The Greening of Urban Space
By Adis Tutusic , Junior Associate, and Zeek Magallanes , Studio Team Leader
Landscape architects are uniquely positioned to advocate for and implement the successful greening of the urban realm. Specifically, trees are the powerhouses of livable, vibrant, and healthy spaces. Our profession moves beyond policy and the written word and is charged with the implementation of this precious resource on every project to achieve the many benefits they provide to our communities. Three of the most critical benefits trees offer are health and wellness, ecological resiliency, and climate adaptation.
Health and Wellness
Trees are essential to promoting the health and wellbeing of individuals and communities, as well as the health and wellbeing of our natural environment. Green streets, for which trees are a critical component, can be considered green infrastructure. Such ‘green’ infrastructure offers environmental, social, and economic advantages in several ways: Skin cancer is mitigated through shading; a green environment reduces asthma, lung infections, colds, and eye irritation by cleaning the air and lessening the impact of smog; trees also calm vehicular traffic, thus enhancing pedestrian and bicycle paths by making them safer and more enticing to use; lastly, trees can remove critical toxins from the environment such as sulfur dioxide, cadmium, nickel, and lead, among others.
Ecological Resiliency
Per the Urban Landscape Institute’s (ULI), 2002 publication Planning for Extreme Heat in San Diego County, roughly 13 percent of all land within American cities is covered by trees when they could support four times this amount. Tree roots are critical to the performance of trees allowing them to absorb and contribute nutrients to the soil, communicate with other trees via mycorrhizal relationships, and respond to localized adverse conditions. Surprisingly, tree roots spread two to four times further than the width of the canopy and are concentrated in the upper 12” of soil. Trees, like all other living things, need food, water, and nutrients to survive. They make their own food through photosynthesis, using energy from sunlight, water, and carbon dioxide (CO2) to create sugar that is used to fuel the rest of the tree. Water is carried from the roots to the leaves through xylem cells. Roots store essential food reserves needed by the tree to produce leaves in the spring. It takes a few years for a tree to complete its roots system, growing surface roots horizontally and only a few anchoring roots vertically.
There are clear advantages to keeping mature trees in urban environments over planting new ones. Mature trees provide instant benefits. Their large canopies offer shade, reduce glare, and enhance aesthetics without waiting for years of growth. Older trees have well-developed root systems that contribute to soil stability, stormwater management, and nutrient cycling. Mature trees also already host various species of birds, insects, and other wildlife due to their complex structure and longevity.
We discussed the topic of mature trees in more detail in our February 2021 news blog article, Trees as a Valuable Resource in Development: “While species are significant, size matters, too. Since carbon storage is directly correlated to the amount of biomass, a key factor in capturing the most carbon in our cities lies in the existing trees because the green infrastructure is already there. The larger the tree, the greater capacity it has to make an impact in the air pollution. Don’t take comfort in planting new trees to replace the demolished ones. The new 24-inch or 36-inch boxes are not going to cut it. Old tree growth is not replaceable.”
Mature trees positively impact property values by beautifying neighborhoods and creating desirable living environments. Therefore, the practice of removing older trees and planting new does not always serve the immediate needs of the community.
Recommended by LinkedIn
Planting in various locations, such as a park, streetscapes, parking lots, and outdoor amenity spaces, proves beneficial for climate mitigation and adaptation, and overall human health. However, during dry periods, compacted soils and manmade toxins can make for unfavorable root growing conditions. This increased soil density and poor growing conditions leads to roots that are less able to penetrate the soil and generally form in a shallow and malformed pattern. Despite challenges such as compacted soils and competition for space, effectively managing root systems can improve the viability of trees.
Adequate root space and soil volume are crucial, requiring thoughtful engineering considerations, root management, irrigation, drainage, aeration, and support measures. In practice, giving trees enough root space through structural support using structural cells or soil can allow trees to be healthy and develop large canopies. Adding root barriers helps to mitigate damage around foundations, concrete, and pavers.
Climate Adaptation
As irregular and unforeseen climate events continue to express themselves across the globe, dense tree canopy and extensive tree roots can serve to help mitigate the negative effects of such events. Established trees, through transpiration and evaporation, mitigate urban heat islands, acting as sustainable coolants. Each well-established tree can have an impact equivalent to 10 air conditioners, contributing to challenging CO2 reduction targets. To meet the Climate Action Plan (CAP) per County of San Diego 2017, targets of 15% canopy cover by 2020 and 35% by 2035, the urban tree canopy assessment data must be analyzed at a fine scale to identify opportunities, as well as programs or strategies, to preserve and grow tree canopy. San Diego recently received $10 million in federal funds to enhance the city's urban forest, and Mayor Todd Gloria said this funding will be put toward the 2035 CAP goal. Specifically, this grant is estimated to plant and preserve 8,400 trees, with an emphasis on underserved communities. City residents can find details about the city’s urban forest plans and also request a free street tree by filling out an application at sandiego.gov/trees.
The integration of green urban spaces, particularly through tree planting, brings multifaceted benefits. Trees serve as effective carbon absorbers, sequestering carbon dioxide and releasing oxygen. Streets with trees are 3°F lower temperature than unforested urban areas per (EPA United States Environmental Protection Agency), additionally UV exposure is significantly lower. A single mature tree annually absorbs 21.6kg of CO2 and provides enough oxygen for two people. They act as natural anti-flood reservoirs, with 100 mature trees capturing 1,137,500 liters of rainwater, reducing stormwater runoff by 2% for every 5% tree cover in a community.
Implementation
Proper planting of trees is critical for their long-term health and resilience. A few key strategies to consider when planting trees are soil biome, soil structure, irrigation, and micro-climate. The soil biome, or soil microbiome, consists of diverse microorganisms like bacteria, fungi, and protozoa, which play a vital role in nutrient cycling, organic matter decomposition, and overall soil health. Soil structure refers to how soil particles clump together. Well-structured soil for trees allows for water infiltration, root penetration, and aeration. If one has compacted or poorly drained soil, it is suggested to amend it by adding organic matter to improve its structure. After planting, adequate water must be provided to help the tree establish its root system. While water conversation is key, and many trees can do well with low water, it is important to water deeply but infrequently to encourage deep root growth. Even after the first few months, weekly irrigation is key until the tree is fully established. Micro-climate considerations include sun exposure, wind direction, tree structure, rooting space, and positioning. Thoughtful planning and proper planting techniques set the stage for healthy, thriving trees that benefit a city’s community and the environment.
McCullough Landscape Architecture endeavors to achieve the highest good in conservation through our implementation of green urban spaces, thus providing more opportunities for our communities to connect with nature. Trees serve a critical role in that pursuit and features heavily in our design work. Through our mindful practice we merge context, creative passion, health and wellness, ecological resiliency, and climate adaptation through the use of urban trees. In this way we seek to root our work in empathy, community, and the human spirit.
Resources