by Marie-Claude Boileau | 17 April 2020
Published in Forest Science 63(1): 62-70. doi: 10.5849/forsci.15-178
Early successional competition among boreal forest tree and shrub species and its effects on growth of commercial tree species have been a major source of uncertainty in establishing efficient precommercial thinning and brushing prescriptions. We examined the effect of prethinning competitor density, postthinning competitor regrowth density, prethinning stem diameter, and the timing of thinning operations on the growth response of black spruce (Picea mariana [Mill.] BSP) and jack pine (Pinus banksiana Lamb.). In addition, we examined the mortality rate of hardwoods after thinning and the number of new shoots produced per surviving thinned stem. For jack pine, growth response was greatest when thinning occurred between 4 and 9 years after establishment, whereas for black spruce we observed no significant relationship between growth response and the timing of treatment. For jack pine, growth response was significantly affected by pretreatment competitor density, posttreatment competitor regrowth density, and pretreatment stem diameter. For black spruce, no significant relationship was observed between growth response and any variables. Mortality rates and production of new shoots in hardwoods varied significantly between species. Considering the high regrowth potential of willow (Salix spp.) and alder (Alnus spp.), we recommend that stands exhibiting low densities of these species should be left unthinned. Our results help foresters identify stands that require precommercial thinning and call for modification of currently used thinning strategies.
by Marie-Claude Boileau | 13 April 2020
Published in New Forests 47(6): 877–895
We studied three hybrid poplar plantations in Quebec (Canada) established on sites with varying soil and environmental characteristics to investigate the effects of logging residues on the water potential, carbon isotope ratio and foliar nutrients of planted trees. On each site, four treatments representing different residue loads, as well as treatments aimed at manipulating specific factors of the environment (Herbicide, Geotextile) were applied to test their effects on seedling water potential, carbon isotope ratio and foliar nutrients. Along with analyses of variance, we used structural equation modelling to infer causal relationships of logging residues on height, basal diameter and foliar nutrition of trees through their effects on soil temperature, soil water content and competing vegetation cover. Logging residues decreased soil temperature at all sites and woody plants cover at one site out of three. Height, basal diameter and unit leaf mass were strongly related to each other. Foliar d13C, N concentration and unit leaf mass increased with decreasing cover of woody plants suggesting an important role of competition for resources. Overall, logging residues had no direct influence on hybrid poplar dimensions after two growing seasons: their effects on the microenvironment of this resource demanding tree species were either cancelling out each other, or were not significant enough to have a significant impact on the growth drivers measured. For example, presence of logging residues might reduce soil temperature, impeding overall seedling performance. Our study highlights the fact that any given silvicultural method aimed at manipulating logging residues has a complex influence involving the interaction of multiple environmental drivers and that the net effect on tree productivity will depend on species and site specific conditions.
by Audrey Verreault | 13 April 2020
Published in Frontiers in Plant Science 7: 1450. doi: 10.3389/fpls.2016.01450
Because of changes in climatic conditions, tree seeds originating from breeding programs may no longer be suited to sites where they are currently sent. As a consequence, new seed zones may have to be delineated. Assisted migration consists of transferring seed sources that match the future climatic conditions to which they are currently adapted. It represents a strategy that could be used to mitigate the potential negative consequences of climate change on forest productivity. Decisions with regard to the choice of the most appropriate seed sources have to rely on appropriate knowledge of morpho-physiological responses of trees. To meet this goal, white spruce (Picea glauca [Moench] Voss) seedlings from eight seed orchards were evaluated during two years in a forest nursery, and at the end of the first growing season on three plantation sites located in different bioclimatic domains in Quebec. The morphophysiological responses obtained at the end of the second growing season (2+0) in the nursery made it possible to cluster the orchards into three distinct groups. Modeling growth curves of these different groups showed that the height growth of seedlings from the second-generation and southern first-generation seed orchards was significantly higher than that of those from other orchards, by at least 6%. A multiple regression model with three climatic variables (average growing season temperature, average July temperature, length of the growing season) showed that the final height of seedlings (2+0) from the first-generation seed orchards was significantly related to the local climatic conditions at the orchard sites of origin where parental trees from surrounding natural populations were sampled to provide grafts for orchard establishment. Seedling height growth was significantly affected by both seed source origins and planting sites, but the relative ranking of the different seed sources was maintained regardless of reforestation site. This knowledge could be used, in conjunction with transfer models, to refine operational seed transfer rules and select the most suitable sites in an assisted migration strategy.
