As long as glacial ice remained in southern Canada, Illinois' forests consisted of open spruce (Picea) and black ash (Fraxinus nigra) woodlands mixed with meadows. Then, a warmer climate caused the ice to retreat. When this happened, plants adapted to the cooler, moister climate declined in number. The annual average temperature continued to rise, and, by 11,000 years ago, a thick deciduous forest of oak (Quercus), elm (Ulmus), ash (Fraxinus), and hickory (Carya) covered Illinois.
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