I am feeling the transition into autumn as trees go from multihued greens to vibrant reds, oranges, and yellows, soon to be barren branches signaling the coming cold weather. With the fall of the leaves I often think about the nutrients the dead leaves will add to the forest floor for next spring’s regeneration. I wonder, what is happening to those nutrients when the leaves are falling onto impervious urban surfaces versus the forest floor?
Leaf litter when deposited on forest floors decomposes and helps to add nutrients necessary to new growth in the future. When that leaf litter is deposited on impervious surfaces, the earth’s ability to absorb those nutrients is compromised and can contribute to increased phosphorous levels in our storm water systems. These nutrients then get deposited into our watersheds. This increased nutrient deposition can contribute to algae blooms which can have serious impacts on the health of stream and lake ecosystems as well as impacting human use.
A study published this spring conducted by William R. Selbig (U.S. Geological Survey- Wisconsin Water Science Center) examined the impact of leaf removal on nutrient concentrations in urban stormwater. The results of this study could have an effect on how our municipalities and homeowners address the removal of leaf litter in the fall. Nutrient deposition can occur when leaf litter is not removed in an efficient fashion as the nutrients can be extracted rapidly by stormwater, which means removal is most effective when done frequently and before any precipitation. So looking at the mass of leaves falling in our cities and towns I have a new appreciation for raking and cleaning up our yards and sidewalks. I hope to be jumping in to some leaf piles while also preventing excessive nutrient deposition in our watersheds this fall.