A large-canopy tree is a very beautiful thing. On this, most people will agree. But is not only beautiful—it also benefits its community. It provides shade and shelter, protects air quality, and reduces air temperatures, water runoff, and human stress. A street lined with such trees is a desirable place to live and work, and a community with many large trees is attractive to visitors, residents, and businesses.
Growing large-canopy trees is a worthwhile investment and a cornerstone of today’s movement toward sustainable communities. Yet the designers of today’s built environments and city planners tasked with creating sustainable, livable, resilient communities continually make mistakes that doom their trees to failure. We wouldn’t hesitate to condemn an engineer who designed a building without being sure the columns would support its weight. Yet we allow designers to populate our landscapes with trees that have little chance to grow to a mature canopy height. Designers sometimes even refer to small stunted tees as “mature,” either an indication that they do not know what a mature tree looks like, or that they are resigned to failure as the price of placing trees in a city.
The success of a tree is fundamentally linked to the soil in which it grows.
Seems obvious, doesn’t it? Yet this simple idea has had trouble taking root (so to speak) in the public realm. To provide benefits to the community, trees must be put into built environments in entirely new ways
MAKE THE SPACE RIGHT FOR THE TREE
Fifteen years ago, James Patterson, then a soil scientist with the National Park Service, speculated that as many as “90 percent of all urban tree problems are soil-related.” Far too often, trees are planted in poor soil, soil with poor drainage, high levels of compaction, chemical imbalances, or other forms of contamination. With such disadvantages, trees are more likely to fail to prosper, grow more slowly and or suffer disease, insect infestation, premature decline, and early death. Many trees decline as a result of poor nursery stock or from inadequate or inappropriate maintenance. Yet in most cases, trees in good soils can overcome these problems. Without question, starting with proper soil is the most important factor in growing healthy trees.
For years, the arboricultural profession has supported putting “the right tree in the right place.” This concept directs a designer to understand the planting site conditions; including soils, drainage, and aboveground conditions, and to find a tree species that matches those conditions. As soil in urban areas becomes increasingly degraded, there are fewer and fewer tree species that will grow in it.
At some point on the road from the forest to Main Street, soil becomes so poor that almost no tree is “right.” The results are city streets with minimal species diversity, and inner-city areas containing limited numbers of poor-quality trees. If the goal is to create a diverse urban forest with healthy trees, we must design soil environments with that goal in mind. After determining which trees best fit a project’s aesthetic, environmental, and technical goals, we must adapt the project site to those trees’ requirements. We must “make the space right for the right tree.”
This approach does not assume a perfect world. Budgets, political issues, and physical restraints on soil improvements will still limit the number and quality of trees. Yet designers should no longer resign themselves to accepting difficult growing conditions. Instead, they must make a strong case so they can grow trees that meet the goals of the design. Budgets for soil and trees will need to be increased to provide a better balance between trees and other elements of the design. Design fewer trees, but make allowances for soil conditions to support each one as a healthy, long-lived specimen. We can thus significantly increase the number of tree species in the urban environment. Trees will live longer, require less maintenance, and provide the benefits expected when the tree was planted.
TREES NEED DIRT
The fates of trees and soil are absolutely interlinked. Trees planted in urban areas by people who ignore their soil needs are likely to fail.
Designers who undertake the planning, design, or installation of trees, must have knowledge of both soils and the biology of trees. Standards must not assume that the professionals who are implementing the larger planning goals at the project level know these things just because they are landscape architects. Unfortunately, despite the name, landscape architects get little training in plants, especially the biology of trees. Coursework in the science of soils is almost nonexistent, although in a few places that is starting to change.
A dead or declining tree is not the only outcome of a poorly designed planting area. The effects of a lack of mature, healthy trees ripple throughout an entire community, impacting local watersheds, water and air quality, property values, street life, vehicle and pedestrian safety, even mortality and public health. An investment in healthy trees supports every element of a highly-functioning, vibrant city, and no street design guidelines will be truly complete without address the provision of soil – and creating adequate planting conditions – for them.
Soil and tree requirements are not details, but a critical component that must be in a larger planning document. It is critical to include the soil and tree requirement discussion in early planning for the following reasons: These components are critical to the overall success of the street concept. Incorporating the science of trees and soil into the design will change the design of other elements as these are large structures, and volumes that need to compete with the other elements in the street for space and funding. Changing a city’s view of trees and soil will require political will and education. Exactly the kinds of challenges a planning document is designed to solve.
What I am proposing is not only a new approach to designing landscapes, but also an approach to thinking about the role of trees and landscapes in your community. A large, healthy tree is an investment in sustaining a healthier, more inviting world.
James Urban, FASLA, is an expert in urban trees and soils. This post is adapted from the first chapter of his book, Up By Roots: Healthy Trees and Soils in the Built Environment.
Flickr credit: sahunhong