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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.

Climate change impacts are more likely to be observed in seedlings and early growth than in mature trees. Temperature and moisture requirements for seed dormancy and germination are often much more critical than habitat requirements of an adult tree. Predicted changes in temperature, precipitation, growing season onset, and soil moisture may alter the duration or manifestation of germination conditions, and individual species will be uniquely affected.

Changes in distribution for individual species is expected to lead to shifts in forest assemblages and tree species may rearrange into novel communities.Major shifts in overstory species composition may not be observable until well into the 21st century because of the long time frames associated with many ecosystem processes and responses to climate change. These shifts, however, may become more apparent along ecotones where boreal species reach the southern edge of their range.

The loss of a traditionally cold climate and short growing season in the region may allow some insect pests and pathogens to expand their ranges northward such as hemlock woolly adelgid and southern pine beetle. Forest impacts from insect pests and pathogens are generally more severe in ecosystems that are stressed by drought and other stressors. Basic information is often lacking on the climatic thresholds that trigger increased populations of many forest pests, and our ability to predict the mechanisms of infection, dispersal, and transmission for disease agents remains low.

Mean annual daily temperature across the region is projected to increase by 8.2 °F (4.5 °C) under the GFDL A1FI (high emissions) scenario and 2.3 °F (1.2 °C) under PCM B1 (low emissions) for the final 30 years of the 21st Century compared to the 1971 to 2000 baseline. The most dramatic increase in temperature is projected to be in winter for the PCM B1 scenario and summer for the GFDL A1FI scenario. No spatial variation in temperature changes across the Chicago Wilderness region is discernable.

Model results project that species currently near their northern range limits in the region may become more abundant and more widespread under a range of climate futures. Results from forest impact models suggest that species such as bitternut hickory, black oak, bur oak, and white oak may have increases in both suitable habitat and biomass, and some deciduous forest types have the potential for productivity increases across the assessment area.

Across northern latitudes, warmer temperatures are expected to be more favorable to individuals near the northern extent of their species’ range and less favorable to those near the southern extent. Results from climate impact models project declines in suitable habitat and landscape-level biomass for northern and high elevation species such as black spruce, balsam fir, red spruce, and paper birch. Forest ecosystems dominated by boreal species, such as spruce-fir or paper birch, are consistently rated as the most vulnerable across numerous regional vulnerability assessments.