Sources: McPherson, E. Gregory; Xiao, Q.; van Doorn, Natalie S.; Peper, Paula J.; Teach, E. “Surface storage of rainfall in tree crowns – Not all trees are equal,” Arborist News; Qingfu, X.; McPherson, E.G., “Surface water storage capacities of 20 tree species in Davis, California,” Journal of Environmental Quality; “Interception: When it comes to capturing rainfall, not all trees are equal,” Landscape Architecture Magazine
Davis, CA (October 2017 ) – In urban settings, a tree’s ability to intercept or slow rainfall reaching the ground can reduce the amount overflowing onto paved surfaces and lost as stormwater runoff. U.S. Forest Service researchers and the University of California, Davis, measured the crown storage capacity of 20 common California urban tree species, and then calculated each species’ potential interception amount over 40 years.
Researchers represented storage capacity as the depth of the water film stored on the leaf, needle or branch. They calculated it by measuring the volume of water retained by the particular piece of vegetation in relation to its surface area.
Blue spruce (Picea pungens) held the most rainwater (1.81 mm) with its many narrow spaces formed by rigid needles, buds and stems. Canary island pine (Pinus canariensis) had the lowest storage capacity for conifers (0.99 mm) because water flowed readily down its long, flexible needles. Overall, Bradford pear (Pyrus calleryana ‘Bradford’) had the lowest storage value (0.51 mm) because of its smooth leaf and stem surfaces, while Chinese pistache (Pistacia chinensis) had the highest storage values for deciduous trees (1.17 mm) because its rigid leaflets and rough stems slowed water flow across its surfaces.
Deciduous trees also were tested without leaves to simulate their storage capacity during autumn and winter. Potential accumulated interception after 40 years was calculated to range from 6 to 17 percent of the amounts that would have been stored during the leaf-on period.
The study stressed the importance of matching foliation periods with local rain patterns so that foliage is present when needed the most. For example, in regions with winter rainfall, evergreens are more effective than deciduous trees for optimal rainfall interception. A summary article of this study can be found in Landscape Architecture Magazine (November 2016).
In a subsequent study published in Arborist News, McPherson and colleagues used Urban Tree Database growth equations that predict dbh from age and leaf area for 20 species to estimate potential interception at five-year intervals for 40 years after planting. The findings reveal that species selection makes a difference, and the difference tends to increase as trees age. Twenty years after planting, there is an estimated 20-fold difference in storage potential between zelkova and crapemyrtle (0.19 and 0.01 m3). After 40 years, the difference nearly doubles to 38-fold. Large-stature trees with substantial leaf- and stem- surface area have capacity to store much more rainfall over the long term than smaller-statured trees.
Arborists can use this research to select trees to maximize rainfall interception at selected periods after planting. For species not described here, the larger a trees’ crown and the more complex its surfaces, the more rainfall it will capture, generally speaking. Rough surfaces, rigid structures, and densely compact clusters of foliage, as with blue spruce, tend to store the most water.