Landscapers, designers and caretakers know all too well the irrigation impacts that some trees play in a managed landscape: Dry shade when trees are leafed out and rainfall interception when it does rain. Trees are an important, yet often overlooked, piece of the stormwater management system. Being able to model the landscape for functionality in terms of stormwater management requires better understanding how which trees do the job.
Surface Water Storage Capacity of Twenty Tree Species in Davis, California was part of a special section in the Journal of Environmental Quality on “The Urban Forest and Ecosystem Services.” The section includes articles about how trees cool the city’s hardscape and the people within it. Articles discussed how trees affect air quality, both good and bad, and the role of trees in taking up and removing pollutants in stormwater among other topics. This open-access article The Urban Forest and Ecosystem Services: Impacts on Urban Water, Heat, and Pollution Cycles at the Tree, Street, and City Scale, includes an overview of all the research featured in the special section.
Which of 20 Davis, California, trees are best?
“Urban trees, and the urban forest as a whole, can be managed to have an impact on the urban water, heat, carbon and pollution cycles. However, there is an increasing need for empirical evidence as to the magnitude of the impacts, both beneficial and adverse, that urban trees can provide and the role that climatic region and built landscape circumstance play in modifying those impacts.”
Trees store rainfall in their canopy in a number of ways: On and between their leaves, and on stems, branches and trunks. The experiment simulated rainfall events onto cut branches. Researchers noted that surface water storage capacity varied threefold among all the species, from Lagerstroemia indica (0.59 mm) on the low end to Picea pungens Engelm. (1.81 mm) on the high end. The mean storage capacity for leaf surfaces was only 0.97 mm. The mean storage capacity for stem surfaces only was 0.25 mm.
Of interest, they noted that trees with rough stem and leaf surfaces held more water. Interstitial spaces between leaves, buds and stems store water and rough surfaces reduce water flow. Evergreens can store more water year-round, since the only structures intercepting water on deciduous trees in the winter months are stems, branches and trunks. The study has many limitations, the authors write, including underestimating the amount of water that would be stored by mature trees.
Surface Water Storage Capacity of 20 Tree Species in Davis, California
- Celtis sinensis Pers.(0.71 mm)
- Fraxinus angustifolia Vahl Raywood (0.60 mm)
- Ginkgo biloba L. (0.64 mm)
- Gleditsia triacanthos L. (0.67 mm)
- Lagerstroemia indica L. (0.59 mm)
- Liquidambar styraciflua L. (0.95 mm)
- Pistacia chinensis Bunge (1.17 mm)
- Platanus hispanica Mill. ex Munchh. (0.87 mm)
- Pyrus calleryana Decne. (0.51 mm)
- Quercus lobata Nee (0.91)
- Zelkova serrata (Thunb.) Makino (0.84 mm)
- Cinnamomum camphora (L.) (0.79 mm)
- Eucalyptus globulus Labill. (0.70 mm)
- Fraxinus uhdei (Wenz.) Lingelsh. (0.78 mm)
- Magnolia grandiflora L. (0.81 mm)
- Quercus ilex L. (0.82 mm)
- Picea pungens Engelm. (1.81 mm)
- Pinus canariensis C. Sm. (0.99 mm)
- Pinus pinea L. (1.04 mm)
- Sequoia sempervirens (D. Don) Endl. (1.16 mm)
“A tree’s total leaf and stem surface area plays an important role in rainfall interception during the leaf-on and leaf-off seasons. During the leaf-on season, a 40-year Japanese zelkova tree can intercept 85, 85 and 62% of rainfall for a 2-, 5- and 25-year storm event, respectively. However, during the leaf-off season, interception drops to 16, 26 and 25% for the same storm events.”
The researchers concluded that the high variability in storage capacity across species is evidence that more study is required to look at additional tree species. Evergreen conifers had the highest storage capacity than broadleaf trees.
NewTerrain May 16, 2016.