One of our urban arborist colleagues is looking for info to help inform an urban forestry bill now before the state legislature. Here in the desert Southwest, there is tension between using trees to reduce the urban heat island effect and our shrinking Colorado River water “budget”. Does anyone know where we can look for good data on the relative impact of urban trees vs. other heat island mitigation strategies (cool roofs and pavements, shade structures, etc)? What strategies are most powerful at reducing urban heat? I'm also looking for clearer data on the effectiveness of trees in reducing air pollution--some trees actually produce VOCs, so I'm having trouble understanding the net benefit. We are trying to assess the health benefits to vulnerable communities vs. the water cost. Appreciate any pointers!
Hi @Joanne Leovy. This is a really good report on the subject, by scientists in Arizona. They note on pages 57 & 59:
Additional water consumption, time to maturity, and the cost of planting and maintaining trees are important factors. For example, water consumption for outdoor landscaping in Phoenix accounts for roughly 45 to 70 percent of total residential water use (Declet-Barreto et al. 2013). This can be decreased by selecting native and drought-tolerant tree species and using green stormwater infrastructure (e.g., bioswales, curb cuts, and rainwater harvesting gardens) to help supplement watering needs…
Planners should pursue heat mitigation strategies to help reduce both chronic and acute heat risk in their communities. These strategies encompass land-use planning, urban design, urban greening, and waste heat reduction.
The report also talks about other shade structures, for example on page 55.
On the question of air pollution, trees help a little bit by intercepting some particulates and absorbing some pollution in leaf stomata. It's a pretty small effect, reducing pollutants by less than 1%, but that can still create a pretty significant health benefit on the macro scale. Some trees do emit VOCs, but that's a small effect. Allergy-triggering pollen is a bigger issue.
I'm working on a lot of materials on this question – the physical and mental health benefits of urban trees. I've got a new slide deck that I'll probably turn into a training page and CCU sometime soon, and also a Yale article on the topic that will probably be published next week, so stay tuned for that 🤓
@Joanne Leovy I'm an arborist and have been involved in urban forestry, and there's lots to consider, but foremost is that urban tree planting is a low-cost tool for adaptation to climate changes - especially if implemented with good science; even when the research has been fuzzy about whether tree planting actually can save the world from full-blown climate chaos. Unfortunately, many municipalities slog along with antiquated tree selection guides, poor and oversized nursery quality, dated planting techniques, and foolish watering regimens. Trees grew fine without people for a long time, so we need to let them do this as much as possible on their own - even as we try to increase urban tree canopy. The benefits have been researched by the USFS, and obviously each region needs to evaluate its weak links. Here in CA several studies have looked at climate ready trees. Planting larger species is also key, as they have a 10-fold advantage over their smaller “cousins” in terms of overall ecosystem benefits. In general, technology cannot ameliorate the existing damage quite like a tree.
@Joanne Leovy Having lived in Phoenix and been aghast at the amount of water that was used for grass and fountains, I know that this is a false argument and an attempt to redirect people's attention away from the real issues.
There's lots of research on replacing foreign trees and plants with native ones and their extra benefit of cooling. Anyone who takes a hike in the desert recognizes this. Plus shade decreases the need for water for the other plants near it. Only watering when/where it is actually needed is key. Another key item is limiting rock exposed to the sun in the “xeric” landscapes and not pruning plants into the little shapes (there are low growing desert plants great for this).
There are some great recent before & after data on the benefits of switching to native landscapes.
As with most things, there's not a one-answer solution. Frank Lloyd Wright showed us years ago with Taliesin West that it is a matter of creating a design that weaves into the natural location paying close attention to the sun's movements.
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