© 1990 Heron Publishing—Victoria, Canada
Changes in the water relations of balsam fir and white birch saplings after thinning
David Pothier (1, 2) and Hank A. Margolis (1)
1. Département des sciences forestières, Faculté de foresterie et de géomatique, Université Laval, Sainte-Foy, Québec, Canada
G1K 7P4 / 2. Centre d’études nordiques, Université Laval, Sainte-Foy, Québec, Canada G1K 7P4 / Received January 10, 1990
Summary
In the autumn of 1987, young balsam fir (Abies balsamea (L.) Mill.) and white birch (Betula papyrifera Marsh.) trees were thinned and their water relations followed during the next two growing seasons. At the beginning of the
first summer following treatment, thinned trees of both species had lower osmotic potentials at full saturation (Ψπ,sat) and at turgor loss point (Ψπ,tlp) compared with controls. At this time, Ψπ,sat was linearly related to the percentage of full sunlight reaching the trees. A higher sugar concentration in leaves was an
important component of the lower Ψπ,sat of thinned trees. For the other two sampling dates during the first growing season after treatment and all three sampling
dates during the second growing season after treatment, little osmotic adjustment of the thinned trees relative to the control
tress was observed in either species. The absence of osmotic adjustment during the second growing season following thinning
suggests that other mechanisms were responsible for the acclimation of the treated trees to the higher atmospheric evaporative
demand. Sapwood permeability (k) of white birch was higher than that of balsam fir, but no differences in k or in sapwood area were found between treated and control trees of either species. Predawn water potentials (Ψpred) of treated trees were less negative than those of
controls.
