In our last Why Trees Matter post we talked about how trees help conserve soil. This week we’ll talk about the other type of conservation trees perform—water conservation.
If anyone else followed PBS’s Earth A New Wild series like us, hopefully you watched their final episode on “Water” which discussed the importance of preserving the limited resource. Not only do trees play an important role in water conservation, but they also help retain something we experience during our monsoons—stormwater runoff.
Trees help preserve and manage water in several ways. First, their leaves and bark intercept and store an amazing amount of water. Although the quantity varies with tree type and size, one tree can store 100 gallons or more of water after 1-2 inches of rainfall. Part of this intercepted water will slowly be released to the soil below, requiring less additional water needed for trees and crops, and part will evaporate into the air. Tree roots also store water which will eventually be released into the atmosphere through transpiration. Transpiration is the process by which stored water in tree roots travel to the leaves and then evaporate into the air through stomata, or leaf pores. An average tree can release 250-400 gallons of water in one day. Through water evaporation and transpiration, trees play a critical role in the water cycle (refer to the diagram below).
Much of the water that travels inland does not come from oceans, which also release moisture into the air that forms rainclouds. Oceanic moisture accounts for rain that reaches only 150 miles from the coast. Through tree evaporation and transpiration, air is remoistened as it travels farther inland from oceans and this accounts for much of the rainfall away from coastal areas. Trees and other small vegetation return nearly 70% of rain back into the air which then maintains the water, or rain, cycle and recharges rivers and lakes. Without trees the interior parts of continents would become deserts, such as in Australia and Africa were deforestation has led to minimal rainfall.
In an opposite but related vein, trees help redistribute and clean water by retaining stormwater runoff (refer to the diagram below). Trees prevent flooding from storms by trapping water in their leaves, trunks, and roots. One mature tree can potentially intercept 1,000 gallons of water a year. Some studies estimate that urban forests can reduce annual runoff by 2-7 percent and with strategic tree landscaping, nearly 65% of storm runoff can be reduced in residential areas. Additionally, trees improve water quality by trapping water that would normally run-off into waterways, carrying with it soil that contains sediments and nutrients. While nutrient-carrying soil may sound beneficial, excessive nutrients like nitrogen or phosphorus can lead to an overgrowth of aquatic plants that reduce oxygen content and disrupt fish life. Finally, trees help collect and filter water for local aquifers and watersheds that replenish the groundwater needed to supply drinking water for much of the world’s population.
Trees play an amazing role in helping conserve and clean the world’s water supply. While the Shade Tree Program stresses the energy savings that trees provide, it’s important to understand the other crucial benefits that trees provide and why trees matter.
http://guernseysoil.blogspot.com/2012/07/your-backyard-woods-water-cycle.html (diagram 1)
http://www.northlandnemo.org/images/800TreeCityUSABulletin_55.pdf (diagram 2)
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