by Marie-Claude Boileau | 28 July 2020
Published in Canadian Journal of Forest Research 19: 432-439
The saturated sapwood permeability (k) of jack pine (Pinus banksiana Lamb.) from stands of different ages and site qualities was measured using a constant water flow apparatus. Saturated sapwood permeability at the base of the live crown (BLC) increased with age and reached a plateau just beyond 4 x 10-12 m2. The rate at which this plateau was reached, however, was dependent on site quality. Such age-related increases in k can partially explain how trees can maintain similar daytime leaf water potentials at different stages of stand development. Within age-classes 15 and 35, k at BLC was greater on the better-quality sites and was strongly correlated with both diameter and height growth. For age-class 120, k at BLC was not significantly different among sites. Saturated sapwood permeability at BLC could be predicted from age and site quality, using a negative exponential function (R2 = 0.66). The ability to predict changes in k with stand development has potential for improving leaf area estimates derived from sapwood area – leaf area correlations. Sapwood conductance from ground level to the upper third of the crown decreased with age for good-quality sites and increased with age for poor-quality sites. It corresponded to the pattern of average annual height growth over the last 5 years (R2 = 0.61). The number of annual rings contributing to the sapwood at a given age was independent of site quality. This suggests that the historical reconstruction of a tree’s leaf area and growth efficiency is possible even after the formation of significant amounts of heartwood.
by Audrey Verreault | 27 July 2020
Published in Tree Physiology 6: 371-380
ln 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 (Yp,sat) and at turgor loss point (Yp,tlp) compared with controls. At this time, Yp,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 Yp,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 trees 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 (Ypred) of treated trees were less negative than those of controls.
by Marie-Claude Boileau | 27 July 2020
Published in Annales des Sciences Forestières 48: 123-132
A precommercial thinning was conducted on young balsam fir (Abies balsamea (L) Mill) and white birch (Betula papyrifera Marsh) trees. Changes in light environment and growth response of the trees were followed during the next 2 growing seasons. The relative growth rate (RGR) of thinned balsam firs increased during both the first and the second growing season. This increase in growth was attributed to a greater net assimilation rate (NAR) which was associated with a higher level of light availability. Thinning tended to positively affect the RGR of white birch during the first summer following treatment but not during the second growing season. Similar fluctuations were noted for NAR even though light levels remained high for thinned white birch trees during both the first and the second growing season. Balsam fir produced more sapwood per unit of additional leaf area than controls during the first summer following treatment but no differences were observed during the second. The sapwood area growth to leaf area growth ratios of thinned and control white birches were similar during both the first and the second summer following thinning. Thus the sapwood area-leaf area relationship appears to be more stable following abrupt changes in environmental conditions for the indeterminate growth species, white birch, than for the determinate growth species, balsam fir.
by Marie-Claude Boileau | 24 July 2020
Published in Canadian Journal of Forest Research 22: 534-540
The calibration of time domain reflectometry, previously established for an organic soil of 0.422 Mg.m-3 bulk density, was established for peat blocks with bulk densities ranging from 0.06 to 0.25 Mg.m-3. An empirical relationship between the volumetric water content and the measured apparent dielectric constant was established in the laboratory. This relationship can be used to estimate volumetric water content between 0.21 and 0.95 cm3.cm-3 with a standard deviation of 0.03 cm3.cm-3. The large variations observed during the calibration were mainly attributed to the size and heterogeneousness of peat samples. When water tables in the field were high, standard deviation decreased to 0.02 cm3.cm-3, which agrees very well with other time domain reflectometry experiments.
by Audrey Verreault | 24 July 2020
Published in Journal of Ecology 81: 719-727
1 The postglacial history of jack pine (Pinus banksiana Lamb.) at its northernmost distribution limit in the upper boreal forest, along the Grande Rivière de la Baleine (northern Québec), was reconstructed by using radiocarbon-dated conifer macrofossils found in dune palaeosols.
2 Black spruce (Picea mariana (Mill.) B.S.P.) was the first conifer species to colonize the area at 6000 BP, immediately after deglaciation. Jack pine first invaded the sandy terraces at 3050 BP, apparently at a low density. The species most likely survived for several centuries at this low density before experiencing a regional expansion between 2400 and 1750 BP.
3 From the period of regional expansion of jack pine to the present, mixed and monospecific stands of jack pine and black spruce developed concurrently, indicating that both species responded positively to fire and climate conditions.
4 The regional expansion of jack pine was not followed by a northward spread of the species into the forest tundra. The formation and expansion of the forest tundra during the last 3000 years restricted jack pine to the boreal forest. A lower fire frequency with colder conditions in the forest tundra may have been responsible for the inability of jack pine to expand northwards.
