by Marie-Claude Boileau | 11 November 2022
Published in The Holocene, 25: 1246-1256. https://doi.org/10.1177/0959683615580863
Eastern hemlock (Tsuga canadensis) is a shade-tolerant tree species of the temperate conifer-hardwood forests of northeastern North America, the northern distribution limit of which coincides with the St. Lawrence River around Québec City (Canada). We analyzed the structure and dynamics of one of the very few old-growth hemlock stands in this area to evaluate its successional status at the Holocene scale. To document the origin and long-term development of the hemlock site, we used conventional forest surveys and macrofossil analysis of woody debris and charcoal pieces at the soil surface and buried in the mineral soil. ‘Rivière-du-Moulin hemlock forest’ is an old-growth forest, at least 1000-years-old, the structure of which has been rejuvenated by recurrent surface fires killing most plants of the shaded forest floor and facilitating hemlock regeneration. According to the number of fires and the corresponding fire intervals, the hemlock site has experienced a sustained fire regime since the mid-Holocene. It first developed as a hardwood forest where beech (Fagus), butternut (Juglans) and birch (Betula) grew, and then for the last 2400–2100 years as a conifer forest where hemlock prevailed during a large part of the period. Our data highlight the influence of fire on the dynamics of hemlock-hardwood stands, a forest ecosystem generally viewed as being controlled by local light and medium canopy-gap disturbances. Soil charcoal analysis of conifer-hardwood forests may be used concurrently with canopy-gap analysis to decipher the influence of stand-scale disturbances and to calculate better forest turnover at several time scales.
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.
