by David Jonas Bardin
We wouldn’t expect the average NBA player to be comfortable flying in coach class. Nor should we expect our street trees to thrive in a similarly cramped environment.
“A tree’s ability to grow and stay healthy is largely dependent on available rooting space,” says Casey Trees, in its 2008 Tree Space Design Report:
This is particularly evident in urbanized areas where many trees exist in small planting spaces with little available soil. Trees in this situation tend to be short-lived, and most never function as vibrant components of a city’s infrastructure.
I have written here previously that expanding DC’s extraordinary tree canopy requires sustained efforts by federal, DC, and non-governmental bodies – and that Mayor Gray’s urban tree canopy goals offer many environmental and social benefits. Those include less storm water runoff and erosion, smaller carbon footprint, better air quality, more wildlife habitat, lower energy bills, higher property values, and enhanced quality of life.
Mayor Gray’s goals require a healthy environment for our street trees, and that means abundant soil volumes in which tree roots can find moisture and oxygen. One city agency has taken steps to address this, but the DC government can do more to ensure that that developers do their share in providing street trees with enough room to grow and thrive.
Tree roots, soil volumes, and canopy
A growing tree extends its roots far and wide to find moisture, air, and nutrients and to anchor the tree. Unless impeded, roots grow way beyond the outer edge of tree foliage (“drip line”). They anchor the tree and bring sustenance, as the District Department of Transportation (DDOT) Urban Forestry Administration (UFA) illustrates.
Indeed, the horizontal expanse of roots far exceeds their depths.
Urban trees have to cope with fundamental facts, as Casey Trees explains:
A growing tree will send roots far into the surrounding soil. In uncompacted soil, the roots of a mature tree can spread to more than twice the width of a tree’s canopy. Trees get nutrient from soil, but roots also need the air and water that occupy voids between soil particles. In uncompacted soil, these voids are abundant.
In dense urban areas where soils are often compacted and covered by pavement, the soil has few voids. Tree roots cannot penetrate highly compacted soil and will not grow in soil that lacks air and water. Roots of street trees frequently grow in the space between the compacted soil and overlying pavement, where air and water are present. As these roots grow, they lift the pavement and cause sidewalk heaving.
In open fields and lawns, tree roots can grow, extending far out, as they do in natural forests where roots of several trees crisscross one another. But in DC, tree roots often meet barriers they cannot penetrate, which limit trees’ survival, growth, vigor, and ability to resist storm winds – thereby constraining extent and density of canopy.
Barriers of concrete, of soils highly-compacted to meet construction standards, and of below-ground facilities (such vaults installed by utilities or building owners) face street trees in DC’s transportation rights of way (ROWs).
How DDOT responded to the challenge
DDOT has sought ways to give tree roots more room to grow while meeting competing demands for other uses of public space in the transportation right of way.
Although some pavements go right up to the building line (with no front set-back), permeable paving can let moisture and oxygen penetrate to sub-pavement soils.
Other DDOT pavements have tree boxes on one side and open spaces on the other. “Structural soil” channels underneath can let roots grow in each direction even if not all sub-pavement space gets filled with cells of soil.
DDOT’s hands-on experience – unmatched by other jurisdictions or public utilities in the region – came through efforts of its staff, contractors, and partners. DDOT mastered landscape architectural solutions – both by its own installations and by overseeing private installations in DC’s public space. But it does not consistently encourage developers to do the same.
Non-DDOT construction in public space
Whereas, DDOT leads the Washington Metropolitan Region in design and installation of permeable paving (including porous brick, porous concrete, and porous rubber) and sub-pavement expansion of soil available to tree roots (including sand-based structural soils, CU-Structural Soils, and soil cells “which are plastic structures to be filled with soil and covered with pavement” as described by Casey Trees), but DDOT has not yet adopted standards on which developers other than DDOT… may rely…[September 2013 ANC 3F resolution]
DDOT regulates non-DDOT uses of DC “public space” – between the curb and the property line – by “private” entities, which include federal agencies and city agencies other than DDOT.
Some developers have received DDOT permits to install sub-pavement soils and permeable paving to enhance root growth opportunities. Some have not.
DDOT’s Public Space Committee (PSC) recently took a new tack, addressing soil volumes explicitly as a means toward maximizing street tree canopy, by imposing the following condition on an amenable developer:
The applicant will consult with the Urban Forestry Administration (UFA) with regards to the maximization of the street tree canopy through soil volumes. [July 31, 2013, Re: 4455 Connecticut Avenue, NW]
The DDOT PSC subsequently required that developer to secure the Urban Forestry Administration’s sign-off on plans to be presented.
Impediments to overcome
DDOT is not the only regulatory agency looking after our urban forest. Other regulatory programs can encourage or discourage more vibrant tree canopy in and over the public space. DC Water and other utilities have not supported widespread use of new technologies to increase soil volumes for tree roots. In my next article, I will explain their fears and concerns and the question of possible impacts on utility rates, set by DC Water’s Board of Directors and the Public Service Commission for Pepco and Verizon.
Volumes of soil penetrated by moisture and oxygen will determine whether tree canopy spaciously shades DC streets, avenues, sidewalks, and public places in the future (paralleling charming ornamental trees) or whether, more and more, our “canopy” under DDOT control must consist of smaller trees alone: ornamental tree species and stunted shade trees.
In other words, if we want our street trees to stand as tall and proud as NBA stars, we need to give them a lot of leg room.
ANC3F Resolution as to 4455 Connecticut Avenue NW public space (Sept. 17, 2013)
DDOT/UFA, Construction Guidelines for Tree Protection (undated) PDF.
Casey Trees, Tree Space Design Report: Growing the Tree Out of the Box (2008) – PDF [to be updated soon].
American Society of Landscape Architects (ASLA), The Great Soil Debate Part II: Structural soils under pavement (2010 Annual Meeting Handout) PDF.
Upchurch, Creating Green Streets in DC (presented at MWCOG Green Streets Workshop April 8, 2013) (DDOT 2013) PDF.
DeepRoot, deeproot.com (especially Silva Cells)