by Marie-Claude Boileau | 30 January 2019
Published in Journal of Forest Research 18: 168-178
Composition, structure, and species-specific patterns of recruitment and growth were characterized in two yellow birch (Betula alleghaniensis Britt.)–conifer stands in Quebec, Canada, to improve our understanding of the dynamics of these complex ecosystems. The mixture of mid- and shade-tolerant species in the canopy, the inverse J-shape stem diameter distribution, and the age distribution were indicative that the two stands were in a late-successional stage. Recruitment of mid-tolerant species above 1.3 m in height appeared to be periodic and synchronized with historical spruce budworm (Choristoneura fumiferana Clem.) outbreaks, while the coniferous component of these mixedwood stands recruited continuously. Results suggest that recruitment of yellow birch and red maple (Acer rubrum L.) requires disturbances of a certain intensity that affect at least 25 % of the forest cover. In contrast, balsam fir (Abies balsamea (L.) Mill.) and red spruce (Picea rubens Sarg.) can recruit under the canopy without relying on moderate or large canopy disturbances. Results suggest that the historical disturbance regime, and differences in shade tolerance between species, largely govern the contemporary composition of these stands. This study improves the comprehension of mechanisms that regulate the dynamics of yellow birch-conifer stands and will be useful for the subsequent elaboration of forest management strategies.
by Claire Morin | 30 January 2019
Published in Forest Ecology and Management 310: 10-18 http://dx.doi.org/10.1016/j.foreco.2013.08.011
Clear-cutting and fire are the two main disturbances affecting the boreal forest of eastern Canada. These two disturbance types exert different effects on forest dynamics, which can have major implications in terms of economic and ecological sustainability. This study compared the long-term effects of these two disturbance types on stand composition, stand density, and merchantable volume on eastern Canadian mesic boreal forests dominated by black spruce and balsam fir. We used 157 permanent sample plots (PSP), 41 of which originated from fire and 116 were from clear-cuts. Model selection with finite-sample corrected Akaike Information Criterion (AICc) was used to understand which factors had the greatest influence on relative stand density and merchantable volume. Our results indicate that merchantable volume was positively influenced by stand density, which in turn was primarily influenced by disturbance origin, with post-cutting stands being denser than post-fire stands. These results indicated that an increase in stand density is an important mechanism through which disturbance acts upon merchantable volume. We also found differences in forest composition between stand-origin categories, with balsam fir being more abundant post-clearcutting; this was likely because advance regeneration was mainly composed of balsam fir, whereas post-fire stands are dominated by black spruce. Differences in merchantable volume between post-fire and post-clearcut stands become non-significant with time, likely because of higher tree growth and lower mortality in the dominant canopy of clearcut-origin stands. Overall, the results indicate that stand origin is an important determinant of stand composition and stand yield, but that the latter effect tends to disappear after a few decades. The higher balsam fir content generally observed in naturally regenerated, clear-cut origin stands could have potentially negative economic and ecological impacts at the landscape scale, which may call for mitigation strategies.
by Audrey Verreault | 30 January 2019
Published in PeerJ https://doi.org/10.7717/peerj.428
Sugar maple syrup production is an important economic activity for eastern Canada and the northeastern United States. Since annual variations in syrup yield have been related to climate, there are concerns about the impacts of climatic change on the industry in the upcoming decades. Although the temporal variability of syrup yield has been studied for specific sites on different time scales or for large regions, a model capable of accounting for both temporal and regional differences in yield is still lacking. In the present study, we studied the factors responsible for interregional and interannual variability in maple syrup yield over the 2001–2012 period, by combining the data from8 Quebec regions (Canada) and 10 U.S. states. The resulting model explained 44.5% of the variability in yield. It includes the effect of climatic conditions that precede the sapflow season (variables from the previous growing season and winter), the effect of climatic conditions during the current sapflow season, and terms accounting for intercountry and temporal variability. Optimal conditions for maple syrup production appear to be spatially restricted by less favourable climate conditions occurring during the growing season in the north, and in the south, by the warmer winter and earlier spring conditions. This suggests that climate change may favor maple syrup production northwards, while southern regions are more likely to be negatively affected by adverse spring conditions.
by Claire Morin | 30 January 2019
Published in Biogeosciences 10: 4627-4639 https://doi.org/5194/bg-10-4627-2013
Nitrogen (N) is a major growth-limiting factor in boreal forest ecosystems. Increases of temperature and atmospheric N deposition are expected to affect forest growth directly and indirectly by increasing N availability due to higher rates of N mineralization. In order to understand the potential impacts of these changes, a mature balsam fir stand in Québec, Canada, was subjected during three consecutive growing seasons (2009–2011) to (i) experimentally increased soil temperature (4 °C) and earlier snowmelt (2–3 weeks) as well as (ii) increased inorganic N concentration in artificial precipitation (3 x current N concentrations using 15NH4- 15NO3). Soil inorganic N was measured using buried ion-exchange membranes (PRS™ probes) and standard soil extractions. Dendrometers were used to monitor the variations in diameter growth and needles were analyzed annually for N to assess the nutritional response of trees. Results from the second (2010) and third (2011) year of treatment are reported.
After three years of treatment, there was no significant increase in soil nitrate (NO3) or ammonium (NH4) availability either in the organic or in the mineral soil as measured with standard soil extractions. Similar results were obtained with ion-exchange membranes, except for NH4 in the forest floor, which increased by an average of 54% over the two years. No effect of treatments was observed on needle N or diameter growth, but an 8-day earlier peak in diameter growth was measured in heated plots in 2010.
We attributed the limited effects of our treatments to the acute soil competition for available N at the site. As a result, the projected modifications of the forest N cycle and concomitant increased forest growth due to an earlier snowmelt, increased soil temperature and N deposition should be considered with caution in similar cold N-poor ecosystems.
by Audrey Verreault | 30 January 2019
Published in Tree Genetics & Genomes 10: 965–975. https://doi.org/10.1007/s11295-014-0735-z
Exotic larch (Larix decidua Mill., Larix kaempferi (Lamb.) Carr. and Larix sibirica Ledeb.) and their hybrids have been introduced into Canada to test adaptation and growth for reforestation purposes. For tree breeding operations, provenance trials and seed orchards of exotic larches and their hybrids have been established in arboreta adjacent to natural forest stands of the indigenous species Larix laricina (Du Roi) K. Koch. This proximity offers an opportunity to measure spontaneous hybridization and the recruitment of hybrids. Using a combination of two chloroplast and three mitochondrial DNA markers, we estimated the rate of spontaneous hybridization taking place between native (L. laricina) and exotic (L. decidua, L. kaempferi, L. sibirica) species of larch in two arboreta. More than 5,000 seeds and 1,000 recruits from the two experimental sites were sampled and genetically identified. The occurrence of spontaneous hybridization between L. laricina and the exotics was found both in seeds and from recruits freely established near the plantations. A low hybridization rate (3 % or less) was observed among the seeds of L. laricina maternal trees. Spontaneous native-exotic hybrids were also observed to establish and survive in the environment under the current climatic conditions at a similarly low rate. However, a much higher and variable rate of establishment was observed for recruits with exotic components (up to 34.6 % at one site). These results indicate that the spontaneous hybridization and the recruitment of hybrid and exotic recruits do occur and should be taken into consideration in the management and establishment of plantations of fast-growing species with exotic components.
