Like soil and other vegetation, trees are an element of the built environment typically associated with the landscape. It makes sense. They are land-based organisms that dominate the natural world. They certainly are nothing like pipes, sewers and other grey infrastructure. Right?
In some very obvious ways, trees are of course very different from, say, pipes. Trees are living things. They contribute to our communities in so many ways that we have attempted to enumerate, though we still fall short. We have a very powerful emotional connection to them.
Yet, trees do meaningfully fulfill some of the same functions that pipes do, for example taking water offline during daily rainfall events. Large trees can intercept between 1/2″ and 3/4″ of rain water in their canopies alone. In addition, the soil that trees grow in also filters the stormwater, removing significant quantities of pollutants and chemicals. Unlike pipes, the older and bigger a tree gets, the better it is actually able to do this job. Trees are especially unique in that they appreciate in value and capacity, rather than depreciate, over time.
One reason they don’t always get enough credit for services like this is that very few urban trees actually get large or mature enough to perform these functions. The average life of a street tree in North American is around 13 years. In these cases, significant contribution to on-site stormwater management is an aspiration rather than a reality. The idea of creating high performance green landscapes whose backbone is a robust urban forest is really only just taking hold. Added to this is the common mis-perception that trees necessarily conflict (or worse, damage) grey infrastructure — something that can indeed happen without planning, but is hardly unavoidable. Given all this, it becomes clearer why it can be hard to persuade people just how significantly green elements like trees and soil can impact our ecological goals and improve how our streets and cities function.
This is part of why we love the term “green infrastructure” so much. It says exactly what it means. Until fairly recently, the word “infrastructure” necessarily meant things like pipes and conduits. Need to manage more stormwater? Bring in a bigger pipe! While grey infrastructure was and remains very important for safe and effective design, it is no longer the only solution, or necessarily the best one, for every site.
A lot of this comes down to is learning to expand our definition of the word infrastructure. As we develop new solutions for building environmentally sound, low-impact streets and cities, our understanding of the role and responsibility of infrastructure must shift and evolve, too. Green infrastructure is here and growing. It is an increasingly essential component of successful design. We must adapt or be left behind.