by Claire Morin | 15 October 2020
Published in The Forestry Chronicle 95(2): 113-123. https://doi.org/10.5558/tfc2019-018
We used the two-step shelterwood cutting to release conifer advance growth and limit the development of trembling aspen (Populus tremuloïdes) suckers in a stratified mixed aspen – conifer stand. This study presents the effects of the final cut, applied 12 years after the establishment cut, on the 5-year response of advance regeneration and aspen sucker development. Suckering was inversely proportional to the intensity of the establishment cut, with 19 000, 14 900 and 6800 stems/ha two years after the final cut, respectively, in the initial removal of 35%, 50% and 65% basal area. By year 5 however, the treatment effect on stem density was no longer significant due to high aspen mortality in the 35% and 50% cuts. At this time, the density of conifer stems taller than 100 cm was comparable to that of aspen stems in the 35% and 50% cuts, while conifers dominated the 65% cut. Standing mortality was higher for hardwood (22–28%) than for conifer (4–9%) advance growth, except in the uncut control (14% and 9%, respectively), while windthrow averaged 4% and 8%, respectively, in the three partial cuts without being related to treatment. Small merchantable conifer stems (diameter at breast height – DBH 9.1–15.0 cm) that were retained were most affected by windthrow, but overall losses were found acceptable considering DBH and height growth of the surviving stems. This study confirms that the two-step shelterwood cutting that secures conifer advance regeneration should be considered to limit hardwood conversion in the boreal mixedwood forest.
by Claire Morin | 14 October 2020
Published in Forest Ecology and Management 458: 117662. https://doi.org/10.1016/j.foreco.2019.117662
In their paper recently published in FEM (“Finding the sweet spot: Shifting climate optima for maple syrup production in North America”), Rapp et al. (2019) suggest that there is a marked “sweet spot” for maple syrup production (i.e., a climatic optimum associated with much higher yield) centered around the 43rd parallel. They also project that this climatic optimum could move 400 km northward in the future, as climate change drives temperatures to increase. As a result of this shift, they also predict that maple syrup production in the northeastern United States will rapidly decline in the next decades, and that the whole maple syrup industry could be at risk in several states. At the same time, they predict that maple syrup production will markedly increase in southeastern Canada, especially in the province of Quebec. Here we show that the predictive model built and used by Rapp et al. (2019) to project future maple syrup yield from climate scenarios is biased and presents several major flaws. We then demonstrate, using a data set of historical data many orders of magnitude larger than the one used by Rapp et al. (2019), that maple syrup yield is remarkably stable across a broad latitudinal and temperature gradient and therefore, that no climate optimum exists. This exercise leads us to conclude that the collapse of the U.S. maple syrup industry predicted by Rapp et al. (2019) is not based on solid evidence.
by Marie-Claude Boileau | 13 October 2020
Published in Agricultural and Forest Meteorology 291: 108063. https://doi.org/10.1016/j.agrformet.2020.108063
On vegetation-covered land surfaces, tree transpiration, compared to soil and canopy evaporation, is a major process that sends large amounts of water back to the atmosphere. While the driving forces of tree transpiration have been studied over a range of tree species across an array of ecosystems, no work has been done on balsam fir and black spruce in the humid boreal forest of eastern Canada. We thus studied the relationships between environmental variables and sap flow velocity (as a proxy for transpiration) for these two boreal tree species located at two forest sites in Quebec, Canada over multiple growing seasons (2004 to 2013 for balsam fir and 2006 to 2009 for black spruce). Our results showed that daily sap flow had a strong non-linear relationship with vapour pressure deficit (VPD) for both species. Sap flow was also strongly correlated to solar radiation (Rad) for both species although with slightly weaker relationships than for VPD. Other variables such as daily maximum temperature and precipitation explain a smaller portion of the variance in sap flow while soil water content (SWC) and wind speed had almost no effect. An analysis of the relationships between sap flow and VPD/Rad on an hourly basis over multiple years showed strong diel hysteresis for both species. Contrary to what has been previously proposed, the magnitude of this hysteresis does not seem to relate to the degree of iso/anisohydricity. Finally, our investigation of sap flow relationships to environmental variables during a drought period at the balsam fir site showed that sap flow was only slightly reduced despite a significant decrease in SWC. On the other hand, VPD and Rad remained the main drivers of sap flow. This study emphasizes that VPD and Rad are indeed the major drivers of transpiration during the growing season as well as during drought in humid boreal region.
by Claire Morin | 8 October 2020
Published in Frontiers in Ecology and Evolution 8:156. https://doi.org/10.3389/fevo.2020.00156
How strong was the anthropogenic imprint in the disturbance regime of eastern Canadian mixed forests during the 20th century? And how did it alter the tree species composition? To answer these questions, we reconstructed the 20th century anthropogenic disturbance regime and analyzed its impact on modern forest composition using historical and modern forest inventory and map data. Between 1895 and 2005, an equivalent of 144% of the study area has been logged and 19% burned. The logging rotation period has shortened from 152 years in 1895–1935 to 47 years in 1965–2005, due to increased industrial capacity. The fire rotation period decreased from 1668 years in 1895–1925 to 200 years during the peak of human settlement (1925–1955), and then increased to 2925 years in 1955–2005. The geographical progression of anthropogenic disturbances in the landscape has reflected the socio-economic context. During the 20th century, logging moved inland from the margins of the main water courses, reflecting the shift in wood transport from log driving on rivers to the densification of the road network in the second half of the 20th century. Most fires were located at low altitude, close to private lands suggesting ignitions from anthropogenic origins. Fire prone species (poplars) are mostly found within burned areas. Despite these disturbances, forest composition remained relatively stable, suggesting resilience of regional forest ecosystems.
by Claire Morin | 8 October 2020
Published in Journal of Ecology. https://doi.org/10.1111/1365-2745.13474
A better understanding of how disturbance impacts tree diversity at different scales is essential for our ability to conserve and manage forest ecosystems in the context of global changes. Here we test the impacts of land use-related disturbances on tree diversity since the 19th century across a broad region (>150,000 km2) of northern temperate forests in eastern Canada. We used a large and unique dataset of early land surveys conducted during the 19th century (>130,000 species lists), along with modern forest inventories (>80,000 plots), to analyse long-term changes in taxonomic and functional tree diversity at several scales (grid cell resolutions ranging from 12.5 to 1,600 km2; we refer to one grid cell as a “landscape”). Our results show that land use-related disturbances have led simultaneously to (a) increased diversity within landscapes and a (b) homogenization at the regional scale (i.e. decreased composition dissimilarity among landscapes). These trends were found for both taxonomic diversity and functional diversity, with temporal changes more pronounced for taxonomic than functional diversity. We also found an increase over time in the strength of correlations between environmental variables and diversity both within and among landscapes. Synthesis. Our results support the idea that human-induced impacts on biodiversity are strongly scale-dependent and not necessarily associated with biodiversity loss. This highlights possible ways that human-driven changes in tree diversity might impact forest resistance and resilience to future global changes.
