Diurnal changes in the stem diameter of Norway spruce in relation to relative humidity and air temperature

Summary Changes in the stem diameter of well-watered seedlings of Norway spruce [Picea abies (L.) Karst.] in a climate chamber were registered using laser technique. Pronounced, reversible changes in stem diameter (maximum 80 m) occurred in connection with shifts between dark and light (280 W/m²) conditions. Diurnal variation in stem diameter (50 m) continued unchanged, in terms of amplitude and phase, after the entire needle mass had been removed. It was found that nearly all of the diurnal variation in stem diameter was caused by changes in relative air humidity at the stem surface. The relationship between stem diameter change and relative air humidity was investigated at three temperatures (5, 15, and 25° C) over a range (40–95%) of relative humidities. In addition, the effect of altering the air temperature (range: 5–25°C) on stem diameter change at a constant relative air humidity (85%) was studied.