Maplewood Mall: The next urban forest?

When Julia Butterfly Hill launched her two-year protest to protect a 1500-year old California Redwood tree from logging, she did so from a platform perched on top of the tree, which she affectionately named Luna. For 738 days, she endured freezing rains and 40 mph winds, helicopter blitzes, a ten-day siege by Pacific Lumber Company security guards, and a horde of angry loggers below. Although California is the most populous state in our county, the tree that Julia pledged herself to protect grows in rural Humbolt County, a mountainous and heavily forested area 200 miles north of San Francisco. Nearly half of all the remaining old growth Coastal Redwood forests are found in Humbolt County, with the majority, approximately 680,000 acres, protected by local, state and national parks and forests. Outside these protected areas, 1,500,000 acres of public and private forests are open to logging, accounting for 20% of all forest production in the state.

Old growth forests once grew here in Minnesota as well. Before loggers and settlers turned a mosquito-filled wilderness into a thriving economy, the trees grew taller, the prairies and wetlands were more numerous, and the water in the Mississippi, St. Croix and 10,000 lakes flowed clearer. The area south of White Bear Lake where Maplewood Mall now stands was once a transitional zone between the Big Woods region, featuring American elm, basswood, sugar maple and red oak, and the Oak Openings and Barrens, which were characterized by red oak, white oak, shagbark hickory, basswood and black walnut, interspersed with prairie along ridgelines and steep, south facing slopes. Now, thanks to an ambitious project of the Ramsey-Washington Metro Watershed District, trees are sprouting right out of the pavement in the parking lot of the Mall.

It should come as no surprise that trees do a better job of capturing rainwater and soaking it into the ground than do asphalt and concrete. In a forested area, about 10% of all precipitation runs off into nearby lakes and rivers, while the rest is absorbed by the trees, soaks into the ground, or evaporates. In commercial areas like that surrounding Maplewood Mall, on the other hand, impervious surfaces like rooftops, parking lots and roads dominate, and it is quite common for 75% or more of the rain that falls in these areas to run off into storm sewer systems, carrying with it a unhealthy mix of sediment, excess nutrients, litter, road salt, and other pollutants. After identifying sources of pollution to Kohlman Creek and Kohlman Lake, the Watershed District determined that the Maplewood Mall parking lot was a major contributor.  They determined that by creating ways for the first inch of any rainstorm to soak into the ground, polluted runoff  would be dramatically reduced and would help to clean up the lake and the stream. Plans were drawn up for features including tree trenches, raingardens, planter areas, a sand filter, porous pavement, and a demonstration cistern, as well as public art, signage and exhibits. Construction of Phase 1 (entrance rain gardens) was begun and completed in 2009 at a cost of $600,000.  Phase 2 and 3 started in summer of 2011 and are nearing completion at a cost of $3 million.  Phase IV is planned for 2012 at a cost of $2.5 million.   

Approximately 80% of the stormwater treatment will be accomplished using tree trenches. Infiltration tree trenches are a high-tech solution for two of the most common problems with urban trees.  Surrounded by a concrete jungle, many parking lot and boulevard trees struggle to grow under harsh conditions with limited root space, not enough water and few buffers from cold, heat and wind. The trees that survive do little to reduce stormwater runoff or provide shade and other benefits until they are 20 to 30 years old. The solution is to design a system of underground reservoirs that provide water and good growing conditions for the trees until they reach maturity and that also retain rain and melting snow so that it soaks into the ground instead of running off into storm sewers.

This summer and fall, Ramsey-Washington Metro Watershed began planting groves of trees in the north, northwest and eastern parts of the Maplewood Mall parking lot. Forming rows between the parking lanes, the trees are rooted in a special mixture of rock and soil that allows room for their roots to grow. Trench drains, swales and catch basins direct runoff from the parking lot into the infiltration trenches where some of the water gets absorbed by the trees, while the rest either soaks into the ground or is filtered and cleaned by the layers of rock before reaching an underdrain that connects to the storm sewer system.

Newly installed tree trenches in the northwest grove of the parking lot at Maplewood Mall

The new trees at Maplewood Mall may never reach the behemoth size of the 200-foot tall redwood, more than 40 feet in circumference, that Julia Butterfly Hill once called home. Within five years, however, they will already be big enough to shade the cars parked beneath their branches. Twenty years from now, their fingertips might touch, creating a leafy bridge for birds and squirrels to travel along. In fifty years, these urban groves might even bear resemblance to the woods that once covered the region. Before this even happens, however, the tree trenches, raingardens and porous pavement at the Maplewood Mall will already be doing their jobs, capturing this year and next year’s rainfall and keeping 30 pounds of phosphorus and 4.4 tons of sediment out of Kohlman Creek and Lake each year. Before too long, the forest will again grow at the Maplewood Mall, and the water downstream will flow clearer too.