by Audrey Verreault | 30 January 2019
Published in The Holocene 25(12): 1912-1922. https://doi.org/10.1177/0959683615591361
Past and present fire frequencies are key factors to evaluate fire-mediated changes in climate, but this metric is difficult to evaluate realistically in paleoecological and climatic reconstructions. Here, we applied charcoal analysis of forest soils to test from direct evidence the stability and resilience of the eastern North-American boreal forest at the Holocene timescale. Current knowledge indicates that the boreal forest is not so stable and resilient in several parts of its range, particularly at its northern and southern edges where it is converted to tundra and woodland communities, respectively. However, it is not known to what degree the structure and composition of the boreal forest situated at the central core of its range (FCR), far from the climate-sensitive edges, have been modified during the Holocene. To address the long-term status of the boreal forest vis-a-vis the Holocene climate, we have used a large dataset composed of 14C-dated and botanically identified charcoal. Long-term fire frequencies in several sites of the FCR were calculated to assess the stability of the main forest ecosystems. The mean fire interval over the last 5000 years of the two principal ecosystems composed of black spruce–jack pine forests and black spruce–balsam fir forests was ~200 and >300 years, respectively. Fires occurred repetitively during this period without fire gaps longer than 400 years (ecosystem 1) and 600 years (ecosystem 2), an indication that the fire regime of the FCR remained stable since the mid-Holocene. Unchanged forest ecosystems during this period also suggest the maintenance of both fire-prone and less fire-prone tree species in their respective sites and the ability of the boreal biome to maintain its structure and function in a repetitive fire-disturbance regime.
by Marie-Claude Boileau | 30 January 2019
Published in Forest Ecology and Management 430: 21-32 https://doi.org/10.1016/j.foreco.2018.07.054
Traditional silvicultural systems such as clearcutting and single-tree selection cutting are critiqued for their tendency to simplify forest complexity. By more closely emulating natural disturbance regimes and increasing the availability and heterogeneity in understory light levels, we pose that systems causing intermediate disturbances such as the single-tree and small group selection cutting system can promote tree regeneration, retain stand structural attributes, and maintain high understory plant diversity in temperate mixedwood stands. To assess this, we implemented a harvest intensity gradient experiment (residual basal area [BA], % removal) consisting of uncut control (26 m2/ha, 0%), light (21 m2/ha, 20%), moderate (18 m2/ha, 31%) and heavy (15 m2/ha, 42%) cutting with retention of legacy trees in uneven-aged yellow birch (Betula alleghaniensis Britton) – conifer stands. We evaluated the effects on gap size, light transmittance, abundance of living and dead trees, plant diversity, and regeneration of target species (i.e. yellow birch, red spruce [Picea rubens Sarg.] and balsam fir [Abies balsamea L.]), during the 8 years postcut. Moderate and heavy single-tree and small group selection cutting treatments triggered changes in microenvironments and in understory plant community. Moderate and heavy selection had greater yellow birch seedling density > 30 cm in height compared to the control. Cutting treatments did not significantly improve red spruce and balsam fir regeneration, despite favorable micro-environmental conditions (e.g. gaps averaging 200–350 m2 and 15–40% transmitted light). The vascular plant community rebounded quickly after disturbance and harvesting did not depress any diversity metric or alter community composition beyond control levels. Tree species richness increased in moderate and heavy selection cuts, while vascular species diversity (H′) was greatest in the heavy selection cut. Observed richness and diversity gains were driven by augmented yellow birch and mountain maple (Acer spicatum Lam.) recruitment into larger classes as well as greater forb, tree, and shrub cover in response to greater cutting intensities. Species richness and diversity were positively correlated with increased light availability, but not with light heterogeneity. Our results show that this hybrid selection cutting system benefits yellow birch recruitment without negatively impacting plant diversity. However, because increasing harvest intensity simultaneously enhanced interfering non-commercial species abundance (e.g. mountain maple), failed to improve red spruce regeneration, and decreased the abundance of large trees (diameter at breast height > 29 cm), we caution to opt for moderate cutting intensity in this forest type. Additional treatments such as enrichment planting in harvest gaps might be necessary to maintain red spruce over time.
by Svetlana Savin | 30 January 2019
Published in New Forests 50: 573–592. https://doi.org/10.1007/s11056-018-9680-2
Advance regeneration of red spruce (Picea rubens Sarg.) is scarce in many forest stands, due to past clearcuts and heavy harvesting of large seed trees in partial cuts. Understory enrichment planting has been proposed as a means to reintroduce the species in managed stands in which regeneration is deficient. Here, we present 5-year results regarding survival and growth dynamics of red spruce seedlings planted under hardwood, mixedwood, and coniferous forest covers following various cuts (each made on 0.5–1.2 ha square area): hybrid single-tree and group-selection, irregular shelterwood (two variants: continuous cover and extended irregular shelterwood), uniform regular shelterwood, harvesting with advance regeneration protection (HARP), and careful logging around advanced growth (hereafter referred to as ‘patch clearcut’). Five-year seedling survival did not vary among cuts except under the coniferous cover, where it was better in the heavier cuts (HARP and patch clearcut; 67%) than under continuous cover shelterwood cuts (24%), in which browsing occurrence was high (around 80%). Under the hardwood cover, height, diameter and biomass growth were greater in the extended irregular shelterwood cut than in other treatments. Under the mixedwood and coniferous covers, seedling growth was greater in patch clearcut, likely because transmitted light (90–100% full sunlight) was higher than in other cuts (around 30%). Results suggest that red spruce could be reintroduced using enrichment planting under a partial forest cover, but that additional treatments could be necessary to manage light and to limit browsing pressure in order to optimize long-term survival and growth performance.
