Seasonal change in tangential strain on the inner bark in white birch (Betula platyphylla var. japonica)

The aim of this paper is to acquire a growth information from the tangential strain due to changes in turgor pressure in the cells of the cambium and phloem. The strains were measured nondestructively and continuously using strain gauges pasted on the surface of the inner bark of the stem and root of white birch (Betula platyphylla var. japonica). They showed seasonal changes. As a repeating seasonal pattern, the tangential strain increased year by year. In the season when secondary growth was most vigorous, the tangential strain increased rapidly and then decreased in July and August. Significant amplitude was not seen before the sprout, then both the strain and its amplitude began to increase rapidly accompanying the sprout. In early July, the diurnal strain amplitude showed a peak coinciding with the peak of the strain. The diurnal amplitude of strain disappeared after defoliation. A phase difference in the periodical strain change could not be seen in terms of the height from the ground. In winter season, marked shrinkage was found in the stem. During this season, the rate of dimensional change in the tangential direction per degree Kelvin was 0.0296(%/K). This value is quite close to the freeze-drying shrinkage of wood, 0.03(%/K). The most probable cause of shrinkage during winter was the frost-shrinkage caused by the freezing-out of cell wall moisture into the cell lumen.