FORMATION AND SEASONAL OCCURRENCE OF XYLEM EMBOLISM IN ALNUS-CORDATA

We investigated the vulnerability of xylem to embolism and the seasona l occurrence of xylem embolism in Italian alder (Alnus cordata Loisel. ) by acoustic and hydraulic methods. Wood anatomy was also studied. Mo re than eighty percent of the vessels were less than 50 mm long and no vessels were longer than 120 mm. Mean vessel diameter was 48 mum. Ult rasound acoustic emissions from root and branch segments dehydrating i n air followed a similar pattern: in both tissues, emission peaks were recorded when the relative water content of the xylem was around 0.2. In branches dehydrating in air, xylem embolism increased linearly as water potential decreased. In trees in the field, more than 80 percent of hydraulic conductivity was lost in the tree crowns during winter. Recovery from winter embolism occurred mostly before bud burst. In sum mer, xylem embolism was low (< 30%) and acoustic emissions from roots, stem and branches of trees in the field were also low.