Determination of thermal expansion of green wood and the accuracy of tree stem diameter variation measurements

The coefficient of thermal expansion in the radial direction for wet fresh wood was determined for two coniferous species Pinus sylvestris and Picea abies and three broad-leaved species Acer platanoides, Betula pendula and Alnus incana. The diameter variation of 7–11 samples of each species was measured in a water heat bath with a linear variable displacement transducer (LVDT). The temperature range was 5–45 °C. The average values for the coefficient of thermal expansion varied between 7.9 x 10–6 and 17.5 x 10–6 °C–1. Heating and cooling gave similar results for all the species and no hysteresis was observed. The results show that the coefficient of thermal expansion for wet green wood is a positive number as for dry wood and timber, contrary to values mentioned in literature. The coefficient is lower than that of dry timber and of the same order of magnitude as the coefficient of many commonly used metals and alloys. In field measurements of tree stem diameter variations an LVDT is usually attached to a metal frame. The similar magnitude of the coefficients means that the accuracy of absolute values of tree stem diameter variations is dominated by the temperature difference between the stem and the surrounding air. However, if both the temperatures are measured, the error in stem diameter variation measurements resulting from the thermal expansion can be corrected.