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When rain falls, it starts its inevitable journey to the sea. If this journey is rapid, the rain carries topsoil and pollutants with it to streams and then rivers, which terminate in the earth’s oceans. If rainwater is slowed by vegetation, more of it seeps into the ground rather than rushing into local streams. Such infiltration not only replenishes water tables but also scrubs the water clean of its nitrogen, phosphorus, and heavy metal pollutants. Moreover, slow and steady discharge from water tables into streams and rivers reduces the destructive pulse of stormwater that scours our tributaries of their biota. By virtue of their copious leaf surface area and large root systems, oaks impede rainwater from the moment it condenses out of clouds. Much of the water intercepted by leafy oak canopies (up to 3,000 gallons per tree annually) evaporates before it ever reaches the ground (Cotrone 2014). All this makes oaks one of our very best tools in responsible watershed management.
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Douglas W. Tallamy (The Nature of Oaks: The Rich Ecology of Our Most Essential Native Trees)