Urban Ecoist

One thing that is unavoidable in a city is compacted soil. Cities are already covered with pavement and concrete, but even the soil in cities can act as a barrier to water. Compacted soil is a natural result from people walking on or vehicles driving on the soil. When a city’s landscape prevents water to flow through, groundwater supplies slowly dwindle, and in a time in our history when water is becoming scarce, cities need to find ways to allow infiltration.

Usually, compacted soil is only a nightmare for gardeners and landscapers. Urban farmers will spend hours digging up compacted soils, trying to break up dense clumps into smaller clumps that will increase the spaces between particles and allow oxygen and other elemental necessities for proper vegetative growth. This is a simple enough idea, but compacted soils also need to be broken up in order to allow water to flow through and down into the groundwater supply. Urban soils are especially compacted, and thus act as an impermeable barrier to water. This barrier is preventing water in the form of rain and snow from flowing through soil.

Well, what do you know, but trees can help break up compacted soils. Trees root systems are excellent at going deeply into the ground, pushing their way into any available space and then just plowing right through into areas not so available, too. The bigger the tree, the bigger the root system, and the more soil the tree will move into, breaking up compacted soil. If the soil is permeable, water in the form of precipitation will freely flow through the soil.

And now, science is proving this theory(?). Researchers from Cornell, Virginia Tech and the University of California at Davis have been conducting experiments “digging” into this issue, and the results are not surprising, but still helpful to city planners.

The lack of infiltration opportunities affects groundwater recharge and has negative repercussions on water quality downstream. Researchers know that urban forests, like rural forest land, can play a pivotal role in stormwater mitigation, but developing approaches that exploit the ability of trees to handle stormwater is difficult in highly built city cores or in urban sprawl where asphalt can be the dominant cover feature.

A group of researchers from Virginia Tech, Cornell, and University of California at Davis have been investigating innovative ways to maximize the potential of trees to address stormwater in a series of studies supported by the U.S. Forest Service’s
Urban and Community Forestry Grants Program. The results of the studies were published in the November-December issue of the Journal of Environmental
Quality.

Virginia Tech scientists used two container experiments to establish that urban tree roots have the potential to penetrate compacted subsoils and increase infiltration rates in reservoirs being used to store stormwater. In one study, roots of both black oak and red maple trees penetrated clay loam soil compacted to 1.6 g cm-3, increasing infiltration rates by an average of 153%. –SPX via Terra Daily

The two other studies furthered the “just plant trees” solution and tried out structured soils, which are engineered to carry a pavement load while remaining permeable as well as geotextiles that will also allow tree root growth to penetrate compacted soils, thus increasing the infiltration rate of water.

I don’t really want to say “duh” here but, it kind of seems like a natural reaction as it is pretty obvious that more trees are better for cities. It is heartening to find out that some university grad students are interested in proving that fact.

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