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Shedding Light on Sap Flow

               I learned a lot this summer. I learned how to build real connections with my mentors, Arboretum employees, guest speakers, and other REU participants without meeting face-to-face. I learned how to manipulate our data in R, finding new ways to organize, visualize, analyze, and share information from several different sources. I learned the basics of sap flow measurements, from why environmental factors can influence it to how we can use it to improve the health of urban trees in the future.

               But perhaps the most important lesson I learned this summer was to be flexible and embrace the fact that research – and life – isn’t always a straight line. At the start of this project, I planned to investigate the relationship between sap flow and temperature and precipitation extremes, but by the end of the project, I was working far more with VPD and solar radiation. In fact, solar radiation ended up being a better predictor for sap velocity across the month of July than any of the other factors we considered!

A quadratic regression of average sap velocity vs. solar radiation (July 2019)
With an R^2 value of 0.8 for this quadratic regression, solar radiation was closely correlated with sap velocity for July, 2019.

               Without a period of free exploration with the data, we would have missed this key trend and the opportunity to understand how the amount and timing of sunlight can affect rates of photosynthesis, transpiration, and sap movement across the whole tree.

               In light of the results we obtained for July (pun intended), I am excited to analyze the data for the entire growing season of 2019. Will the trends be the same in May or June? What about August or September? I would also like to see if the trends we observe hold true in other years and other species. The Tree Observatory Program has sap flow sensors set up for Norway spruce, juniper, and Buckley oak, and it would be fascinating to see if other species respond in the same way as our American sycamores.

               Going forward, I hope to continue my involvement with tree observation and research. From expanding our sap flow findings to integrating them with other sources of observational data, there is still so much left to discover about the hidden complexities of trees. I learned a lot this summer, including this: what I want to do is continue to learn more.