drought stress, which could occur later in the growing season, may make trees susceptible to attacks by boring insects such as bronze birch borer and two-lined chestnut borer and to diseases such as Botryosphaeria canker.

Overall vulnerability of trees in the Chicago region can be estimated by considering the impacts on individual trees using model projections or changes in heat or hardiness zone, together with the adaptive capacity of trees as described in the previous section. Two vulnerable species are nonnative (Japanese red pine and Katsura tree). Vulnerable species tend to be native to mountainous or northern areas. Examples include black cherry, red and white pine, balsam fir, quaking and big tooth aspen, white spruce, gray and paper birch, and Douglas fir.

To understand the capacity of tree species and cultivars in the area to adapt to other effects of climate change like changes in disturbances, pests, and diseases, we relied on a scoring system developed by researchers. Scores were generated for all species listed as occurring in the Chicago area according to the recent Regional Tree Census. In addition, we included several other species that are being considered on planting lists for the City of Chicago and the Chicago Region Trees Initiative. For species that are only present in a cultivated setting, we only generated one score.

Species that are currently marginal for hardiness zone (lowest zone is 5 or higher) may experience benefits from milder winters. Species that are marginal for heat zone (highest zone is 7 or lower) may experience negative effects from hotter summers. Based on this method, 24 species (23 percent of those evaluated) may experience a positive effect from an increase in hardiness zone over the next century (Brandt et al., 2017, table 4). Twelve of these species are currently found in the region according to the Regional Tree Census (Nowak et al.