Below ground. City silviculture and the art and science of urban soils.

INTRODUCTION

I pick the tree-lined streets in my cycle commutes across London. The air is cooler, the glare is less, and if there’s a breeze, the ear can tune into a gentle rustle of leaves over the humdrum of the city.

I know certain groups of trees well – some now for over half a century – having lived in the borough all my life. They are majestic friends, heroic in their resilience to almost all weathers, with lofty canopies and textured or fissured bark encrusted with lichen.

Planted by our forefathers and with the benefit of their foresight, London’s large trees give natural scale to the city. They are our urban forests, planting vast volumes of biomass in amongst the inertia of built form, rich in biodiversity and providing a free service by enhancing our quality of life. I cannot think of any man-made innovation that matches the performance of a large specie tree in terms of cooling the city, carbon sequestration, oxygen giving, storm water interception, biodiversity or property uplift.

These trees are London’s living heritage, some older than the buildings around them. Their health, vigour and stature is directly related to the soils that sustain them. What graces the eye relies on what lies beneath our feet.

THE GREAT STORM OF 1987

I remember walking to the studio in Shoreditch in the aftermath of the hurricane in October 1987, now thirty years ago, when 15 million trees were blown over overnight in the Southern counties. We couldn’t cycle – there were trees down everywhere across the roads. We lamented the loss of many wonderful specimens, leaving what seemed like huge raw bald patches in the urban fabric.

However, in Kew Gardens, this sad aftermath led to some positive critical observations: many trees that had not fallen completely began to sprout vigorous new branches. It was as if they had been given a shot of oxygen. The more popular the tree, the more it had suffered from the trodden ground of millions of admiring visitors, and the ongoing compaction had starved the roots and stunted the tree’s growth.

Another revelation was new evidence that about 90% of the root plate of most trees grew in the top 600mm of soil, as trees all over the country toppled by the wind now presented their root systems for all to see.

The more frequent occurrence not only of high wind events but also storm water surges, increasingly in summer when the soil is dry and hard-baked, has confirmed the need for critical drainage strategies; smart urban water management that slows the flow from sky to sea despite the impermeability, not just of our streets, but also of London’s parks and green spaces.

Limited resources to regularly decompact soils in the most popular parks means that storm water, a precious resource, runs too quickly over – rather than through – the ground. I have walked Hampstead Heath with the Superintendent who described much of the ground performing more like concrete than soil, with the magnificent and significant tree canopy showing signs of stress particularly in periods of drought.

London’s soils need to be able to perform naturally as life giving super-sponges: absorbing, attenuating, supporting, sustaining.

WHAT ARE SOILS | SECRETS OF THE NATURAL & ANTHROPOGENIC

Soil is the foundation of all life, of all biodiversity. It is a dynamic living medium, complex physically, chemically and biologically. Soil is our unsung hero, so often unnoticed, unappreciated, neglected. Even the words ‘dirt’ and ‘soiled’ conjure up negative connotations.

In cities, soil needs particular protection and nurture, especially during the construction process. Thus over the course of a project, we take on stewardship of the soil. We’ll call on Tim – our sharp-eyed soil scientist – to join us on site with mini auger in hand. There is no denying what he draws out of the ground – a deep plug of soil that presents a narrative of care or disregard that would otherwise remain concealed beneath the surface.

Soils are sensual. They have texture, scent, colour. They are the living skin of the planet. When Tim grabs a handful and lifts it to his nose with his eyes closed to take in the health or otherwise of a sample, he is reading a living dynamic, with the potential for holding more microorganisms in that one handful than people on earth. He observes how the material rolls, whether it binds into a cohesive joint or a crumbling one, his fingers registering any granular quality and moisture.

