Time lags for xylem and stem diameter variations in a Scots pine tree

Diameter variations in the xylem and whole stem (i.e. over bark) stem of a Scots pine (Pinus sylvestris L.) tree were measured at four heights over a 23 d period at 5 min intervals. Cross-correlation analysis was used to calculate time lags between the measurements. Xylem diameter measurements at the different heights had time lags varying from 10 to 50 min, measurements at the lower heights lagging behind the most. This result was in good agreement with the cohesion theory of transpiration. For the whole stem diameter measurements, the treetop lagged behind all other heights and the shortest lags were midway along the stem. Changes in whole stem diameter always lagged behind those of xylem stem diameter (30-110 min), and at all heights. The considerable differences in the behaviour of xylem and whole stem diameter support the Munch hypothesis of phloem flow. Time lags calculated separately for the shrinkage (morning) and swelling (afternoon) periods indicated shorter time lags during the swelling periods. The non-destructive methods used show promise in the simultaneous study of flow dynamics of xylem and phloem in trees.