As we begin to remove forest canopy and replace it with roads, parking lots, driveways, homes, patios, pools (impervious surfaces) and even grass, we immediately have impact on watersheds and receiving streams (or lakes). With the increased amount of impervious surfaces, water runs off the land, traveling on the surface towards the streams. As this ‘storm water runoff’ travels to the streams it collects pollutants and increases speed. The changes to the landscape, not only increase the volume of water that goes to the stream, it also shortens the amount of time it takes the water to get to the stream. These increased or peak flows cause water to move quickly to the streams. This leads to flooding, streambank erosion, widening of streams, sediment deposited in streams, a loss of fish habitat, and decline in water quality. In Pennsylvania there are over 12,200 miles of polluted streams and over 3,000 miles of streams that are impaired by storm water runoff.
Trees and forests play an incredible role in reducing stormwater in several ways and removing or filtering pollutanting that would otherwise wind up in our waterways.
Tree canopies intercept and capture rainfall, reducing the amount that reaches the ground. In urban and suburban settings, a single deciduous tree can intercept between 500 and 760 gallons per year, while a mature evergreen can intercept over 4,000 per year.
Tree roots and forest soils allow for better infiltration of rainfall with rates of up to 15 inches per hour. The leaf littered forest floor acts like a gaint sponge, allowing for slow infiltration into soils befre releasing it to natural channels and recharging ground water.
Tree consumer stormwater through a process called evapotranspiration. Water is taken up by roots and move up through the tree until it is transpired back into the atmosphere as water vapor. A single mature oak tree can consume (transpire) over 40,000 gallons of water each year.
Trees are very good at removing pollutants such as nitrates & phosphates; and other contaminates such as heavy metals, pesticides, solvents, oils, and hydrocarbons that are found in stormwater.
Trees and Riparian Forests protect and buffer streams and are critical to maintaining healthy, clean streams. Tree roots provide streambank stability, reducing erosion, filter out sediments, remove nutrients, shade and cool the water, provide habitat for many different species, and provide the primary food source for aquatic insects that are a critical part of the aquatic food chain.
Until recently, stormwater management strategies focused on detaining large volumes of water in basins that had little to no effect on removing the pollutants in the stormwater. In December 2006, PA DEP unveiled new stormwater best management practices (BMPs) that work to protect water quality and put stormwater back into the ground where it fell. One of the 10 principles in the BMP manual is to preserve and utilize natural systems such as forests, trees, and native soils.
The Role of Trees and Forests in Managing Stormwater, a Penn State Factsheet
Green Roofs reduce stormwater and help building save energy by reducing summer temperatures. Witness the installation of a green roof at PECO Energy Building in Philadelphia.
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