Diurnal variation in xylem hydraulic conductivity in white ash (Fraxinus americana L.), red maple (Acer rubrum L,) and red spruce (Picea rubens Sarg,)

Xylem hydraulic conductivity and percentage loss of conductivity (PLC) were measured on a ring-porous (Fraxinus americana L,, white ash), a diffuse po rous (Acer rubrum L., red maple) and a coniferous (Picea rubens Sarg., red spruce) tree species in a temperate deciduous forest in central Massachuset ts, USA. Measurements were made on current and 1-year-old branch segments i n the afternoon and on the following morning. Afternoon PLC was 45 to 70% f or the current year's extension growth in both white ash and red maple. Mor ning PLC was significantly lower (10-40%). Conductivity also varied diurnal ly suggesting, on average, a 50% recovery from cavitation overnight. Red sp ruce showed lower PLC and conductivity and a less pronounced night-time rec overy, Diurnal variation in hydraulic conductivity and PLC suggests that em bolism removal occurred in all three species despite the existence of tensi on within the xylem, Further evidence for embolism removal was observed wit h an in situ double-staining experiment in which dyes were fed to a transpi ring branch during the late afternoon and the following morning. Examinatio n of stem cross-sections showed that a larger number of vessels were conduc tive in the morning than on the preceding afternoon, Results of this study suggest that hydraulic capacity is highly dynamic and that conductivity mea surements reflect a balance between two processes: cavitation and embolism removal.