Recherche

Bergeron, J.A. Colin; Pinzon, Jaime; Odsen, Sonya; Bartels, Samuel; Macdonald, S. Ellen; Spence, John R.; Bergeron, J. A. Colin — 2017-05-19 The extent to which past states influence present and future ecosystem characteristics (ecosystem memory (EM)) is challenging to assess because signals of past ecological conditions fade with time. Using data about seven different taxa, we show that ecological gradients initiated by wildfires up to three centuries earlier affect biotic recovery after variable retention harvest in the boreal mixedwood forest. First, we show that fire history over the last 300 years is reflected in pre-harvest species-specific stand basal area (BA), with longer times since high severity fire associated with proportionally higher BA of shade-tolerant softwood species than shade-intolerant hardwoods. Second, using patterns in the BA of pre-harvest tree species we link fire history to species composition of pre-harvest assemblages of bryophytes, herbs, shrubs, regenerated trees, songbirds, spiders and carabid beetles. Finally, we use variance partitioning to compare the importance of species-specific pre- versus post-harvest BA for explaining the structure of these seven biotic assemblages two, five and ten years after harvest. We detected persistent significant effects of pre-harvest BA in all post-harvest biotic assemblages up to ten years after harvest. Pre-harvest BA was more strongly associated with early post-harvest understory plant and carabid beetle assemblages than was post-harvest BA, but the opposite was true for spiders, songbirds and regenerated trees. EM effects were detected two, five and ten years after harvest but temporal patterns varied according to taxa. Thus, EM of fire history can persist at least ten years after variable retention harvest and such effects appear to be stronger for understory plants than for animals. We conclude that management of biological legacies to increase post-disturbance EM will increase overall resilience and sustainability of these mixedwood forests.

Bartels, Samuel F.; James, Ryan S.; Caners, Richard T.; Macdonald, S. Ellen — 2019-02-15 1. Site moisture is an important component of the forest landscape for maintaining biodiversity, including forest-floor bryophytes, but little is known about its role in shaping understory responses to harvesting. 2. We investigated the influence of site wetness, determined using a remotely-sensed, topographic depth-to-water (DTW) index, on responses of bryophyte cover, richness, diversity, and composition to variable retention harvesting (comparing: 2% [clear-cut], 20%, and 50% dispersed green tree retention and uncut controls [100% retention]) in three boreal forest cover-types (broadleaf, mixed, and conifer forests) in western Canada. The DTW index provides an approximation of depth to water at or below the soil surface, and was derived from wet-areas mapping based on discrete Airborne Laser Scanning data acquired over an experimentally harvested landscape located in northwestern Alberta, Canada. 3. The effectiveness of leaving retention (versus clear-cutting) for conserving bryophyte communities depended on site wetness, as indicated by DTW, with the specifics varying among forest types. In broadleaf forests, bryophyte cover and richness were generally low and not much affected by harvesting but drier sites had higher richness and a few more unique species. In mixed and conifer forests, leaving retention (versus clear-cutting) on wetter (versus drier) sites was more effective for conserving bryophyte cover, wetter sites had higher total species richness, and more species were exclusive to wetter sites. 4. Synthesis and applications. Site wetness, as indicated using the remotely-sensed topographic site wetness index "depth-to-water," mediates bryophyte responses to variable-retention harvests. Specifically, our results suggested that in conifer and mixed forests it would be more beneficial to target wetter sites for retention patches or dispersed retention whereas in broadleaf sites there might be a slight advantage to targeting drier sites. Our study demonstrates that this tool could be used to inform management decisions around leaving dispersed or patch retention.28-Jan-2019

Bartels, Samuel F.; Macdonald, S. Ellen — 2023-01-10 <p>1. Retention harvesting is advocated as an alternative to intensive timber harvesting, such as clear-cutting, to better maintain or facilitate recovery of biodiversity and other ecological values in managed forests. However, it is not clear how long the benefits of retention harvests persist.</p> <p>2. We investigated responses of understory vascular plant cover, richness, diversity (inverse Simpson index), and composition to a gradient in dispersed retention (2% [clear-cut], 10%, 20%, 50% and 75% retention; unharvested reference [100% retention]) at 3, 6, 11, and 17 years after harvest, in four boreal mixed wood forest types (deciduous (broadleaf)-dominated, deciduous-dominated with conifer understory, mixed, and conifer-dominated) in western Canada.</p> <p>3. Understory cover and richness tended to increase in the short-term (3 years), peaked at 6–11 years with differences following the gradient of harvesting intensity, then plateaued or declined in the second decade (17 years), by which time there were minimal or no differences among harvesting levels, including the reference. Responses for diversity were minimal. In contrast, composition varied along the gradient of harvesting intensity and showed little recovery towards the unharvested condition over the 17-year period. Generally, for plant community composition, clear-cut and lower retention treatments (10%, 20%) were similar to one another but differed from the higher retention and unharvested reference treatments.</p> <p>4. Synthesis and applications. Retention harvests can moderate the negative impacts of harvesting and facilitate the recovery of biodiversity. Our results suggest that for the cover, richness and diversity of understory vascular plants, this moderating influence is weak and short-lived. However, higher levels of retention can temper changes in understory composition relative to the unharvested forest, but full recovery is likely to be slow and will be complicated by post-harvest regeneration dynamics.</p>

