Relationships between diurnal xylem diameter variation and environmental factors in Scots pine

We analyzed the variability in diurnal xylem diameter in Scots pine (Pinus sylvestris) by comparing daily maximum, minimum and average diameters and daily amplitudes of variation with environmental variables related to transpiration and water uptake from the soil. The measurements were carried out during six summers, 1999–2004 at SMEAR II station in Hyytiälä, southern Finland. We found that the daily maximum, minimum and average diameters and amplitudes were closest related to daily average water vapor deficits (VPD) and soil water contents in A-horizon ([theta]A). However, the highest degree of determination was only 37%. That was found for daily minimum diameter by VPD. The degree of determination and correlation could be improved by making up variables that took into account the variation in factors related to transpiration and soil water availability. In that case the highest correlation (negative correlation) and coefficient of determination was found for daily minimum diameter by VPD scaled with [theta]A so that VPD was enhanced when soil water content was low (VPD* = VPD*([theta]Amin/[theta]A)). This could explain 50% of the variation in daily minimum diameters. The results emphasize the importance of both inflow- and outflow-related factors in explaining the water tension and water status of tree stems.