On December 17th, 2020, BirdHouse Ecological Programs Manager Cameron Miller and Horticulture student at Mt. San Antonio College, was awarded a scholarship by Street Tree Seminar, Inc., “an organization dedicated to the development, health care and management of street trees in rural and urban areas within southern California.” His essay will also be published in the upcoming Street Tree Seminar newsletter for January-February 2021.
Arborists and urban foresters have developed sophisticated models to calculate the value of street trees. Various metrics help us arrive at a dollar figure per tree, according to the amount of carbon they will store in their lifetimes, the filtration of automotive exhaust and emissions they provide, or the cooling effect they bring to the ‘heat islands’ of our urban landscapes. These quantitative factors are incredibly important for the significant task of convincing public officials and policy-makers that the value of the street trees we plant will far exceed the costs they incur.
As we approach the rainy season in Southern California, let us consider how trees benefit our public infrastructure, by reducing stormwater runoff otherwise destined for stormwater drains and sewers. Trees can absorb as much as the first 30% of precipitation through their leaf systems, meaning that the falling precipitation never even reaches the ground. In a landscape like Los Angeles, where as much as 80% of rainfall ends up following a route from compacted soils to non-permeable pavement, to sewer, to river, to ocean, this is hugely significant. Not only do trees absorb and then transpire that initial water back into the atmosphere, they also absorb as much as the next 30% of precipitation through their roots (Burden). As they accumulate organic matter in the soils where they’re planted, soil structure improves, leading to greater water infiltration. We need this infiltration to begin recharging our groundwater and slowly moving away from our dependence on water from the Colorado River and other diverted rivers, which are channeled to Los Angeles via aqueducts. Street trees are a multifunctional aid in this goal.
In addition to the many quantifiable factors by which we can describe the importance of street trees, there is another piece that needs to be accounted for, which is harder to quantify but which underwrites every other demonstrable benefit: Humans co-evolved with trees, and our interactions with trees shaped our history, our foodways, our settlements, and our very cultures. It is deeply hardwired in us to live among trees, in mutually beneficial arrangements. We have been in these relationships with trees for much longer than we have lived in cities, and if we are to continue living in urban centers, we must make a space for trees, and continue participating in this human-arboreal relationship.
As we consider the role of street trees in our lives, we must consider the care they require in order to thrive. The stream of benefits they offer in our direction must be matched by the level of consideration and thoughtful maintenance that flows back to them. This needs to be mirrored in policy, in ample budgets for their ongoing needs, and in excellent research, education, and practice of tree care.
