The Maple Sap Season in
Underhill Ctr., Vermont: Spring 2011

Welcome to TreeMet, a webpage which illustrates some of the complex physiology of the maple tree during the sugaring season. Data shown on the graphs are collected in real time from several large (22"-26" dbh) sugar maples located in the woods about 600' southeast of the Proctor lab, and these measurements are updated every 15 minutes, so you can follow the sugaring season from start to finish.

graph of temperatures
graph of sap pressure
graph of trunk temperatures

Temperature measurements are made using fine-wire thermocouples, which are extremely accurate probes that can be inserted into small holes in the wood or soil, or suspended in the air. The branch temperatures shown on the first graph are from the center of a small branch located 54' off the ground. The trunk temperatures in the third graph are measured at a height of 4.5' using probes at the depths and aspects indicated on the graph (the tree is 22" in diameter).

Sap Pressure measurements are made by drilling a 1.5" deep taphole, inserting a standard 5/16" plastic spout, and connecting the spout to a small pressure sensor. Over the course of the season, many episodes of both positive and negative pressure are typically recorded. Negative pressure occurs when small branches freeze and pull water into the wood fiber cells of branches, where it accumulates as ice crystals. This force, or suction, is transmitted through the trunk to the roots, which then take up more water during the process. Positive pressure occurs when the ice crystals in small branches thaw, resulting in several forces, including gravity, bubble expansion, and osmosis, acting together. Pressures shown on the graph are in PSI, or pounds per square inch. Note that pressures (and sap flow) are measured on both the south and north sides of the tree, as temperature and pressure are often very different on these sides. Trees in which pressures are measured are designated N 58, N60 etc. on the graph, with the number referring to a specific tree.

Sap Flow measurements are made by tapping trees (5/16" spouts, 1.5" deep tapholes) on the north and south sides, and collecting the sap by gravity in chambers that are fitted with sensors to record the increasing depth. The lines on the graph represent cumulative flow in liters; the flow rate can be estimated by the slope of the lines (periodically the flow totals are reset to zero so the line will not go off the graph). When there are two trees with sap flow measurements, they are designated sapflow N 1 and sapflow N2 etc.

The data that you see have traveled from the tree, to a datalogger in the woods, to a shed in the woods with a computer where the data are graphed and graph images created, to a computer in our lab via fiber optic cable, and finally to the UVM webserver. Occasional malfunctions in this electronic stream will cause new data to not be shown for a day or two until the problem can be located and corrected.

Questions or Comments about TreeMet, please email