In building our cities we have managed to create the harshest possible conditions in which plants can grow. Trees are faced with increased temperatures, modified hydrology, substantial surface impermeability and infrequent irrigation. They contend with these challenges from the limited available space left for them between hard surface areas, and underground they compete with pipes and cables for soil. This is the urban condition we have created for plants, and from that perspective it is surprising that our cities are as green as they are.
The benefits of trees to cities and the people who live in them are well documented. If we really want more trees, then we need to find space for them.
Historically, urbanisation has followed a formula when it comes to planting. For several reasons, it involved removing topsoil and replacing it with subsoil covered in concrete. Subsoil does not have the necessary chemical and physical properties to support plant growth. Plants need the soil to physically support them and stimulate essential biological processes. Key important factors can include the absence of air gaps in the soil, sufficient water availability, and minerals. Then there are the active microbial constituents of topsoil – like fungi and other microorganisms that synergistically enable exchanges between the soil, water, nutrients, and roots. Trees cannot, in other words, grow in anything.
Trees also face competition for space, both above and below ground. Electricity, water, sewers, and communications services tend to end up in the same spaces earmarked for trees and other vegetation. In this competition the engineering services and traffic requirements usually win out. Trees are thought of as nice to have, rather than a necessity of urban living.
For example, in a current project involving an inner-city brownfields redevelopment, we are working with the council to install trees that meet the requirements of biodiversity (a maximum number of trees of each species, genus and family) with sufficient growing volumes as determined through the Elke Soil Volume Simulator to attain a satisfactory canopy coverage (a 40 percent minimum coverage in a street and park environment), while mitigating a heavily contested sub-surface environment (including the provision of future-proofed services and their offsets). Projects like these can quickly become extremely complicated.
With the council preferring structural soils to be used to allow future access to underground services, the resultant growing volumes, once access to underground services are considered, almost giving us the resultant canopy coverage required - but not quite. The reduction of any component requirement will have a downside. We know that we can create optimal growing environments for trees using a combination of these measures where space is available.
When space is at a premium, we lose trees. Reduced tree numbers give us less pleasant cities, hotter cities, and unhealthier cities. Reduced growing volumes result in smaller trees with shorter lifespans that may not give us the canopy coverage we hope for in the long-term. Reduced clearances to services may increase long-term maintenance costs and impact on the health of trees when roots are disturbed.
Techniques like soil vaults solve part of the equation, but the essential problem remains: in many key urban environments there is not enough space.
Are we up to a compromise?