Is tree planting in desert cities a climate solution or water waste?

Climate change makes cities consider adaptation versus mitigation

Diane E. Pataki, Foundation Professor and Director of the School of Sustainability

Tree planting is a climate solution whose moment seems to have arrived. There are municipal programs all over the world to plant a billion trees, national programs to plant a billion trees, and global programs to plant a billion trees in order to slow the pace of climate change through biological carbon sequestration.

At the same time, climate change is reducing the availability of water needed to support trees in dry regions such as the southwestern U.S, especially Arizona State University’s home metropolitan area of Phoenix. In these areas, trees and forests can’t grow without supplemental water, creating tradeoffs between nature-based solutions to climate change and water conservation programs. In Arizona and elsewhere, some have suggested that planting trees in dryland cities may be a luxury we can’t afford in a time of severe drought.

So, what does science have to say about tree planting in dryland and desert cities? Is tree planting actually a climate solution in the Southwest and similar regions? Or should dry cities cut back on tree planting to save water? The answer depends on whether the goal is to mitigate or avoid climate change, or to help cities and their residents adapt to rising temperatures.

Let’s start with the science of climate mitigation in the form of biological carbon sequestration. Trees are made up of about 50 percent carbon, which they obtain from the atmosphere through photosynthesis. When it comes to carbon sequestration in trees, what you see is what you get. Larger trees store more carbon, and the more trees there are on land, the more atmospheric carbon is locked up in forests.

Natural forests cover more than a third of the global land area, so they are very important in regulating global climate. Cities, on the other hand, cover less than 1% of the land surface. But greenhouse gas emissions are highly concentrated in cities — so concentrated that within cities, the amount of carbon taken up through photosynthesis is a small fraction of the amount emitted through fossil fuel combustion. For example, per unit of land area, Los Angeles emits more than ten times the amount of carbon fixed by natural forests in photosynthesis.

These numbers do not bode very well for planting urban trees as a climate mitigation measure. Natural forests have continuous tree cover, but there is little available space in cities to reach the tree densities found in natural forests. While extensive tree planting efforts at large regional and continental scales may have an impact on the global carbon cycle, it’s unlikely that municipal tree planting programs will remove enough carbon from the atmosphere through photosynthesis to lower the concentration of CO2. And in the end, we must lower atmospheric CO2 concentrations to avoid climate change.

However, the situation is very different when considering options for urban climate adaptation, and for helping people cope with rising temperatures. In this case, the effect of urban trees can be quite significant.

Trees influence both local climate and the human experience of climate, which is called thermal comfort. Trees lower air temperature at and near the ground surface through shading effects, and also through the cooling effects of water evaporated at the leaf surface, which acts as a heat sink. The shading effect of trees also lowers the mean radiant temperature — a measure of how people experience temperature based on the exchange of solar radiation. You already know how much cooler it feels to stand in the shade of a tree rather than in the full sun, and that effect will be increasingly important as cities get hotter. The more cities make shade and evaporative cooling available, the more urban residents will be able to tolerate the climate without air conditioning.

But what about the water use of trees? Will we have enough water in desert cities to plant and maintain urban forests in future climates?

Research has shown that the answer to this question is yes. Outdoor landscapes are responsible for the majority of household water use in southwestern U.S. cities. However, most outdoor water use in these cities is attributable to lawns, not trees. For example, in Los Angeles 70% of landscape water use comes from lawns. Lawns have a much higher leaf area than trees, and they’re also very shallowly rooted, so they have to be irrigated very often. Deeply rooted trees are comparatively efficient in using water. There are differences among tree species, but for the most part, urban forests can be sustained with a much lower water allocation than current household water use.

Municipal tree planting programs can be particularly effective for climate adaptation by focusing on the locations where cooling will have the most impact, such as pedestrian walkways and bike paths, bus stops, transportation routes and sites that shade buildings. Some of these applications will require more research — for example, the influence of trees on indoor air temperatures has not been directly measured in many studies. But overall, the current evidence shows that urban tree planting can be a climate solution in dryland and water scarce cities, even as water conservation becomes increasingly important.