Edinburgh Napier University

  The diameter of five dominant Sitka spruce (Picea sitchensis) trees at Griffin Forest (near Aberfeldy, Scotland) were monitored at two minute intervals with dendrometers between April 2008 and October 2012. This experiment was designed to investigate how changing daily and seasonal conditions can effect tree growth, but they also revealed the daily cycle of the tree swelling and shrinking due to moisture movement over the course of a day, within the growing season. This is a well-known phenomenon in tree physiology and previous studies of this have concluded that these daily fluctuations are a result of high transpiration rates during daylight, when absorption of water through the roots cannot supply enough water to the stem resulting in water being drawn from tissue causing the stem to shrink. The stem swells back again at night when the stem re-saturates with water. This experiment corroborated the findings of previous researchers. At Griffin the trees reach a peak in size each day at about 6 am and at a minimum size in the evening at about 5pm. This mechanism of the shrinkage and swelling in the living tree is not the same as when the wood is cut, but the biomechanics are part of the explanation for wood properties. In the living tree, cohesion forces keep the water column continuous and transpiration in the canopy creates a water tension gradient. Because of the wood elasticity, the wood volume decreases under this internal tension.