by Marie-Claude Boileau | 13 April 2020
Published in For. Ecol. Manage. 400: 408-416. https://doi.org/10.1016/j.foreco.2017.06.026
Temporal changes in stand growth dominance, i.e. a measure of the relative contribution of differentsized trees to the total stand growth, may play a role in the commonly observed decline in forest productivity over time through a shift in resource acquisition and utilization between dominant and nondominant trees. We hypothesized that the expected decreases in both growth dominance (GD) and relative growth rate (RGR) over time were related to decreases in leaf biomass of dominant trees or increases in allocation to leaf biomass of non-dominant trees. To better understand these potential relationships, we quantified stand growth dominance and some functional components (specific leaf area,
leaf weight ratio, net assimilation rate, nitrogen per unit leaf area and nitrogen use efficiency) of the relative growth rate of dominant and non-dominant trees along forest development stages in the eastern Canadian boreal forest using a 1067-year-long post-fire chronosequence. As expected, stand growth dominance decreased with stand development, and was closely related to differences in RGR between dominant and non-dominant trees. Decline in both growth dominance and differences in RGR between 100 and 200 years after fire was related to greater biomass partitioning to leaves in non-dominant trees, coupled to better light acquisition capacity of non-dominant trees, which appeared in stands that were >75-years-old. In old-growth stands, the growth advantage of non-dominant trees over dominant trees involved other mechanisms, such as higher photosynthetic rates and better resource use efficiency in the non-dominant trees. Overall, the observed decrease in stand growth dominance with increasing age was explained mainly by declining resource acquisition and utilization in dominant trees rather than through improved resource acquisition and utilization of non-dominant trees.
by Audrey Verreault | 13 April 2020
Published in Journal of Vegetation Science 28: 57-68. doi: 10.1111/jvs.12466
Aims: Our study aimed to: (1) document the preindustrial (1925) forest composition prior to extensive logging; (2) document the magnitude of changes from 1925 to 2005; and (3) identify the relative influence of logging and natural disturbances as drivers of the present-day forest composition.
Location: Boreal forest in central Quebec, eastern Canada.
Methods: We used a dense network of georeferenced historical (~1925) forest plots (n = 30 033) to document preindustrial forest composition. We evaluated the magnitude of changes with the present-day using modern plots (1980s to 2000s). We reconstructed a long-term, spatially explicit history of logging, spruce budworm outbreaks (Choristoneura fumiferana [Clem.], SBO), and fire using historicalmaps and field surveys.
Results: In the preindustrial period, late successional coniferous taxa (Abies balsamea and Picea spp.) dominated the landscape, whereas early successional deciduous taxa (Betula spp. and Populus spp.) were confined to recently burned areas. In the present-day landscape, large areas dominated by late successional coniferous taxa have been replaced by early successional deciduous taxa. Forest communities dominated by early successional deciduous taxa increased sharply throughout the study area. Logging has been a minor driver of these changes compared to fire and SBOs.
Conclusions: This study demonstrates the importance of documenting the long-term history of both anthropogenic and natural disturbances in order to assess their relative contributions to the development of the present-day forest ecosystems. Natural disturbances have remained the main drivers of forest composition during the 20th century, whereas logging played a less important role. In the current context of global change, long-term experimental research is required to help forecast impacts of natural disturbances and forest management on boreal forest composition.
