Diurnal Patterns of Cavitation in Red Maple, Paper Birch and White Ash at Harvard Forest 2011-2012

Previous work at Harvard Forest has suggested that woody plants cavitate and re-dissolve embolisms in xylem on a daily basis. Here we investigated the common assumption that severing stems and petioles under water preserves the hydraulic continuity in the xylem conduits opened by the cut when the xylem is under tension. In red maple and white ash, higher PLC in the afternoon occurred when the measurement segment was excised under water at native xylem tensions, but not when xylem tensions were relaxed prior to sample excision. Bench drying vulnerability curves in which measurement samples were excised at native versus relaxed tensions showed a dramatic effect of cutting under tension in red maple, a moderate effect in sugar maple, and no effect in paper birch. These results suggest that sampling methods can generate PLC patterns indicative of repair under tension by inducing a degree of embolism that is itself a function of xylem tensions at the moment of sample excision.