Measuring the Effect of Urbanization on Water Use by Trees in Riparian Settings

Urban forests provide measurable benefits to both human and ecological communities. What is less clear are the effects of urbanization and climate change on vegetation, and on riparian systems in particular. As urbanization increases runoff and municipal water leakage into inner-city creeks, many experience regionally atypical perennial streamflow. This extra water source impacts vegetation growth and water use strategies which cascade to effect soil wetness and erosion, stream dynamics, and temperature and humidity in and around creeks. This study investigates the total-canopy effects of an urbanized creek on two species of large riparian trees in Austin, Texas. We designed, built, and deployed Loggers for Ecological and Atmospheric Factors (LEAF) at four canopy levels in bald cyprus (Taxodium distichum) and durand oak (Quercus durandii) in an urban creek where it runs through the University of Texas at Austin campus. We compare the response of these trees to trees of the same species in a less urbanized creek, Onion Creek, as determined by land cover use and the relative abundance of impervious cover. The effect of sustained municipal baseflow on vegetation growth is supported by a preliminary dendrochronology study in the same area which demonstrates that Onion Creek riparian trees are more strongly coupled to past drought and precipitation records compared to the trees at Waller Creek. We hypothesize that this is due to urban leakage which will also result in differences in vegetation function and water use strategy.

The LEAF system measures transpiration, canopy throughfall, branch movement, relative humidity, temperature, barometric pressure, and stream-bank soil moisture. This high resolution and species-specific comparison of the physical hydrological and atmospheric processes around storm events in urbanized and unurbanized riparian systems gives us new insights into ecohydrological coupling and the effects of anthropogenic alterations to the hydrologic cycle. Better understanding of these effects will allow predictions about the changing climate in cities and the habits of vegetation that impact the water cycle and budget in urbanized areas.