Echiverri, Laureen Francesca Inocian; Macdonald, S. Ellen; Nielsen, Scott E. — 2022-04-20 <p>The influence of edges on forest biodiversity is an important environmental effect associated with habitat fragmentation, as edge effects can further reduce the remaining ‘interior’ habitat. However, extrapolating the influence of edges across the broader landscape has been difficult, especially regarding how to treat multiple edges in close proximity, where edge effects might interact. In this study, we examined the interaction of edge effects for multiple edges from a dense network of narrow (3-8 m wide) and low-severity linear disturbances called ‘seismic lines’. Seismic lines are created during oil and gas exploration and are responsible for severe dissection of boreal forests in western Canada. Specifically, our objectives were to: (1) to compare the edge influence of “wide” (~8 m) and “narrow” (~3 m) seismic lines; and (2) to determine whether edges in close proximity show interaction of edge influences, i.e. do multiple narrow seismic lines have a stronger or weaker edge influence than a single narrow seismic line. We sampled vegetation along transects perpendicular to seismic lines in treed moderate-rich and poor fens. We used randomization tests of edge influence to calculate the magnitude and distance of edge effects. In moderate-rich fens, we found a positive edge influence on understory diversity from both wide and narrow seismic lines. We also found a weakening edge interaction on diversity, i.e., single narrow seismic lines had a stronger edge influence on diversity than multiple narrow seismic lines. In treed poor fens, multiple narrow seismic lines had a negative edge effect on tree density, understory abundance, richness, and composition. In addition, we found strengthening edge interactions in treed poor fens on tree density, graminoid cover, and understory composition. Our results show how assessing the edge influence of multiple disturbances can provide a better understanding of the cumulative effects of habitat fragmentation.</p>

Aubin, Isabelle; Cardou, Françoise; Munson, Alison; Anand, Madhur; Arsenault, André; Bell, F. Wayne; Bergeron, Yves; Boulangeat, Isabelle; Delagrange, Sylvain; Fenton, Nicole J.; Gravel, Dominique; Hébert, François; Johnstone, Jill; Macdonald, S. Ellen; Mallik, Azim; McIntosh, Anne C.S.; McLaren, Jennie R.; Messier, Christian; Morris, Dave; Shipley, Bill; Sirois, Luc; Thiffault, Nelson; Boisvert-Marsh, Laura; Kumordzi, Bright B. — 2022-04-18 <p class="MsoPlainText">Intraspecific trait variability (ITV) provides the material for species adaptation to environmental changes. To advance our understanding of how ITV can contribute to species adaptation to a wide range of environmental conditions, we studied five widespread understory forest species exposed to both continental-scale climate gradients, and local soil and disturbance gradients. We investigated the environmental drivers of between-site leaf and root trait variation, and tested whether higher between-site ITV was associated with increased trait sensitivity to environmental variation (i.e. environmental fit).</p> <p class="MsoPlainText">We measured morphological (specific leaf area: SLA, specific root length: SRL) and chemical traits (Leaf and Root N, P, K, Mg, Ca) of five forest understory vascular plant<span style="font-family:KievitWeb , sans-serif;font-size:medium;"> </span>species at 78 sites across Canada. A total of 261 species-by-site combinations spanning ~4300 km were sampled, capturing important abiotic and biotic environmental gradients (neighbourhood composition, canopy structure, soil conditions, climate). We used multivariate and univariate linear mixed models to identify drivers of ITV and test the association of between-site ITV with environmental fit.</p> <p class="MsoPlainText">Between-site ITV of leaf traits was primarily driven by canopy structure and climate. Comparatively, environmental drivers explained only a small proportion of variability in root traits: these relationships were trait-specific and included soil conditions (Root P), canopy structure (Root N) and neighbourhood composition (SRL, Root K). Between-site ITV was associated with increased environmental fit only for a minority of traits, primarily in response to climate (SLA, Leaf N, SRL).</p> <p class="MsoPlainText">Synthesis. By studying how ITV is structured along environmental gradients among species adapted to a wide range of conditions, we can begin to understand how individual species might respond to environmental change. Our results show that generalizable trait-environment relationships occur primarily aboveground and only accounted for a small proportion of variability. For our group of species with broad ecological niches, variability in traits was only rarely associated with higher environmental fit, and primarily along climatic gradients. These results point to promising research avenues on the various ways in which trait variation can affect species performance along different environmental gradients.</p> https://creativecommons.org/publicdomain/zero/1.0/

