DIURNAL CHANGES IN THE RADIUS OF A SUB-ALPINE NORWAY SPRUCE STEM - THEIR RELATION TO THE SAP FLOW AND THEIR USE TO ESTIMATE TRANSPIRATION

Diurnal changes in the stem radius of a subalpine mature Norway spruce were measured simultaneously with the flow of sap in xylem. Matric po tentials in the soil were >-35 kPa. The kinetics of the flow were clos ely related to the changes in the radius of the stem resulting from de pletion of its extensible tissues. The radius of the stem oscillated d aily and, fairly independently of this, fluctuated over several days. The daily shrinkage (Delta S-d) was correlated with the daily flow thr ough its base (Q(d)), When the crown transpired little and was nearly saturated during rainy days, Delta S-d tended to increase relative to Q(d). Using a linear relation, the estimates of flow by Delta S-d devi ated less than +/-10% from the values measured by heat balance, provid ed that the periods of calibration in their ratio of dry to rainy days were comparable to those estimated. If the two periods differed in th is respect, the estimates of flow deviated up to 42%. A quadratic rela tion yielded estimates that depended less on weather. It reduced maxim al deviations to +/-22%. Since Delta S-d additionally may represent th e time pattern of the daily transpiration better than Q(d), analysing changes in the radius of stems may supplement or partly replace measur ements of sap flow in stems.