Using a Laser Instrument to Measure Small Changes in Stem Diameter

In industry, laser equipment is used in order to accurately measure material thicknesses. For ten days, tests were made in a climate chamber to study whether similar equipment (Lasermike, model 183-00) could be used to measure small changes in stem diameter of a growing spruce. The surrounding air temperature was +10 degrees C and the water vapour pressure 10 hPa on average during six hours of darkness. During the light part of the day the temperature was 18 degrees C and the water vapour pressure 15.5 hPa at a measured irradiance of 60 W/m(2) and 11.4 hPa at an irradiance of 260 W/m(2). The total contribution of the laser instrument and other equipment to the variation in stem diameter was +/-3.5 mu m. By means of the instrument it was possible to register hydrostatic changes in stem diameter, i.e., shrinking and swelling associated with changes in moisture content in the substrate, with smaller but marked changes depending on shifts between light and darkness and down to the changes in stem diameter caused by small and rapid variations in temperature and air humidity conditions. The true stem diameter growth per day could also be obtained from registrations during periods when the moisture content of the substrate in the pot was kept constantly at field capacity. The growth increased considerably when the irradiance increased. Growth also took place during periods when the stem was shrinking in size. The laser equipment could be rapidly and easily set up for use and was also easily operated. A considerable amount of developmental work is necessary before the instrument can be used in all types of weather in the field.