Sapwood Temperatures in Hemlock Trees at Harvard Forest HEM Tower 1997-1998

Sapwood temperatures in four large hemlocks, 130-200 years old and about 40 to 90 cm basal diameter and 25 to 29 m tall, were measured in order to estimate sapwood respiration in these trees as part of a physiological model for hemlock forest carbon exchange. As expected, sapwood temperature varied most in small branches and least in the main bole of the tree near the ground. An additional finding from these measurements is that sapwood begins to freeze at -1 to -2 degrees C, as shown by a stabilization and in some cases an increase of about 1 degree C in sapwood temperature when air temperature cooled continuously to below this temperature range.