Parking lots have significant environmental impacts that can be partially mitigated by planting trees. Growing trees in parking lots presents some challenges that can be addressed by giving them enough space. Structural soils can give tree roots the space they need while supporting parking loads and maximizing the number of parking spaces.
Environmental Impacts of Parking Lots
On a naturally vegetated landscape west of the Cascades, rainfall is intercepted by trees or soaks into the ground where much of it is taken up by plant roots. Studies by Chris May at University of Washington show that there is virtually no runoff from a forested landscape.
When it rains on asphalt or concrete, water runs off rather than soaking into the ground. This runoff carries pollutants from brake linings, tires, and automobile fluids, as well as other pollutants from the urban landscape. Nat Scholz at NOAA Fisheries has shown that copper from brake linings is very toxic to our native salmonids. Approximately 20% of impervious area in urban watersheds comes from parking lots.
Storm drains from parking lots are generally connected to a storm sewer system. In many areas, storm sewers dump into the nearest stream. Besides carrying pollutants to these streams, the runoff causes streams to rise quickly during storm events, eroding banks and stirring up sediments and legacy pollutants from the stream bed.
In many larger cities, including Portland, storm sewers connect to the sanitary sewers and go to a wastewater treatment plants. During storms, the combination of sewage and runoff is greater than the capacity of the system and a “combined sewer overflow” occurs, dumping into the nearest river.
Parking lots also contribute to the heat island effect. On a hot summer day, the temperature of an asphalt parking lot can be 50F warmer than the surrounding area. The heat gain from a lighter colored concrete parking lot is less severe.
Overcoming Difficulties of Growing Trees in Parking Lots
Many trees are not adapted to the high temperatures that occur in asphalt parking lots. Soil compaction that typically happens with any paving project is not conducive to tree growth. Trees also need protection from cars, trucks, shopping carts and other equipment that might collide with a tree in a parking lot.
Numerous studies correlate tree growth with the volume of soil available to them. One way of accommodating tree growth in parking lots is to provide large tree wells. Where land costs are prohibitive, “structural soils” can provide a healthy growing medium that can support parking loads. Trees thrive in structural soils 3’-4’ deep and 10’ or more wide.
Cornell University has been a pioneer in structural soils, with more than a decade of research documenting their performance. Structural soils contain load supporting crushed rock of sufficient size to provide voids for soil, air and water. Screened organic soil is mixed with the crushed rock for the roots to grow in. A soil binder is mixed with the coil and rock to keep the soil in place.
Cornell’s CU structural soil is a patented mix that is licensed to vendors across the country and comes with quality control assurances. Other recipes have been developed across the country using local materials. The University of California at Davis has developed a structural soil using locally available lava rock. Lava rock contains voids for air and water and is not as strong as other rock, so the Davis mixture uses rock of smaller size than other structural soils.
The City of Olympia Washington constructed a successful demonstration project in 2001 using structural soils in a linear tree well 100’ long x 10’ wide x 3’ deep, covered with concrete sidewalks. Three trees reside in 4’ x 4’ cutouts in the sidewalk. Across the street from this demonstration, 4 trees were planted in compacted native soils as a control for the experiment. The results as shown in this Google Earth image 8 years later are impressive.
Trees in parking lots need protection from collisions. Wheel stops are the most typical protection methods. Placement of wheel stops should consider whether a long-bed pickup truck might back into that parking space.
The 5000 Acres Initiative
Tualatin Riverkeepers is working with various partners to initiate a program to plant trees in parking lots for stormwater mitigation. The Tualatin River basin has more than 5000 acres in parking lots that cause runoff that erodes and pollutes streams. One of the goals of this project is to retrofit parking lots without losing parking capacity. Structural soils and linear tree wells designed by Maria Cahill of Green Girl Land Development Solutions a primary feature of the first two proposed project sites. Stormwater monitoring for the first year at these two sites is being contributed by Clean Water Services. Matt Stine of Ash Creek Forest Management is the consulting arborist for this program.
One of the proposed sites is at the Sunset Swim Center, owned by Tualatin Hills Parks & Recreation District (THPRD). THPRD had already planned to replace a ½ acre asphalt parking lot with pervious concrete. Tualatin Riverkeepers was interested in testing the stormwater interception capabilities of native conifers. High temperatures in asphalt parking lots are not conducive to the survival of native conifers, so this site with a cooler concrete parking lot is ideal for this trial.
The other proposed site is at the Mission of Atonement, a congregation of Lutherans and Roman Catholics interested in improving their environmental stewardship. Their current parking lot of gravel and asphalt produces significant runoff that pools offsite on a THPRD site. Linear tree wells with structural soils appear to be an ideal solution for runoff reduction on this site.
Tualatin Riverkeepers is looking for additional local partners for this project. We need to raise an additional $90,000 for the first year of the project and grant applications are pending for most of that. There are research opportunities for graduate students or others wishing to study rainfall interception of trees or performance of linear tree wells and structural soils. Potential partners for funding, in-kind donations, research or installation of trees on a parking lot are encouraged to contact Brian Wegener, Watershed Watch Coordinator of Tualatin Riverkeepers at 503-620-7507 or firstname.lastname@example.org.