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.
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.
Over the past century (1916 to 2007), the frequency of extreme and exceptional droughts in Illinois and Indiana decreased. Exceptional droughts are the most severe form of drought experienced in the region, and extreme droughts are the second most severe. Until the recent drought of 2012, all of the exceptional droughts were prior to 1970, and the majority of them occurred during the dust bowl era of the 1930s. In general, more recent drought events have been less intense in their severity, duration, and spatial extent compared to earlier in the 20th Century.
Model projections that show future changes in habitat for many tree species do not account for migration constraints, longevity of current species, or differences among age classes. Because mature trees are expected to remain on the landscape, and recruitment of new species is expected to be limited, major shifts in species composition will not likely be observed by the middle of the century, except along ecoregional boundaries and in areas that undergo major stand-replacing disturbance events.
Model projections that show future changes in habitat for many tree species do not account for migration constraints, longevity of current species, or differences among age classes. Because mature trees are expected to remain on the landscape, and recruitment of new species is expected to be limited, major shifts in species composition will not likely be observed by the middle of the century, except along ecotonal boundaries and in areas that undergo major stand-replacing disturbance events.
The uncertainty of future precipitation patterns makes it difficult to determine whether conditions may become dry enough to increase moisture stress for plants in the Northeast. Forests that are affected by moisture deficits and drought are more likely to experience reduced tree vigor or increased mortality, both of which can affect forest composition and structure. Further, warmer temperatures can drive or enhance drought-induced mortality by disrupting plant physiology . This “hotter drought” can also interact with other forest stressors to cause tree death and forest die-off .
