Where do urban trees get their water?

PI: Ian Smith, PhD student, Department of Earth and Environment, Graduate School of Arts & Sciences; BU URBAN Program
Co-PIs: Lucy Hutyra, Professor, Department of Earth and Environment, College of Arts & Sciences; Pamela Templer, Professor, Department of Biology, College of Arts & Sciences

Read a Q&A with Ian Smith and watch a spotlight video.

Cities face an array of environmental challenges and disparities, most of which are poised to become more problematic with climate change. In response, cities often turn to nature-based climate solutions such as tree planting due to the suite of ecosystem services that city trees offer. Despite dedicated resources to expanding urban canopies, the benefits of trees in cities can only be realized if trees are healthy and mortality rates are low. A key factor in promoting tree health and survival is access to water, however, it is currently unknown where and how trees in cities access their water.

Understanding where trees source their water is critical for managing the health of urban trees and evaluating the feasibility of nature-based climate solutions in cities. If city trees primarily access water from precipitation, they may become vulnerable to forecasted increases in the frequency and severity of drought with a changing climate. If city trees primarily access water from irrigation, cities may need to modify management practices and water conservation measures may begin to limit services from urban trees. If trees can tap into underground anthropogenic water sources, such as sewer and stormwater pipes, to supplement their water demands, they may reduce their susceptibility to drought, but this may come at the cost of damaging critical infrastructure.

Smith, Hutyra, and Templer will leverage differences in the isotopic composition of various water sources in Boston (precipitation, irrigation, groundwater, and wastewater) to determine the relative contributions of each water source to water found in tree tissue. Isotopes are variants of the same element with an equal number of protons, but varying numbers of neutrons, resulting in different atomic masses while maintaining most chemical properties. Various chemical reactions and phase changes, like evaporation, are selective in the isotopes involved, resulting in the isotopic compositions of water not being uniform. Therefore, they expect that the isotopic compositions of precipitation, irrigation, groundwater, and wastewater will be distinguishable.

Samples of various water sources and tree tissue will be collected between May and August 2021 to answer three central questions: (1) What are the isotopic signatures of potential water sources for trees in Boston (precipitation, irrigation, groundwater, and sewer water)? (2) How is tree water use partitioned across the four water sources? (3) Do tree water sources shift across the growing season? The answers to these proposed research questions have important implications for tree management and planting in cities and will improve our understanding of the benefits that can be expected from initiatives that aim to tackle urban heat resiliency by improving the size and health of Boston’s urban forest under changing climatic conditions.

Publications

Winbourne, J. B., Jones, T. S., Garvey, S. M., Harrison, J. L., Wang, L., Li, D., … & Hutyra, L. R. (2020). Tree transpiration and urban temperatures: Current understanding, implications, and future research directions. BioScience 70(6), 576–588.

Selected as editor’s choice for July 2020 issue and featured on Bioscience Talks podcast.

See more of our 2021 Early Stage Urban Research Award recipients