Trees and Stormwater: How are trees in cities different from forests?
By Steven Quick
What can we learn from comparing water use in urban trees to water use in forest trees?
I grew up on the Eastside and graduated from the Science and Math Institute in Tacoma. As a kid, I used to love it when my mom drove us up to the North End. I loved driving under the bigleaf maple and red maple canopies, and I always wondered how those trees were different from their forest-borne counterparts. Twenty years later, I’m now a forest ecologist and I’m studying these trees… or at least, trees just like them.
The trees I study are down at The Evergreen State College, in the parking lot, which mimics urban activity and impervious surface cover like what we see in Tacoma. The labs I work with are interested in the way that trees use water in urban areas. Researchers know a lot about tree water use in plantations and natural forests, which have wide open air above their canopies and a lot more soil to spread their roots. This allows the sun to evaporate more water from the canopy, and for the roots to supply the canopy with more water. But those maple trees that amazed me as a kid, peppered along the sides of Union Avenue, surrounded by pavement, remain less understood. Recent policies promoting canopy expansion have resulted in extensive planting of broadleaf deciduous trees. The problem is, we don’t know how water use in these trees compares to conifers in urban areas, and uninformed decisions can lead to dysregulated ecosystems.
To figure this out, we designed a remote device that measures the speed of water moving up the tree stem. We plugged these into several trees in the parking lot at Evergreen and compared them to trees in a nearby forest (Figure 1). What we found was that conifers and broadleaf trees don’t move water all that differently, but they do move water at different times of year and some species move a lot more than others.
Figure 1. A thermal dissipation probe installed in a Douglas-fir. Photo credit: Steven Quick.
Broadleaf trees move more water at the beginning of the year while conifers do more of the work at the end of the year (Figure 2). On day 150 in 2023 (June 1), bigleaf maples were moving upwards of 200 liters of water per day while Douglas-firs were moving about 75 liters per day. Compare that to day 200 in 2024 (late July), where the bigleaf maples were moving about 50 liters of water per day and the Douglas-firs were moving about 125 liters per day. The situation in late 2023, where bigleaf maples show greater transpiration at the end of the year, was an anomaly largely driven by temperature differences between years. If the temperatures were the same, this is exactly what we would expect.
Figure. 2. A time series plot showing average sap velocity (points and lines) by species (colors) in 2023 and 2024. Species codes are as follows: Bigleaf maple, Acer macrophyllum (ACMA); Red maple, Acer rubrum (ACRU); Honey Locust, Gleditsia triacanthos (GLTR); and Douglas-fir, Pseudotsuga menziesii (PSME).
When we compared our parking lot trees to the forest trees directly, we found that they were moving water at similar rates, but dramatically different volumes (Figure 3). This is likely an artifact of canopy or leaf area. Since the parking lot trees had smaller canopies, there was less leaf area to evaporate water from, and less access to soil and water, thus we expected lower volume. It was a surprise, however, that the velocity was not significantly different from the forested trees. This made us suspect that the parking lot trees were more stressed—and we were right. Notice in figure 3, that sap velocity approaches 0 for Douglas-firs in plot 3 (the parking lot) around day 250, while the forested Douglas-firs (plot 1 and 2) approach 0 around day 200. This suggests that these trees are less resilient to environmental pressure because they couldn’t regulate their water loss as quickly as trees in the forest.
Figure 3. Time series plot faceted by plot number, showing average velocity (above) and volume (below) in points and lines, with species indicated by color. Plots 1 and 2 are forested, and Plot 3 is urbanized.
These conclusions are consistent even when we control for tree size, too. So, size doesn’t matter here, but that’s a good thing! It means that scientists can easily estimate tree transpiration based solely on its species. If this were not the case, then urban foresters would need to go out and measure each individual tree across the whole city. This work will help improve urban forestry practices and hopefully compel municipalities to plant more conifer trees.
But what does this mean for the community? I recommend that communities work to plant and protect conifers, especially large conifers which have greater influence on total transpiration. When planting, we should aim to give these trees as much bare soil and open air as possible. When protecting, we should aim to conserve soil water in the summer months by laying mulch around small trees, and we should look for appropriate ways to water the deep roots of large trees. I also recommend that communities focus planting efforts on native species, which transpire significantly more volume compared to species bred for urban applications like the red alder and honey locust (Figure 2). These introduced species demand resources all the same but are less effective at mitigating stormwater.
Steven Quick is a graduate student in the Master of Environmental Science program at Evergreen State College. Since 2016, Steven has been working on urban and remote land conservation and restoration projects aimed at restoring salmon populations across the state. His interests in forests and ecosystem services have led to his current work investigating trees as stormwater solutions and improving urban forestry research methods. He currently works as a research assistant for the Fischer Lab of Plant and Community Ecology at Evergreen, and a data management technician for Mount Saint Helens Institute.