Diurnal changes in the radius of a subalpine Norway spruce stem: their relation to the sap flow and their use to estimate transpiration

Abstract

Diurnal changes in the stem radius of a subalpine mature Norway spruce were measured simultaneously with the flow of sap in xylem. Matric potentials in the soil were > -35 kPa. The kinetics of the flow were closely related to the changes in the radius of the stem resulting from depletion of its extensible tissues. The radius of the stem oscillated daily and, fairly independently of this, fluctuated over several days. The daily shrinkage (Δd) was correlated with the daily flow through its base (Qd). When the crown transpired little and was nearly saturated during rainy days, ASd tended to increase relative to Qd. Using a linear relation, the estimates of flow by Δd deviated less than ± 10% from the values measured by heat balance, provided that the periods of calibration in their ratio of dry to rainy days were comparable to those estimated. If the two periods differed in this respect, the estimates of flow deviated up to 42%. A quadratic relation yielded estimates that depended less on weather. It reduced maximal deviations to ±22%. Since Δd additionally may represent the time pattern of the daily transpiration better than Qd, analysing changes in the radius of stems may supplement or partly replace measurements of sap flow in stems.