by Marie-Claude Boileau | 26 May 2020
Published in Forest Ecology and Management 168: 177-186
A precommercial thinning trial, conducted in 1978 in a 20-year-old balsam fir (Abies balsamea [L.] Mill.) stand in the province of Québec, Canada, produced a broad range of stand densities over 60 permanent plots. Plots were measured at 5-year intervals until 1998 and the data were analyzed to verify the principal effects that are expected from this treatment in terms of individual tree growth and production per hectare. The diameter growth during the 20-year period following thinning was inversely related to the residual density. The increase in diameter of trees in thinned plots accumulated mainly during the first 10-year period. One explanation why thinning did not increase the diameter throughout the entire 20-year inventory period involves a spruce budworm (Choristoneura fumiferana [Clem.]) outbreak that occurred during this period. Total volume per hectare after 20 years was proportional to residual stand density, while merchantable volume per hectare was similar through the range of densities. If trees larger than 15 cm in diameter at breast height are considered exclusively, then thinning significantly increased 20-year stand yield. Therefore, the most important effect of precommercial thinning is to increase the volume per hectare of large diameter trees. Any conclusions on the impact of this treatment on stand yield must take into account the minimum usable diameter that is considered.
by Marie-Claude Boileau | 26 May 2020
Published in Tree Physiology 22: 363-371
Experiments were conducted on 1-year-old western red cedar (Thuja plicata Donn.) seedlings to determine the response of illuminated foliage to reversible changes in total photosynthetic foliage area (La). Reductions in La were brought about by either shading the lower foliage or by reducing the ambient CO2 concentration (ca) of the air surrounding the lower part of the seedling. ln the latter case, the vapor pressure was also changed so that transpiration rates (E) could be manipulated independently of photosynthetic rates (A). We hypothesized that following such treatments, short-term compensatory changes would occur in stomatal conductance (gs) and A of the remaining foliage. These changes would occur in response to hydraulic signals generated by changes in the water potential gradient rather than changes in the distribution of sources and sinks of carbon within the seedling. When a portion of the foliage was shaded, there was an immediate reduction in whole-seedling E and a concomitant increase in gs, A and E in the remaining illuminated foliage. However, the inter-cellular CO2 concentration did not change. These compensatory effects were fully reversed after the shade was removed. When the lower foliage A was reduced to < 0 mmol m-2 s-I, by shading or lowering ca, and E was either unchanged or increased (by adjusting the vapor pressure deficit), there was no significant increase in gs and A in the remaining foliage. We conclude that compensatory responses in illuminated foliage occur only when reductions in LA are accompanied by a reduction in whole-plant E. The relationship between the reduction in whole-seedling E and the increase in A is highly linear (r2 = 0.68) and confirms our hypothesis of the strong regulation of gs by hydraulic signals generated within the seedling. We suggest that the mechanism of the compensatory effects is a combination of both increased CO2 supply, resulting from increased gs, and a response of the rate of carboxylation, possibly related to the activity of Rubisco.
by Marie-Claude Boileau | 26 May 2020
Published in Wetlands 22(2): 225-233
Low water tables typically found in peatlands during dry summer periods or in the vicinity of drainage ditches may lead to moisture deficiency in porous surface peats. Episodes of drought stress might compromise the growth benefits brought about by lower ground-water levels. We examined the water relations of black spruce (Picea mariana) trees on a natural peatland during relatively wet (1990) and relatively dry (1991) summers. Seasonal patterns of pre-dawn and mid-day shoot water potentials and stomatal conductance were not related to peat water content or to water-table depth. There was no evidence of water stress or osmotic adjustment in sampled trees during wet and dry growing seasons. Our soil moisture data showed that although water-table levels were as low as —66 cm in 1991, water availability in the root zone remained high. Even with the absence of mid-day water stress during the summer of 1991, a 50% reduction in stomatal conductance as compared with the previous year was found. We suggest that signals from the bulk of the roots located in dry peat top layer contributed to the regulation of stomatal conductance.
by Audrey Verreault | 11 May 2020
Published in Journal of the Torrey Botanical Society 133(4): 519-527.
The genus Populus comprises some of the most commercially exploited, pioneer forest trees distributed throughout the northern-hemisphere. The high level of morphological diversity, extensive inter-species hybridization, and low level of DNA sequence variation among species in this group have impeded the progress of taxonomic and phylogenetic studies. We used nuclear genomic data based upon inter simple sequence repeat (ISSR) variability, a highly variable class of molecular markers to determine the genetic relationships among species of the genus Populus. Species of the section Populus (Leuce) clustered together suggesting monophyly of the section Populus. The Eurasian members of section Populus (P. alba, P. davidiana, and P. tremula) showed closer genetic relationships to each other than to two North American aspens (P. tremuloides and P. grandidentata) of the same section. In contrast to previous phylogenetic studies, P. nigra showed a close genetic relationship to species of the section Tacamahaca. This relationship is in agreement with various phenotypic traits, interfertility and chemistry of bud exudates and serves as evidence for introgression between P. nigra and species of the section Taccamahaca. Overall, the genetic relatedness estimates based on nuclear ISSR data were congruent with phylogenetic trees based on other molecular (RFLP and DNA sequence) and morphological data, but provided better resolution in assessing the genetic relatedness among closely related taxa, and provided genetic evidence for previously suspected introgressions.
by Audrey Verreault | 5 May 2020
Published in Canadian Journal of Forest Research 39: 264-284
A critical component of assessing the impacts of climate change on forest ecosystems involves understanding associated changes in the biogeochemical cycling of elements. Evidence from research on northeastern North American forests shows that direct effects of climate change will evoke changes in biogeochemical cycling by altering plant physiology, forest productivity, and soil physical, chemical, and biological processes. Indirect effects, largely mediated by changes in species composition, length of growing season, and hydrology, will also be important. The case study presented here uses the quantitative biogeochemical model PnET-BGC to test assumptions about the direct and indirect effects of climate change on a northern hardwood forest ecosystem. Modeling results indicate an overall increase in net primary production due to a longer growing season, an increase in NO3– leaching due to large increases in net mineralization and nitrification, and slight declines in mineral weathering due to a reduction in soil moisture. Future research should focus on uncertainties, including the effects of (1) multiple simultaneous interactions of stressors (e.g. climate change, ozone, acidic depositon); (2) long-term atmospheric CO2 enrichment on vegetation; (3) changes in forest species composition; (4) extreme climatic events and other disturbances (e.g., ice storms, fire, invasive species); and (5) feedback mechanisms that increase or decrease change.
