An investigation into the factors affecting street tree rainfall interception

As cities expand to meet the requirements of the growing population, construction of impervious infrastructure causes stormwater runoff during rainfall events. This runoff may contain toxic contaminants and pathogens are is hazardous to people and the environment. Rainfall interception, the process of precipitation retention on tree surfaces that is subsequently evaporated back into the atmosphere, has gained attention for the implications trees may have on stormwater runoff mitigation in urban environments. Although interception has been documented on forest-stands dating to the early twentieth-century, assuming similarities between forest-stands and urban trees is likely inappropriate due to inherent differences in tree physiology and micro-meteorological conditions. Therefore, this dissertation addresses the current literature gap by investigating some of the variables that affect urban tree rainfall interception. The research objectives are separated into three studies: Study 1 (i.e., Chapter 2) evaluates the association between sub-millimeter surface morphology and interception storage capacity of the honeylocust street tree surfaces (Gleditsia triacanthos L.); Study 2 (i.e., Chapter 3) quantifies rainfall interception of the honeylocust in relation to the four canopy phenophases; and, Study 3 (i.e., Chapter 4) examines the interspecific differences of storage capacities and interception depth among ten common street tree species in New York City (NYC). In Study 1, the association between sub-millimeter surface morphology and interception storage capacity of the honeylocust foliar and bark surfaces were evaluated through the integration of bioimaging microscopy techniques, three-dimensional (3D) surface mapping, and laboratory-based interception storage capacities. Results demonstrated a relationship between microrelief and storage capacity among the three canopy phenophases and tree height for the foliar and bark surfaces, respectively. 3D surface mapping revealed insight that greater surface microrelief may facilitate retention by trapping water in reservoirs formed from microstructures (i.e., trichome projections, vein contours, and valley depressions). The dominance of the bark surface was also observed, which consistently had five to six times greater roughness and storage capacity than the foliar surface. Implications from this study raises the question as to the significance of bark and foliar surfaces on the seasonal variation of rainfall interception among the canopy phenophases. The influence of canopy phenophase (i.e., leafless, emergent, full leaf, senescent) on rainfall interception were quantified through field-based monitoring of forty-one rainfall events for three honeylocust street trees in Study 2. While interception percent (i.e., 25.7 - 38.8 %) and depth (i.e., 3.3 - 3.4 mm) varied, ANOVA determined that interception did not statistically differ among the phenophases. Rather, a stepwise regression analysis indicated that interception was most significantly predicted by event-based cumulative precipitation. The limited role of canopy phenophase also may be attributed to the underscoring dominance of the bark surface, which estimated between six and five times greater storage capacity and surface area than the foliar surface, respectively. Irrespective of foliage presence or absence, findings suggest that street trees can effectively intercept rainfall year-round in urban environments, and bark surface characteristics may dictate interspecific interception among street tree species. Study 3 examined the interspecific differences in the bark and foliar storage capacities and up-scaled interception storage depths among ten common species of street trees in NYC. Results showed that bark and foliar storage capacity substantially varied among the species, although mean bark storage capacity was an order of magnitude greater than the foliage. Up-scaled interception storage depth ranged between 1.0 - 4.0 mm among the species, demonstrating that thick-bark species generally facilitated greater water retention than thin-bark species. Specifically, the bark represented > 90 % of the entire interception depth yet contributed 42 % of the surface area. In addition to suggesting that street trees can retain considerable quantities of rainfall, this study emphasizes the critical role bark surfaces have on interspecific interception storage depth and the potential importance of species selection in cities to improve stormwater runoff management.