My name is Katelyn McBride, and this summer, I worked with sap flow measurements in American sycamores (Platanus occidentalis L.) at The Morton Arboretum. Sap flow readings integrate information about photosynthesis, transpiration, water transport, and wood condition in a tree. As a result, measuring sap flow can give us crucial information about the overall health of a tree, similar to measuring blood pressure in a human.
Sap flow readings can be dramatically affected by environmental factors. This summer, we focused on four key factors: solar radiation, precipitation, temperature, and vapor pressure deficit (VPD), or the atmospheric “pull” on the water in a tree’s leaves. Our analysis was made more difficult by the complex relationships shared by all four variables. For example, precipitation is often correlated with a decrease in solar radiation due to increased cloud cover, and precipitation is also correlated with a decrease in VPD due to the increase in the relative humidity during a storm.
Despite the challenges posed by our data, we discovered a number of interesting relationships. First, sap flow in our studied trees was closely correlated with solar radiation for the month of July 2019. In fact, solar radiation explained over 80% of the variation we observed during July. However, American sycamore sap flow changes in how it responds to environmental factors throughout the 2019 growing season. Going forward, we plan to expand our analysis to the entire growing season, which will allow us to determine exactly when these trends change.
Although this experience took place in the midst of a global pandemic, the members of the Tree Observatory Program at the Morton Arboretum understood that we must continue to safely and deliberately prepare for upcoming challenges if we hope to overcome them. Urban trees are particularly vulnerable to the effects of climate change, as they face a number of unique challenges, including decreased soil availability, increased temperatures, and isolation from other trees. In addition to examining relationships between sap flow and environmental factors, we plan to investigate the variation in sap flow patterns between trees living in groups and trees living alone. When combined with other measurements from the Tree Observatory Program, research into sap flow can help us build a complete profile of tree health and keep the trees in our neighborhoods healthy for years to come.
Sap flow sensors on one of our American sycamores. Photo taken by Samantha Panock at The Morton Arboretum.