Point dendrometers are high precision devices that allow detecting stem radius changes at resolution of micrometers. These stem radius changes show irreversible seasonal stem growth processes on the one hand and reversible diurnal shrinkage and expansion of the bark indicating missing water in the tree on the other hand.
Stem growth is mainly attributed to the activities within the cambium, a layer of cells between the bark and the wood. The cambium produces new cells towards the center of the stem, mainly differentiating to xylem (wood), and towards the periphery of the stem, mainly differentiating to phloem (bark).
The water related processes are driven by varying (under-) pressure conditions inside the tree, mainly induced by transpiration. These changing water tensions in the xylem lead to a dehydration of living tissues in the stem. Thus, during the daytime when transpiration is elevated, the stem delivers water from elastic tissues additionally to the water coming from the roots. This water loss from the stem (mainly the bark) is proportional to a reduction in stem radius. During nighttime, when transpiration ceases, elastic stem tissues are replenished and stem radii increase again.