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A Sappy Topic

With the Covid-19 pandemic still in full swing, the ability to conduct our research from home has become more useful than ever. Fortunately, sensors installed as part of the Tree Observatory Program have been collecting sycamore sap flow data since 2018, and now is the perfect time to analyze their readings!

My cat plays with her toy on my desk.
My coworker got her degree in cuteness from DePaw University. Unfortunately, it didn’t include data analysis.

So why collect sap flow data?

Measuring sap flow lets us understand the circulatory system of a tree, just like measuring blood pressure lets understand a person’s circulatory system. By summarizing the effects of temperature, water availability, and wood condition, sap flow provides a window into the overall health of a tree.

What does sap flow data tell us about a tree?

Just like humans, trees need to drink water in order to survive. Trees use xylem to bring water and essential nutrients from the roots up to the canopy, similar to how we use suction to drink through a straw. Water travels from the roots in the form of sap, so a tree’s water usage can be measured through sap flow.

Sap flow measurements can also help us to understand how trees breathe. Tree leaves are filled with stomata, which are tiny pores involved in gas exchange. Stomata allow plants to bring in carbon dioxide, which is needed for photosynthesis, and send out gases like oxygen and water vapor. The evaporation of water vapor through a tree’s stomata is known as transpiration. Since levels of sap flow and levels of transpiration are closely correlated, sap flow can be a convenient way of measuring transpiration when the canopy is difficult to access directly. Therefore, sap flow is useful for understanding both how trees drink and how trees breathe.

What parts of the environment affect sap flow?

Temperature can affect sap flow by changing how much a tree breathes. As temperature increases, so does the vapor-pressure deficit, or VPD. VPD is the difference between the amount of water vapor currently in the air and the maximum amount of water vapor the air can hold.

Consider a glass of water.
You can picture VPD using a glass of water. The glass is how much water vapor the atmosphere can hold. The water is the amount of water vapor currently in the air. VPD measures how much water you could add without the glass overflowing.

Just like smell of baking bread spreading through a kitchen, water will move from areas of high concentration to areas of low concentration. Since the water vapor in the atmosphere is less concentrated than the water in the leaf (picture the leaf as a small, full glass and the atmosphere as a larger, emptier glass), the water in the leaf will “want” to travel through the stomata and into the atmosphere. So to put it more simply, VPD describes how much the water in the leaf wants to leave the leaf and enter the atmosphere. As the temperature increases, the air can hold more water vapor (picture an even larger glass). The water vapor in the air becomes less concentrated, and the water in the leaf wants to enter the atmosphere more. As a result, both transpiration and sap flow increase as the temperature rises. In my research, I will be examining sap flow on days with extreme high and low temperatures in order to gain a better understanding of the effects of temperature and VPD on sap flow.

The amount of moisture in the soil also has a big effect on sap flow. After all, if there’s no water in the soil, there’s no water for the sap to transport from the roots! In fact, on days when a tree doesn’t have enough water, it may choose to keep its stomata closed and not transpire at all. This may reduce the amount of water lost to the atmosphere, but it also prevents the tree from making food through photosynthesis. By measuring sap flow during flooding and drought events, we hope to gain a better understanding of the relationship between soil moisture and sap flow.

How can we use sap flow data?

Trees in cities face a number of unique challenges, including decreased soil availability, increased temperatures, and isolation from other trees. The last element I will examine in my research is the differences 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 and happy for years to come.