Chicago-area residents are noticing leaves starting to change color on some trees already, and experts from the Morton Arboretum say the region’s recent weather is having an impact.
According to Dr. Christy Rollinson, PhD candidate in forest ecology at the Arboretum, early to mid-September is a critical time to determine how quickly leaves will change their colors.
“It’s kind of a critical season. There are always trees that color a little early, but we’re getting a little more color than normal,” she said. “If that trend continues, it’s going to be driven by the dry weather we’re having.”
Rollinson said hot weather itself doesn’t affect how quickly leaves will change color, but it can help dry out the soil, which limits moisture flowing to leaves and can encourage them to change color and die more quickly.
“Warmer weather will affect fall foliage more toward the end of fall,” she said. “Warm weather will affect that by the moisture in the available soil. Drought is a combination of how much water is available to plants, and how much they need, and how much they need is controlled by temperature.”
According to the US Drought Monitormost of the Chicago area is currently considered “abnormally dry” due to low amounts of precipitation in recent weeks.
The hot and dry weather may continue for at least the next week or two, too National Weather Service’s Climate Prediction Center calls for above average temperatures and near normal precipitation over the next 8-14 days.
Leaves are green because of the chlorophyll they use to absorb energy from sunlight during photosynthesis, but as temperatures drop and days get shorter, trees receive less direct sunlight and the chlorophyll in the leaves begins to break down.
When that occurs, trees begin to shut off the veins that transport water and nutrients to and from their leaves. When that process is complete, the leaves begin to die and weaken at the stem before they fall to the ground, according to experts.
According to American Forests, mottled leaf coloration will begin to appear late this month, with colors nearing their peak in mid-October. Most of the Chicago area will have passed its color topo by the end of October.