by Audrey Verreault | 30 January 2019
Published in Ecoscience 21(3-4): 340-373. https://doi.org/10.2980/21-(3-4)-3696
This study aims to demonstrate that contemporary landscape vegetation heterogeneity is controlled by a combination of natural disturbances with other sets of explanatory variables. Integration of these drivers should be considered the key to explaining vegetation changes along ecological gradients characterizing the boreal forest. Forest inventory plots and maps produced from about 1970 to 2000 were used to characterize a large area (175 000 km2) according to 3 vegetation themes constituting distinct aspects of forest community composition (tree species, forest types, and potential vegetation‒successional stages) and 4 sets of explanatory variables (climate, natural disturbances, physical environment, and human disturbances). Canonical ordinations were performed to define ecological gradients as well as the overlap between vegetation themes and sets of explanatory variables along each gradient. For each vegetation theme, we quantified the relative proportion of vegetation variation explained by unique as well as combined sets of explanatory variables. The landscape vegetation heterogeneity described by species and potential vegetation‒successional stage was mostly explained by natural disturbances and climate in association with other sets of explanatory variables. The influence of physical environment was higher for landscape vegetation heterogeneity related to forest types than for the other themes, but this theme also was dominated by natural disturbances and climate. Compared to natural sets of explanatory variables, human disturbances played a secondary but significant role in the 3 vegetation themes. This research contributes to a better understanding of the relationship between vegetation and the factors underlying its development in the boreal forest and represents an important step toward ecosystem-based management.
by Audrey Verreault | 30 January 2019
Published in Scientific Reports 5: 13356. https://doi.org/10.1038/srep13356
Climate, vegetation and humans act on biomass burning at different spatial and temporal scales. In this study, we used a dense network of sedimentary charcoal records from eastern Canada to reconstruct regional biomass burning history over the last 7000 years at the scale of four potential vegetation types: open coniferous forest/tundra, boreal coniferous forest, boreal mixedwood forest and temperate forest. The biomass burning trajectories were compared with regional climate trends reconstructed from general circulation models, tree biomass reconstructed from pollen series, and human population densities. We found that non-uniform climate, vegetation and human drivers acted on regional biomass burning history. In the open coniferous forest/tundra and dense coniferous forest, the regional biomass burning was primarily shaped by gradual establishment of less climateconducive burning conditions over 5000 years. In the mixed boreal forest an increasing relative proportion of flammable conifers in landscapes since 2000 BP contributed to maintaining biomass burning constant despite climatic conditions less favourable to fires. In the temperate forest, biomass burning was uncoupled with climatic conditions and the main driver was seemingly vegetation until European colonization, i.e. 300 BP. Tree biomass and thus fuel accumulation modulated fire activity, an indication that biomass burning is fuel-dependent and notably upon long-term co-dominance shifts between conifers and broadleaf trees.
by Audrey Verreault | 30 January 2019
Published in Frontiers in Plant Science 6: 877. https://doi.org/10.3389/fpls.2015.00877
The predicted climate warming and increased atmospheric inorganic nitrogen deposition are expected to have dramatic impacts on plant growth. However, the extent of these effects and their interactions remains unclear for boreal forest trees. The aim of this experiment was to investigate the effects of increased soil temperature and nitrogen (N) depositions on stem intra-annual growth of two mature stands of black spruce [Picea mariana (Mill.) BSP] in Québec, QC, Canada. During 2008–2013, the soil around mature trees was warmed up by 4°C with heating cables during the growing season and precipitations containing three times the current inorganic N concentration were added by frequent canopy applications. Xylem phenology and cell production were monitored weekly from April to October. The 6-year-long experiment performed in two sites at different altitude showed no substantial effect of warming and N-depositions on xylem phenological phases of cell enlargement, wall thickening and lignification. Cell production, in terms of number of tracheids along the radius, also did not differ significantly and followed the same patterns in control and treated trees. These findings allowed the hypothesis of a medium-term effect of soil warming and N depositions on the growth of mature black spruce to be rejected.
by Marie-Claude Boileau | 30 January 2019
Published in Forests 9(10): 588. https://doi.org/10.3390/f9100588
Many northern hardwood stands include several low-vigor trees as a result of past management. To restore these degraded stands, partial cuts are applied with partly validated tree classification systems that are based upon apparent stem defects. We sampled 214 sugar maple (Acer saccharum Marsh.) and 84 yellow birch (Betula alleghaniensis Britt.) trees from six sites covering the northern hardwood forest zone of the Province of Quebec, Canada. We evaluated their vigor with a four-class system, and quantified the growth efficiency index and several indices that were based solely upon radial growth. The growth efficiency index increased non-significantly with increasing tree vigor class. The five-year basal area increment (BAI-1-5) was significantly different between the lowest and highest tree vigor classes. Yet, temporal changes in BAI-1-5 helped classify correctly only 16% of high-vigor trees that became poorly vigorous 8–10 years later. Overall, these results suggest that the tree classification system is weakly related to actual tree vigor and its application likely generates few significant gains in future stand vigor. Modifying and simplifying the tree vigor system must be considered to facilitate the tree marking process that is required to improve the vigor of degraded stands.
by Marie-Claude Boileau | 30 January 2019
Published in Forestry 91(1): 62-72. https://doi.org/10.1093/forestry/cpy031
We used 11 silvicultural trials in forests dominated by sugar maple (Acer saccharum Marsh.) to compare the development of structural attributes between unmanaged stands and stands subjected to two selection cuttings. Results showed that managed stands had significantly lower amounts of large live and dead trees, lower downed coarse woody debris (CWD) volume and lower density of large trees with wildlife cavities compared to unmanaged stands. Despite these differences, both managed and unmanaged stands had sufficient amounts of large trees and large trees with wildlife cavities to maintain excavator bird habitat based on minimum target guidelines from the literature. In both managed and unmanaged stands, minimum target guidelines to emulate old-growth conditions were not met: at least 30 live trees ha−1 >49.0 cm in diameter at breast height, and at least 11 m3 ha−1 of downed CWD volume with large-end diameter >44.0 cm. We discuss management options to promote the development of structural attributes in managed, uneven-aged sugar maple stands.
