by Marie-Claude Boileau | 11 November 2022
Published in Forest Ecology and Management, 433: 376-385.
Analysis and ¹⁴C dating of charcoal fragments ≥ 2 mm buried in mineral soils make it possible to obtain a stand-scale portrait of Holocene fires that occurred in well-drained, fire-prone environments, as well as changes in forest stand composition over time, based on botanical identification of charcoals. Yet, it is not always possible to reconstruct all fire events, due to disturbances that have altered soil stratigraphy. To evaluate the efficacy of this approach, we conducted a comparative analysis with a proximal environment that presents an a priori continuous stratigraphy of charcoal fragments. For two sites in the coniferous boreal forest of eastern North America, the charcoal record of a forest soil was compared with that of an adjacent peatland margin situated at a distance of 20 m. For both types of sedimentary environments, a similar fire history was reconstructed for a most of the Holocene. The greatest differences were for the early Holocene period, for which a smaller number of fires were detected in the forest soil compared to the peatland soil. Retracing the oldest fires using mineral soils in a fire-prone environment is more difficult, given charcoal decay that results from repeated fire events. Yet, forest soils reveal a relatively accurate fire history for subsequent millennia if the number of charcoals being dated is sufficiently large. Any accurate reconstruction of the fire history of proximal peatland environments is strongly dependent on continuous stratigraphic units of peat and charcoal. Indeed, the age of charcoal fragments in peat may be different from that of the sedimentary layer in which they are buried due to allogenic disturbances such as erosion events caused by deep burning of the organic horizon and other mass-wasting events. Despite the large number of ¹⁴C dates it requires, analysis of soil macro-charcoal yields a realistic picture of the fire history at the stand scale. The concurrent analysis of macro-charcoal from adjacent peatland deposits may be used as a complement to more accurately record the oldest fire events.
by Marie-Claude Boileau | 11 November 2022
Published in Botany, 95: 697-707 https://doi.org/10.1139/cjb-2016-0319
Plant species are unique in their biological traits and biogeographical history, resulting in distinctive species distributions. Continuous and fragmented ranges of varying size and shape have captured the interest of biogeographers. Fragmented distribution into isolated populations is a common pattern of temperate and boreal species caused by contraction and expansion processes. Jack pine (Pinus banksiana Lamb.), a North American tree species, is among a multitude of species showing fragmented distributions to isolated populations. Whether disjunct jack pine forests are remnants of larger Holocene populations or newly established populations due to long-distance transport remains unanswered. We used a retrospective approach based on soil macro-charcoal analysis to address the question of residency of a disjunct population in the boreal forest. The studied forest forms a disjunct population of a former regional population that has contracted since the mid-Holocene. Short to moderately long-fire intervals have occurred over the last 6000 years to maintain the species in a fire-prone sandy environment, thereby assuring its regeneration and survival. Disjunct distributions similar to the studied pine population are often caused by regional extirpation of populations in which environmental contraction produces small ecological refugia where local conditions remain suitable through time for a species to complete its life cycle.
by Audrey Verreault | 8 November 2022
Published in Physiologia Plantarum 174(6): e13798. https://doi.org/10.1111/ppl.13798
Under climate change, the increasing occurrence of late frost combined with advancing spring phenology can increase the risk of frost damage in trees. In this study, we tested the link between intra-specific variability in bud phenology and frost exposure and damages. We analysed the effects of the 2021 late frost event in a black spruce (Picea mariana (Mill.) BSP) common garden in Québec, Canada. We hypothesised that the timing of budbreak drives the exposure of vulnerable tissues and explains differences in frost damage. Budbreak was monitored from 2015 to 2021 in 371 trees from five provenances originating between 48° and 53° N and planted in a common garden at 48° N. Frost damages were assessed on the same trees through the proportion of damaged buds per tree and related to the phenological phases by ordinal regressions. After an unusually warm spring, minimum temperatures fell to −1.9°C on May 28 and 29, 2021. At this moment, trees from the northern provenances were more advanced in their phenology and showed more frost damage. Provenances with earlier budbreak had a higher probability of damage occurrence according to ordinal regression. Our study highlights the importance of intra-specific variability of phenological traits on the risk of frost exposure. We provide evidence that the timings of bud phenology affect sensitivity to frost, leading to damages at temperatures of −1.9°C. Under the same conditions, the earlier growth reactivation observed in the northern provenances increases the risks of late frost damage on the developing buds.
by Marie-Claude Boileau | 7 November 2022
Published in Journal of Quaternary Science, 26: 571-575.
The use of macrofossil soil charcoal as a palaeoecological tool to reconstruct past vegetation, climate or fire history has gained much interest in recent years. Yet, little is known about the taphonomy of charcoal in soils. Here, we assessed the putative loss of palaeoecological information due to charcoal fragmentation after burial. We found no significant loss of charcoal mass with time. Instead, we found a significant positive relationship between the mass of charcoal particles and their age. Permineralization of charcoal particles older than ca. 5200 a explained the increased charcoal mass with time in mineral soils. The permineralization process increased the density of charcoal particles (resulting in a two-fold increase particle mass) and, thus, offers a protection against subsequent degradation. Our results suggest high stability of palaeoecological information from charcoal macrofossils buried in mineral soils at least over the Holocene timescale.
by Marie-Claude Boileau | 7 November 2022
Published in Ecosystems, 21: 1183-1195. https://link.springer.com/article/10.1007/s10021-017-0211-3
Balsam fir (Abies balsamea) and black spruce (Picea mariana) forests are the main conifer forest types in the North American boreal zone. The coexistence of the two species as well as their respective canopy dominance in distinct stands raises questions about the long-term evolution from one forest type to the other in relation to environmental factors including climate and stand disturbance. We tested the hypothesis that repetitive fire events promote the succession of balsam fir forests to black spruce forests and vice versa. Post-fire chronosequences of one black spruce (BSP) and one balsam fir (BFI) site were reconstructed based on the botanical composition and ¹⁴C-dated soil macro-charcoals. The results support the hypothesis of successional dynamics. The BSP site has been affected by fire for the past 7600 years, whereas the BFI site, after having been affected by several fires during the first-half of the Holocene, evolved in a fire-free environment for the last 4400 years. Periods of fire activity facilitated the dominance of black spruce forests. The cessation of fires around 4400 cal. years BP on the BFI site marks the beginning of the transition from black spruce to balsam fir stands. This succession is a long process, due to the ability of black spruce to regenerate by layering in the absence of fire. The resulting balsam fir stands are ancient and precarious ecosystems, since fire generally leads to the return of black spruce. The increase in balsam fir to the detriment of black spruce in boreal forests is a response to a decrease in fire frequency.