Samples provide evidence from site that are verified at the laboratory to confirm what a good soil scientist already knows. The trial pit is preferably dug by hand, or if suitable mechanical plant is on site, Tim will take advantage of a deeper excavation, deep enough to expose a profile of humus, topsoil, subsoil and possibly even the parent material. The characteristics of each illuminate the resource as an asset or a liability, a below ground narrative of physical and human geography. He interprets the science into horticultural potential. A recipe that reflects the specific needs of the plant species selected and prevailing environmental conditions, that measures and balances soil composition; assesses the texture of the soil which has a strong bearing on how the soil behaves; considers soil amelioration to enable the slow-release of nutrients and trace elements, beneficial soil microbes, water-retention, nutrient retention, structural development; analyses soil porosity and structural stability to ensure the potential for proper growth and function of root systems.

In London, there are few places that have never been developed. These ‘brownfields’ tell a story of occupation and land-use, a cultural history of a place, and the soils are very much anthropogenic in composition. Often the ecosystem is surprisingly diverse, as it has been left to its own devices, more than so-called ‘greenfield,’ where land management prescriptions typically exert a control over nature. These unprepossessing soils may actually provide a good basis for a tree rooting medium or urban tree soil, able to be engineered to give the correct balance of air, water, plant nutrients and microbes while providing stability to support overlying pavement constructions.

There are many treasures in the geology beneath the oldest metropolis in the world. There is also a plethora of ducts, cables, mains, pipes and tunnels. There is no reason that London’s street trees can’t continue to coexist within this context, but there is a pressing need for a more creative and equitable cross disciplinary collaborations as tree roots compete for space with the services that make the city tick.

Soils are developed; they are not merely an accumulation of debris resulting from decay of rock and organic materials ... In other words, a soil is an entity -- an object in nature which has characteristics that distinguish it from all other objects in nature. C.E.Millar & L.M.Turk, 1943

THE ROOTING ENVIRONMENT

The natural environment for trees is woodland, so a city’s microclimate and pavements present relatively hostile conditions for growth and pedogenisis. This is the challenge facing the makers of city soils. For trees, the important components of a rooting medium are a good subsoil or substrate with the right structural and drainage characteristics allowing adequate gaseous exchange.

There is much innovation in the field notably in Netherlands, Sweden, the USA and the UK developing load-bearing substrates that provide air and water for root growth while supporting pavements above. These ‘engineered’ soil profiles, or urban tree soils can be a recipe of mineral and organic components, including recycled composts, sands, crushed brick and concrete. A reconfigured and reconstructed ‘soil’ or rooting medium, the subject of long terms trials; or they can be an open cellular system that take the full weight of highway loadings rather than the soil they are filled with, to recreate a kind of below ground forest floor scenario. Trials now over many decades show the benefit of these systems in terms of enhanced health, stature, performance value and longevity of street trees, providing mounting evidence through the investment in long-term monitoring.

ACTION

All natural soils, are weathered bedrock, the natural signature of a landscape, telling a story that extends much further back than human existence. Now we are realising that our survival relies on a change in attitude to the stewardship of soils, that it is time to bust silo thinking of what is urban and what is rural; rather for a catchment approach to soil resource management to become second nature. To do this we need to reengage communities in natural processes, and realise that in the face of climate change and intensification, everything is connected.

London has a wonderful climate for growing. As professionals in the field we have a duty to protect, retain and enhance city soils, soils that are being rapidly and extensively compacted, eroded, and excavated under intensifying use for increasingly deeper basements and more massive tunnelled infrastructure. We need to exert influence, and support a stronger mandate for, and greater priority to, large specie trees and our urban forest in and around the capital, so fundamental to London’s identity.

What better time to emphatically express the economic and ephemeral value of the majestic street tree, the urban forest, and the soils on which it all depends. The street is, after all, the social conduit for city life. This matter concerns, therefore, the sustainability of communities, the economic success of the metropolis, and ultimately the wellbeing of all Londoners.

About the authors

Johanna Gibbons and Tim O’Hare have worked together over the last 25 years tackling the challenges of the urban landscape, advocating for urban tree soils, particularly related to the establishment of large specie trees for the long-term. All photos in this blog are auhored by Jo Gibbons.

This is an extract from a booklet, Below Ground, that touches on the interests of both disciplines; landscape architecture and soil science in a discussion on the art and the science of soil in relation to urban forestry, reflected in the poetry of Steven Fowler, Poet in Residence at J & L Gibbons. It is available here