Relation between air water potential and cambial electrical resistance of balsam fir and white spruce after budbreak

Cambial electrical resistance in 10 balsam fir (Abiesbalsamea (L.) Mill.) and 10 white spruce (Piceaglauca (Moench) Voss) trees in Québec was measured 42 times during July and August 1985. Simultaneous measurements of relative humidity and temperature were also taken so that water potential in air (ψ a ) could be determined. In both species, cambial electrical resistance, corrected to a constant temperature of 15 °C ( CERCOR ), was negatively correlated with ψ a . On the other hand, 10 measurements on 14 balsam fir trees that had been severely defoliated by the spruce budworm (Choristoneurafumiferana (Clem.)) in 1983 and 1984, and 12 measurements on 14 protected trees showed that the correlation was stronger for sprayed trees than for those that had been defoliated. We propose that the correlation coefficient between cercor and ψ a may be a useful index of tree vigor. However, in situations where plots can be visited only once during the season, cambial electrical resistance measurements should be made in the morning when the relative humidity is high.