by Claire Morin | 30 January 2019
Published in Forest Ecosystems 5(1): 30 https://doi.org/10.1186/s40663-018-0148-9
Background: In the context of ecosystem management, the present study aims to compare the natural and the present-day forested landscapes of a large territory in Quebec (Canada). Using contemporary and long-term fire cycles, each natural forest landscape is defined according to the variability of its structure and composition, and compared to the present-day landscape. This analysis was conducted to address the question of whether human activities have moved these ecosystems outside the range of natural landscape variability.
Methods: The study encompassed a forested area of 175 000 km2 divided into 14 landscapes. Using a framework that integrates fire cycles, age structure and forest dynamics, we characterized the forest composition and age structures that resulted from three historical fire cycles (110, 140, and 180 years) representative of the boreal forest of eastern Canada. The modeled natural landscapes were compared with present-day landscapes in regard to the proportion of old-growth forests (landscape level) and the proportion of late-successional forest stands (landscape level and potential vegetation type).
Results: Four landscapes (39%) remain within their natural range of variability. In contrast, nine landscapes (54%) show a large gap between natural and present-day landscapes. These nine are located in the southern portion of the study area, and are mainly associated with Abies–Betula vegetation where human activities have contributed to a strong increase in the proportion of Populus tremuloides stands (early-successional stages) and a decrease of oldgrowth forest stands (more than 100 years old). A single landscape (7%), substantially changed from its potential natural state, is a candidate for adaptive-based management.
Conclusion: Comparison of corresponding natural (reference conditions) and present-day landscapes showed that ten landscapes reflecting an important shift in forest composition and age structure could be considered beyond the range of their natural variability. The description of a landscape’s natural variability at the scale of several millennia can be considered a moving benchmark that can be re-evaluated in the context of climate change. Focusing on regional landscape characteristics and long-term natural variability of vegetation and forest age structure represents a step forward in methodology for defining reference conditions and following shifts in landscape over time.
by Claire Morin | 30 January 2019
Published in Forest Ecology and Management 375: 172-181. https://doi.org/10.1016/j.foreco.2016.05.037
Fire is considered the major disturbance in boreal forests. Nonetheless, in several areas logging has become the primary driver of forest dynamics. In many areas of the boreal forest, stands may undergo paludification (i.e. the accumulation of thick, poorly decomposed organic layers over the mineral soil) in the prolonged absence of fire, which reduces forest productivity. Whereas high-severity fires (HSF) may restore forest productivity by burning the soil organic layer (SOL), low-severity fires (LSF) mainly burn the soil surface and do not significantly reduce SOL thickness. In the Clay Belt region of eastern Canada, an area prone to paludification, forest stands have historically been harvested by clearcutting (CC), but concerns about the protection of soils and tree regeneration lead to the replacement of CC by careful logging (CL). Whereas CC disturbs the SOL and is thought to favor tree growth, CL has little impact on the SOL. Furthermore, it has been suggested that prescribed burning after clearcut (CCPB) could also be used to control paludification. Using a retrospective approach, this study sought to understand how CC, CL, and CCPB compare to LSF and HSF with respect to soil properties, SOL thickness, vegetation ground cover, tree nutrition, and stand height in paludified black spruce stands of the Clay Belt region. HSF led to significantly taller trees than CL and LSF, but did not differ from CC and CCPB. Foliar N was significantly higher in HSF and CCPB sites relative to CL and LSF, with an intermediate value in CC sites. Ground cover of Rhododendron groenlandicum was significantly lower in HSF and CC sites relative to LSF, with intermediate values in CL and CCPB sites. Sphagnum spp. ground cover was significantly lower in HSF and CCPB sites relative to CL, with intermediate values in CC and LSF sites. High-severity fire sites had a significantly thinner SOL than the four other disturbances. Finally, regression tree analysis showed that SOL thickness represented the best predictor of tree height, whereas segmented regression showed that tree height was negatively correlated to SOL thickness and revealed a cut-off point circa 23 cm, which suggests that tree growth is impeded beyond this threshold. These results support the idea that management strategies intending to regenerate paludified forests should primarily aim at reducing organic layer thickness, either through mechanical disturbance or combustion.
