City Trees

For Urban Forests, Watershed Management Starts at the Treetop Key Terms Canopy water storage capacity: Maximum amount of precipitation water that can be stored on the surfaces of leaves, bark, and any (epiphytic and par- asitic) plants living within the canopy. Net precipitation: Total amount of pre- cipitation that reaches the soil surface beneath plant canopies. This is the sum of throughfall and stemflow. As the surface in forests can have a litter layer, this term differs from "effective precipitation" which describes the amount of net precipita- tion that actually enters into the soil. Precipitation interception: Storage and evaporation of precipitation by plant can- opies. This process reduces the amount of precipitation that reaches the surface and is typically just called "intercep- tion" in the hydrology community. Stemflow: Precipitation which drains down leaves and branches until reaching the trunk, whereafter this water con- tinues to drain in rivulets until reaching the surface near the trunk. Throughfall: Precipitation that passes through gaps or drains along leaves and branches until reaching a point where it drips to the surface. This can create (a) dry spots where precipitation was diverted away to another location and (b) concentrated drip points. Because we live on the land surface, the effects of urbanization on stormwater runoff are literally right beneath our noses. As a result, much is known about runoff pro- cesses and watershed management options exist to control runoff (and its associated nutrients and pollutants). We rarely look upward during a storm and, therefore, it comes as no surprise that the processes in urban forest canopies that precede runoff are much less researched and hardly con- sidered in watershed management. I say "hardly considered" because one role of urban trees is somewhat included in water budgeting: the interception of precipitation by canopies. Given the right canopy condi- tions, interception can reduce precipitation by 25% in large storms, and >90% in small storms! Broad, and sometimes problematic, assumptions are often made to estimate interception for an entire urban forest. For example, it may be assumed that all trees with one kind of canopy (i.e., needle- leaves) can store x amounts of precipitation, while all trees with another kind of canopy (i.e., broadleaves) can store y amounts of precipitation. Of course, as arborists are no doubt aware, canopy structure (and there- fore the amount of precipitation that can be stored on a canopy) can vary dramatically between individual trees of the same species in different urban settings—and even more so between species with similar leaf types. Thus, broad assumptions about the urban forest's canopy water storage capacity can result in erroneous estimates of interception. This is important as errors in interception are carried through all subsequent water budget calculations, including runoff and infiltra- tion. With this in mind, something as simple as sharing tree age and canopy structural information (e.g., whether branches or dead- wood were pruned) with watershed managers may help improve not only interception esti- mates, but reduce uncertainty for other more common hydrological processes of interest. Now, what about the precipitation that reaches the surface beneath canopies—the net precipitation? One may think, since net precipitation is what's left over after interception, that with an interception estimate you can begin making hydrolog- ical calculations and predictions at the surface … but, the story is a little more complicated than that. The pattern of gaps, leaves, and branches (all at different angles) force net precipitation into a pattern of wet and dry spots at the surface. A stemflow pathway has begun Lagerstroemia indica tree in storm progressed, this stemflow a highway capable of transporting (26 gallons) of water to the soil stem in a single rain event. Photo After a heavy rain event in Asheville, induced runoff was so large that leaf litter from around its stem, 22 CityTREES

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