Modeling the mysterious flow of rainwater and nutrients through urban tree canopies: a laser-focused integration of hydrology and ecology

Tree canopies have large surface areas that interact with rainfall and contribute substances that affect the quantity and quality of stormwater runoff. For example, trees can (a) redirect rainfall back to the atmosphere through evaporation of water that gets held up on leaves or branches, known as interception, (b) redirect rainwater into soil by funneling it down their trunks, known as stemflow, or (c) allow the rainwater to fall or drip through leaves and branches below the canopy, known as throughfall. In cities, these flow pathways can prevent rain from landing directly on pavement, which can overload storm-sewers and carrying pollutants into rivers. In this project, scientists will investigate how different tree species with different canopy traits such as smooth bark or rough bark partition rainfall into interception, stemflow, and throughfall. The team aims to determine which canopy traits might be most useful for filtering stormwater and reducing floods in urban areas. They will use uniquely integrative methods from different disciplines, drawing from ecology, hydrology, and remote sensing to quantify how rainfall is affected by the canopy traits of species such as Oak and Maple that are commonly found in American cities. The team expects their findings to increase understanding of how to optimize the ecosystem services provided by urban trees. They will train graduate and undergraduate students and collaborate with community members and local officials such as land and water managers to improve management of stormwater through optimized planting and maintenance of trees. This research will focus on about 30 tree species in an arboretum near Cleveland, Ohio. Scientists will use terrestrial laser scanning and other methods to measure characteristics of canopy form such as leaf waxiness, leaf and branch angles, bark roughness, and the surface area of leaves and branches. Hydrological measurements beneath each tree will quantify canopy interception, stemflow, and throughfall. The nutrients in samples of stemflow and throughfall will be monitored to inform how trees mediate the flow of airborne nutrient pollution into soils and stormwater systems. The high spatial and temporal resolution of hydrological measurements, coupled with novel applications of hydrological models, will yield a view of tree-rainfall interactions that is unprecedented in its accuracy and utility. Statistical relationships between canopy traits and hydrological flows beneath trees will be described to optimize selection of tree species that minimize stormwater runoff into sewers. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.