City Trees

Most net precipitation drips from the canopy or through its gaps as "throughfall." Still, throughfall can concentrate large amounts of water at drip points, where the surface can receive up to 10x more water than pre- cipitation in the open. The location where throughfall drip points occur may be mean- ingful for an area. For example, do throughfall drip points direct most net precipitation to impervious or permeable surfaces? Trees have another trick up their trunk for re-directing large amounts of precipitation to tiny spots at the surface: stemflow. If a tree's canopy has enough branches, inclined at the right angles, and with the right bark (not so rough, flaky, or spongey), hydrologists have observed stemflow waters at the surface that are 100x greater than open precipitation! Although stemflow is one of the least under- stood pathways for precipitation through tree canopies, it has often been reported to supply >10x more water per unit area to near-stem soils than open precipitation. Depending on the surface and subsur- face soil properties, stemflow rates can be so high that they overflow urban tree planters and become runoff, or preferen- tially infiltrate deep into the subsurface. Because of this, it is arguably worthwhile knowing the stemflow patterns in an urban forested watershed. Importantly, since stemflow and throughfall drip points bathe the most canopy surfaces, they can be loaded with as much nutrients as runoff. So, let's start looking upward during storms— or at least, let's start thinking about how precipitation may interact with the trees we plant and with our modifications to the existing canopy. Although these interactions have not been very systematically researched compared to other processes, we can improve interception estimates and make a few gen- eralizations about throughfall and stemflow. Anecdotally, tree species with steep branches and smooth bark have been observed to generate more stemflow, while species with multiple branch bends or confluences (which may also be due to pruning) may create throughfall drip points. If trees are selected or pruned in such a way as to favor stem- flow, one should ensure that the near-trunk soils are capable of infiltrating high water supply rates. Given how little these processes have been considered to date, perhaps their consideration in future watershed manage- ment will yield surprising new advances. —John T. Van Stan, II, Ph.D., Director of the Applied Coastal Research Laboratory, Georgia Southern University, Savannah, GA and Associate Professor, Dept of Geology & Geography, Georgia Southern University, Statesboro, GA begun on the trunk of a Statesboro, GA. As the stemflow pathway grew into transporting nearly 100 liters soil at the base of the Photo by John Van Stan Asheville, NC, stemflow- that it washed away the stem, exposing bare soils. Throughfall drip points and their branchflow pathways, frozen in time during a rain-to-ice storm in Statesboro, GA. Photo by John Van Stan www.urban-forestry.com 23

• Cover