Martin Simard

Fire-induced reductions in biomass limit the likelihood of reburning during early vegetation recovery in several crown-fire ecosystems; however, it is unclear how this negative fire-fuel feedback will buffer the rising fire activity expected with climate change. Although the feedback weakens with increasing time since the last fire and with increasing fire-weather severity, how the spatial configuration of fuel influences fire-return intervals remains unknown. Using a detailed 204-y fire history reconstruction along a 300-km longitudinal transect in eastern Canadian boreal forests, we show that interior areas of large fires (>8 km from the fire edge) are much less likely to reburn at short intervals than edges (<2 km from the edge), indicating a strengthening of the feedback within large burns. This process results in fewer fires in immature stands that are vulnerable to poor tree regeneration, thus conferring substantial resistance against compositional changes. However, the reduction in short-interval fires also fosters the development of large patches of mature forest that become extremely fire-prone after 4 to 5 decades and thereby predisposes landscapes to recurrent large fires. This study highlights an overlooked mechanism by which increases in fire size due to climate change may expose boreal forests of eastern Canada to large fires burning at longer intervals rather than to more frequent short-interval fires, which should sustain their capacity to recover.

Applying a centennial or millennial perspective to disturbance regimes permits an understanding of how these events have varied in the past in relation to climate change. Correctly interpreting this variability is crucial when preparing sustainable forest management practices for future warming. The eastern spruce budworm (Lepidoptera) is the most important biotic disturbance in the eastern Canadian boreal forest. Adult moths are covered by chitinous scales, and lepidopteran scale records in lake sediments have been analyzed to reconstruct Holocene spruce budworm populations. However, the magnitude of these scale accumulations has yet to be calibrated using an independent proxy. Here, we determine whether the impacts of spruce budworm defoliation are recorded by both sedimentary lepidopteran scale accumulations and tree-ring widths. Agreement between proxies was found at five of nine sites and strongest between the proportion of affected trees and scale accumulations while agreement in signal synchronicity was found at six of nine sites and strongest when comparing scale accumulations to a growth suppression index. A species-based composite chronology relying on white spruce produced the clearest outbreak record for both proxy records. Peak scale accumulations correlated well with smaller tree-ring widths, demonstrating that larger scale accumulations correspond to more severe defoliation events. Therefore, lepidopteran scales provide reliable records of spruce budworm abundance serving as a proxy record ameliorating our understanding of how budworm impacts have fluctuated at centennial and millennial time scales in the context of past climate change.

Climate change has increased the size and frequency of wildfires across the boreal biome. Severe wildfires in boreal forests have been found to trigger shifts from evergreen to deciduous canopies, which has cascading effects on carbon and nitrogen cycling. Ecosystem productivity and carbon uptake in boreal forests are strongly linked with nitrogen, and Earth system models increasingly depend on our understanding of the nitrogen balance to predict post‐fire carbon uptake. To investigate the post‐fire boreal nitrogen balance, we combined a mass balance approach and literature synthesis to estimate rates of nitrogen accumulation and nitrogen inputs across a network of 18 boreal wildfire chronosequences that varied in both wildfire regime and post‐fire canopy type, comprising 527 forest stands. We found that deciduous‐ or mixed‐dominance boreal forests establishing after severe, stand‐replacing fires had the highest nitrogen accumulation rates (15.7 ± 3.8 kg ha⁻¹ year⁻¹), while evergreen‐dominated forests establishing after surface‐ or mixed‐severity fires had the lowest nitrogen accumulation rates (1.4 ± 1.1 kg ha⁻¹ year⁻¹). Annual known inputs from nitrogen deposition and biological nitrogen fixation combined, estimated from published data, largely failed to explain the rate of nitrogen accumulation, particularly in deciduous or mixed‐dominance forests establishing after stand‐replacing fires, suggesting that the origins of most nitrogen in these forest types remain poorly understood. As the frequency of severe wildfires increases across the boreal biome and shifts toward deciduous canopies become more common, our study reveals a large knowledge gap in the resulting nitrogen balance that needs to be resolved in order to improve predictions of forest carbon uptake.

Characterizing disturbance regimes over long time scales is paramount for describing and identifying their variability. The most important biotic disturbance in the eastern Canadian boreal forest is the defoliation caused by the eastern spruce budworm, a moth of the insect order Lepidoptera. Lepidopteran scales have recently been used to reconstruct spruce budworm population fluctuations throughout the Holocene. However, this novel proxy has yet to be compared to an independent proxy. This study aimed to determine whether lepidopteran scales found in the surface sediments of boreal lakes tracked large spruce budworm populations, that is, outbreaks, using yearly aerial surveys (1967–present) of spruce budworm defoliation as an independent proxy. Scales were extracted (1 cm resolution) from the top 20 cm of ²¹⁰Pb-dated sediment cores recovered from nine lakes. To identify significant abundance peaks of scales in the time series, we removed background noise using a modified version of CharAnalysis. A 100-year smoothing window width combined with a 60th percentile threshold yielded the highest true positive and true negative occurrences, and the lowest false positive and false negative occurrences, with values of 0.69 and 0.70 for Cohen’s Kappa and Matthews correlation coefficient, respectively. Our findings demonstrate that lepidopteran scales are a suitable proxy for identifying spruce budworm outbreaks in the sediment record enabling the reconstruction of budworm and other lepidopteran species outbreak dynamics at millennial timescales.

Climate change has resulted in a widespread increase in primary productivity in northern regions. This “greening” can alter terrestrial ecosystems dynamics and trigger positive climate feedbacks but often exhibits spatial heterogeneity. Few studies have focused on the differential responses of plant functional groups to warming as a potential driver of heterogeneity in greening rates. Our objective was to determine whether climate–growth relationships are species-specific, and whether they can explain the heterogeneity of the subarctic greening rates. We compared climate–growth relationships and associations to normalized difference vegetation index (NDVI) of the dominant tree ( Picea mariana) and shrub ( Betula glandulosa) species at two locations in the forest–tundra ecotone in Nunavik (Québec, Canada). Correlation coefficients of the climate–growth relationship varied between species and locations, being higher for P. mariana and at the northernmost location. The lower association between NDVI and P. mariana radial growth at the northernmost location appeared to be compensated by a stronger association between NDVI and B. glandulosa radial growth. Our results show that response to climate varies spatially and between species, both of which could potentially be used to explain the heterogeneity of subarctic greening rates. This improved understanding of the species-specific response to climate change will help predict forthcoming changes in primary productivity and their potential positive feedback on climate change.

Mis en service en 2018, le port de Kribi au Cameroun est un exemple de grand projet d’infrastructures initié par un État africain pour moderniser son économie. La région de Kribi, située au sud-ouest du pays, a ainsi été affectée par de grands travaux dans une zone localisée en bordure du Golfe de Guinée. Cet article vise à comprendre les impacts sociaux reliés au port, en particulier les effets du programme de relocalisation de villages. À cette fin, nous avons effectué une série d’entrevues en 2021 afin de saisir les bouleversements vécus par les populations ciblées. Nos résultats démontrent que le programme de relocalisation a connu d’importantes difficultés : consultations limitées, indemnisations entachées d’irrégularités et nouveaux sites villageois inhabités. Une large part de ces insuccès s’explique par le fait d’avoir ignoré les perceptions et les représentations des populations.