By Angie Hong
We all know that the leaves of deciduous trees turn color and drop in the fall, but have you ever wondered why?
Minnesota winters can be brutal, and not just because of the cold and snow. During the winter, the Earth’s axis tilts away from the sun, giving us fewer hours of daylight. At its most extreme, we have only eight hours and 46 minutes of daylight on December 21 (winter solstice), compared with 15 hours and 37 minutes of daylight on June 20 (summer solstice). That’s nearly seven hours fewer hours of light, which is a big challenge when you’re a giant living being that depends on the sun to create your food.
To conserve energy and water, deciduous trees will drop their leaves in the fall when sunlight begins to fade, and then grow new leaves in the spring when the snow melts. During the winter, the trees are essentially resting in a state of dormancy, much like when an animal hibernates.
Though it is sad for us to look around and see bare, brown trees, the fallen leaves also play a critical role in the nutrient cycle of forests. Leaves are the primary source of organic matter in a woodland ecosystem and release essential nutrients like phosphorus and nitrogen as they decompose. With the help of fungus, microbes, and other decomposers, these nutrients eventually help to fuel new growth in the spring.
In recent years, you might have heard horticultural experts urging you to “leave the leaves” on your lawn and in your gardens. The reason is that leaves act as free fertilizer and also provide warm nooks and crannies for smaller animals like toads, salamanders, fireflies, and shrews to shelter during the winter. In urban and suburban environments, however, fall leaves can also be a major source of stormwater pollution.
The problem begins when leaves fall onto driveways, sidewalks and streets. From here, they can clog storm drains, or decompose and send a pulse of nutrients into nearby lakes, rivers and streams. This, in turn, contributes to algae blooms during the summer.
Over the past 10 years, researchers from Minnesota and Wisconsin have examined the impact of fall leaves in urban settings. In 2016, William Selbig (UW Madison) studied two neighborhoods in Madison, Wisconsin, and found that nearly 60% of the annual phosphorus yield in the stormwater from these neighborhoods was coming from leaf litter in the fall. He also found that it was possible to reduce phosphorus concentrations in the stormwater by 80%, simply by removing leaves from city streets in a timely manner.
A follow-up study by Selbig, et. al., published in 2020, looked at the effectiveness of municipal street sweeping in neighborhoods in Madison, Fond du Lac, and Oshkosh, Wisconsin, as a strategy to reduce phosphorus and nitrogen in stormwater runoff. The research team found that weekly street sweeping during the fall reduced phosphorus loading to local water bodies by 65-70%, but had only negligible impacts on nitrogen. It is important to note, however, that there was almost no benefit to collecting leaves without also sweeping the streets.
Here in Minnesota, the University of Minnesota Water Resources Center, Minnesota Sea Grant, and Minnesota Pollution Control Agency have developed a Clean Sweep Program, which provides resources and training to communities to create street sweeping programs for cleaners lakes and rivers.
In Chisago and Washington County, local watershed districts and the Lower St. Croix Watershed Partnership have also provided funding to help Afton, Lake Elmo, Bayport, Baytown, Forest Lake, Lakeland, Lakeland Shores, Lake St. Croix Beach, Linwood, Marine on St. Croix, North Branch, Oak Park Heights, Rush City, Scandia, Stillwater, St. Mary’s Point, West Lakeland, Woodbury, and Wyoming implement enhanced street sweeping programs.
This November, you can help to protect lakes and rivers from stormwater runoff pollution by raking and sweeping leaves out of the street in front of your home and disposing of them in your yard waste collection or compost. Please note that it is illegal to dump leaves in your garbage or into nearby wetlands, ditches, or ravines.