Warmer temperatures can lead to decreased soil moisture even without an associated decrease in precipitation, resulting in a temporary inability for a tree to meet water demand.
Some studies have examined the impact of climate change on forest productivity within the region, but they disagree on how other factors such as species composition, stand age, disturbance, or pollution may interact to influence productivity. Changes are not expected to be consistent within a species, and the diversity of forest conditions across the landscape suggests that changes will be spatially variable.
Seedlings are more vulnerable than mature trees to changes in temperature, moisture, and other seedbed and early growth requirements; they are also expected to be more responsive to favorable conditions.
Forest impact model results predict that habitat and biomass of individual tree species will change, and that tree species will respond uniquely. However, few studies have specifically examined how assemblages of species may change.
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.
Of 70 native species examined for the Chicago Wilderness region, suitable habitat for 15 of them was projected to decline under both climate scenarios. One species projected to decline in habitat suitability, black cherry, is one of the most common species in the Chicago region according to the Regional Tree Census. Other common species projected to decline are white oak, eastern white pine, quaking aspen, and paper birch.
Chicago and areas right around Lake Michigan in Indiana are in hardiness zone 6a (mean annual lowest temperature -10 to -5 °F; -21 to -23 °C), and most of the surrounding area is currently in hardiness zone 5b (mean annual lowest temperature -15 to -10 ° F; -26 to -23 °C). By the end of the century, hardiness zones are expected to shift to between 6a and 6b under a low emissions scenario up to potentially 7b under a high emissions scenario. However, it is important to keep in mind that hardiness zones are based on a 30-year average and some very cold winters could still be possible.
