Xylem acoustic signals from mature Pinus sylvestris during an extended drought

Abstract

Mature Scots pine (Pinus sylvestris L.) were subjected to an 11 month imposed soil drought, from November 1994. Control plots received rainfall plus irrigation. The extent of cavitation in the xylem of branches and boles was compared using ultrasonic acoustic emission (UAE) measurements. Supporting measurements of the relative water content (Rw) of the bole and shoot were recorded. Acoustic emissions were detected in both treatments. An effect of water-stress, in both boles and branches, at the height of the drought, was noted. Differences in vulnerability were found and suggest a reduced vulnerability to cavitation of remaining functional conduits, after loss of the most vulnerable conduits, in the water-stressed trees. Bole Rw was lower than branch Rw. In 1-year-old shoots of droughted trees a significant increase in xylem embolism was found. Seasonal changes in shoot Rw were interpreted as the occurrence and recovery (refilling) of emboli in xylem tissues: an active role of precipitation is hypothesised. It was estimated that 15% of tracheids were cavitated without affecting above-ground hydraulic resistances. It is suggested that runaway cavitation was avoided through stomatal closure maintaining leaf water potential (indicative of xylem tension) below the cavitation threshold, and thereby little effect of drought was evident in above ground tissues.