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Tree Canopy is Key in Keeping Water Clean

By Emily Lardner

Imagine Olympia in ten years time, downtown streets lined with trees. Parking lots around the city host clumps of healthy conifer trees. As you ride the bus or bike through town, you admire new plantings along mature swaths of trees. You realize you can't go anywhere in the city, not even to the malls, without noticing the presence of trees. You feel cooler, more relaxed, and breathe more easily.

Not a fantasy - but the consequence of steady community pressure on the Olympia City Council to make canopy cover goals a critical element of the new comprehensive plan. In the past, without accurate aerial photography, canopy cover goals could have been construed as statements of value, rather than measurable goals. Cities hoped to increase the percentage of land within city limits covered by tree canopy, but tools for assessing progress with respect to reaching these goals were limited.

The situation has changed. The City of Olympia now has access to sophisticated technology that provides images accurately depicting landforms: tree cover, including distinguishing between conifers and deciduous trees; impermeable surfaces, including streets, parking lots and roofs; and streams. With this technology, Olympia has the opportunity to update its tree ordinance. The current tree ordinance is based on "tree units" - a tree is worth a unit, and big trees are worth more units. Depending on location, developers are required to keep 30 tree units per acre, or in the Green Cove area in northwest Olympia, 220 tree units per acre. In 1992, when the tree ordinance was passed, it represented forward thinking. That's no longer the case.

Other Washington cities have set canopy cover goals, including Seattle and Vancouver. The argument for increasing the total area within city limits covered by canopy is supported in a number of ways, ranging from aesthetics, to helping offset the heat-island effect attributable to climate change, to improving air quality and establishing urban wildlife corridors. In our region, another reason to set canopy cover goals is because trees play a critical role in the health of Puget Sound.

The Link Between Trees and Stormwater

Trees and water quality go hand in hand. Non-point source pollution - pollution without a single point of origin - significantly detracts from the health of Budd Bay, and storm water is a serious culprit. Rain falls, hits pavement, picks up pesticides, motor oil, anything else dissolvable or moveable, and carries it out through streams into the Bay. Keeping more rain where it falls will help clean Budd Bay because those raindrops won't become carriers of pollution as they travel down streets, into streams, and ultimately into Budd Bay.

Trees help keep rain where it falls. At a presentation to Olympia's Utility Advisory Committee in December 2009, former city forester and current planning supervisor for the Storm and Surface Water Program, Joe Roush, provided this scenario: Imagine a hundred raindrops falling on a conifer tree. About thirty of them will be "handled" by the tree. Some drops will stay on the tree and evaporate, some will run down stems to the trunk, some will be absorbed by the roots, and the roots themselves create space in the soil so more drops can be absorbed. Big trees handle more water than little ones, and conifers are more adept at "handling" rain than are deciduous trees, so planting and protecting big conifers is a good strategy for managing storm water. As Rich Hoey, Director of Water Resources of the City of Olympia said in the same meeting, "good storm water managers are also good forest managers."

The new technology Olympia has access to provides sophisticated layering of aerial photos and GIS maps which allows us to "see" the complex interplay of factors that transform ordinary rain into "storm water" and then into a non-point source of pollution. Using this technology, we can also identify strategic places for plantings of trees to maximize rain absorption, and stands of trees could play a more prominent role in Olympia's updated stormwater plan.

However, urban conditions and urban/suburban development put trees at risk. Establishing canopy cover goals as part of an overall comprehensive plan would be helpful. When cities have accurate canopy data, explicit goals to increase canopy cover will drive development and zoning decisions in environmentally friendly ways. Explicit canopy goals also articulate a commitment to a healthy Budd Bay, to healthy watersheds, to healthy ecosystems. With updates to Olympia's comprehensive plan and stormwater plan currently underway, perhaps we can move forward.

Emily Lardner is a columnist for the Green Pages.


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Updated 2015/01/07 21:14:22