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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.

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.

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 .

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.

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.

Changes in climate may allow some nonnative plant species, insect pests, and pathogens to expand their ranges farther north as the climate warms and the growing season increases. The abundance and distribution of some nonnative plant species may be able to increase directly in response to a warmer climate and also indirectly through increased invasion of stressed or disturbed forests. Similarly, forest pests and pathogens are generally able to respond rapidly to changes in climate and also disproportionately damage-stressed ecosystems.