SUMMARY

Xylem embolism was monitored from mid-winter to mid-summer in four co-occurring species: Betula cordifolia (Reg.) Fern., Fagus grandifolia Ehrh., Abies balsamea (L.) Mill., Picea rubens Sarg. The study site was a west-facing slope in the northern Green Mountains of Vermont, U.S.A.; Betula and conifers were sampled at 914 m; Fagus was collected at 827 m near its local altitudinal limit. Embolism was quantified by the percent the hydraulic conductivity of branch segments was below the maximum obtained following removal of air embolism in xylem conduits. Between early February and early May, the deciduous species averaged 60 to 84% embolism compared to 15 to 60% for the conifers. From April 24 to May 25, embolism in Betula dropped from 81 to 8%; this recovery was associated with root pressures up to 86 kPa as measured with manometers at the lower trunk. Betula trees in which root pressure was eliminated by overlapping saw cuts still showed 75% embolism in June; only 4% was present in control trees cut in a similar fashion after leaf flush. Root pressure was weak (3 kPa) and uncommon in Fagus, and trees remained 80% embolized through June showing considerable dieback. Fagus at lower elevations (60 m) were 33% embolized in June with no dieback. Embolism in the conifer species decreased from 40% embolism in late April to 6% in late June despite no detected positive xylem pressures. The mechanism for conifer recovery is unknown.