Macdonald, S. Ellen; Bartels, Samuel F.; Johnson, Derek; Caners, Richard T.; Spence, John R. — 2017-08-23 1. Variable-retention harvest is widely recognised as an alternative to more intensive methods such as clear-cutting. However, present information is inadequate to judge impact of variable-retention on biodiversity of indigenous forest organisms intolerant of canopy removal, such as forest-inhabiting bryophytes. 2. We examined how bryophyte species cover, richness, diversity and composition change with time in response to a broad range of dispersed retention harvest treatments (2% (clear-cut), 10%, 20%, 50%, 75% retention of original basal area) contrasted with uncut controls (100% retention)) in broadleaf deciduous, mixedwood and conifer-dominated boreal forests in NW Alberta, Canada. Bryophytes were studied in 432 permanent sample plots within 72 compartments before harvest and at three, six and eleven years after harvest. 3. Clear-cut and lower (10% and 20%) retention levels resulted in lower cover and richness of bryophytes than in unharvested control compartments in mixed and conifer-dominated forests, but less so in deciduous-dominated forests, which generally supported low cover and richness. Species composition in each forest type varied along the gradient of harvesting intensity; clear-cuts and lower levels of retention supported similar composition, as did control plots and those representing higher retention levels. Over time the retention harvest treatments became more similar to uncut controls. 4. Synthesis and applications. Increased retention moderated the negative impacts of harvesting on bryophyte assemblages across all forest types, and our results suggest that even 10% retention will facilitate faster post-harvest recovery of bryophytes.02-Aug-2017 https://creativecommons.org/publicdomain/zero/1.0/

Franklin, Caroline M.A.; Macdonald, S. Ellen; Nielsen, Scott E.; Franklin, Caroline M. A. — 2018-06-14 Retention harvesting (also called tree retention or structural retention), in which live mature trees are selectively retained within harvested stands at different retention levels and in different patterns (aggregated to dispersed), is increasingly being used to mitigate the negative impacts of forest harvesting on biodiversity. However, the effectiveness of combining different patterns of retention harvesting for conservation and recovery of understory vascular plants in the long-term is largely unknown. To address this gap, we compared understory vascular plant diversity, abundance, and composition between aggregated retention and five levels of surrounding dispersed retention (0% = clearcut, 10%, 20%, 50%, 75%) 15 years post-harvest. We also investigated the influence of dispersed retention on the ability of embedded retention patches to support plant communities characteristic of unharvested forests, and whether it varies by patch size of aggregated retention (0.20 ha or 0.46 ha) and position within patches (edge or interior). Species richness, diversity, and cover were higher in the dispersed retention than in the patch retention as the harvested areas favored early-seral plant species. Graminoid cover was greater at the edges than in the interior of large patches. Retention patches as small as 0.2 ha more effectively supported shade-tolerant (forest interior) plant communities when they were surrounded by higher levels of dispersed retention (as compared to patches retained within clearcuts). Overall, the combined use of both aggregated and dispersed retention within a given cutblock benefits both late- and early-seral plant species and thus could effectively conserve understory plant assemblages in harvested landscapes. Sustainable forest management should therefore consider using a range of retention patch sizes combined with varying levels of surrounding dispersed retention in harvest designs to achieve objectives for plant conservation. https://creativecommons.org/publicdomain/zero/1.0/

Harper, Karen A.; Macdonald, S. Ellen; Mayerhofer, Michael S.; Biswas, Shekhar R.; Esseen, Per-Anders; Hylander, Kristoffer; Stewart, Katherine J.; Mallik, Azim U.; Drapeau, Pierre; Jonsson, Bengt-Gunnar; Lesieur, Daniel; Kouki, Jari; Bergeron, Yves — 2015-03-23 1. Although anthropogenic edges are an important consequence of timber harvesting, edges due to natural disturbances or landscape heterogeneity are also common. Forest edges have been well-studied in temperate and tropical forests, but less so in less productive, disturbance-adapted boreal forests. 2. We synthesized data on forest vegetation at edges of boreal forests and compared edge influence among edge types (fire, cut, lake/wetland; old vs. young), forest types (broadleaf vs. coniferous) and geographic regions. Our objectives were to quantify vegetation responses at edges of all types and to compare the strength and extent of edge influence among different types of edges and forests. 3. Research was conducted using the same general sampling design in Alberta, Ontario and Quebec in Canada, and in Sweden and Finland. We conducted a meta-analysis for a variety of response variables including forest structure, deadwood abundance, regeneration, understorey abundance and diversity, and nonvascular plant cover. We also determined the magnitude and distance of edge influence using randomization tests. 4. Some edge responses (lower tree basal area, tree canopy and bryophyte cover; more logs; higher regeneration) were significant overall across studies. Edge influence on ground vegetation in boreal forests was generally weak, not very extensive (distance of edge influence usually < 20 m) and decreased with time. We found more extensive edge influence at natural edges, at younger edges and in broadleaf forests. The comparison among regions revealed weaker edge influence in Fennoscandian forests. 5. Synthesis. Edges created by forest harvesting do not appear to have as strong, extensive or persistent influence on vegetation in boreal as in tropical or temperate forested ecosystems. We attribute this apparent resistance to shorter canopy heights, inherent heterogeneity in boreal forests and their adaptation to frequent natural disturbance. Nevertheless, notable differences between forest structure responses to natural (fire) and anthropogenic (cut) edges raise concerns about biodiversity implications of extensive creation of anthropogenic edges. By highlighting universal responses to edge influence in boreal forests that are significant irrespective of edge or forest type, and those which vary by edge type, we provide a context for the conservation of boreal forests.