Pierre J. H. Richard

Wildfire responses to abrupt climate change in North America

It is widely accepted, based on data from the last few decades and on model simulations, that anthropogenic climate change will cause increased fire activity. However, less attention has been paid to the relationship between abrupt climate changes and heightened fire activity in the paleorecord. We use 35 charcoal and pollen records to assess how fire regimes in North America changed during the last glacial–interglacial transition (15 to 10 ka), a time of large and rapid climate changes. We also test the hypothesis that a comet impact initiated continental-scale wildfires at 12.9 ka; the data do not support this idea, nor are continent-wide fires indicated at any time during deglaciation. There are, however, clear links between large climate changes and fire activity. Biomass burning gradually increased from the glacial period to the beginning of the Younger Dryas. Although there are changes in biomass burning during the Younger Dryas, there is no systematic trend. There is a further increase in biomass burning after the Younger Dryas. Intervals of rapid climate change at 13.9, 13.2, and 11.7 ka are marked by large increases in fire activity. The timing of changes in fire is not coincident with changes in human population density or the timing of the extinction of the megafauna. Although these factors could have contributed to fire-regime changes at individual sites or at specific times, the charcoal data indicate an important role for climate, and particularly rapid climate change, in determining broad-scale levels of fire activity.

Holocene biomass burning and global dynamics of the carbon cycle

Fire regimes have changed during the Holocene due to changes in climate, vegetation, and in human practices. Here, we hypothesise that changes in fire regime may have affected the global CO2 concentration in the atmosphere through the Holocene. Our data are based on quantitative reconstructions of biomass burning deduced from stratified charcoal records from Europe, and South-, Central- and North America, and Oceania to test the fire-carbon release hypothesis. In Europe the significant increase of fire activity is dated approximately 6000 cal. yr ago. In north-eastern North America burning activity was greatest before 7500 years ago, very low between 7500-3000 years, and has been increasing since 3000 years ago. In tropical America, the pattern is more complex and apparently latitudinally zonal. Maximum burning occurred in the southern Amazon basin and in Central America during the middle Holocene, and during the last 2000 years in the northern Amazon basin. In Oceania, biomass burning has decreased since a maximum 5000 years ago. Biomass burning has broadly increased in the Northern and Southern hemispheres throughout the second half of the Holocene associated with changes in climate and human practices. Global fire indices parallel the increase of atmospheric CO2 concentration recorded in Antarctic ice cores. Future issues on carbon dynamics relatively to biomass burning are discussed to improve the quantitative reconstructions.

Variability in Fire Frequency and Forest Composition in Canada'sSoutheastern Boreal Forest: A Challenge for Sustainable ForestManagement

Because some consequences of fire resemble the effects of industrial forest harvesting, forest management is often considered as a disturbance having effects similar to those of natural disturbances. Although the analogy http://www.ecologyandsociety.org/vol2/iss2/art6/ (1 of 11) [9/5/2008 10:08:21 AM] Conservation Ecology: Variability in fire frequency and forest composition i...s southeastern boreal forest: a challenge for sustainable forest management between forest management and fire disturbance in boreal ecosystems has some merit, it is important to recognize that it has limitations. First, normal forest rotations truncate the natural forest stand age distribution and eliminate over-mature forests from the landscape. Second, in the boreal mixedwoods, natural forest dynamics following fire may involve a gradual replacement of stands of intolerant broadleaf species by mixedwood and then softwood stands, whereas current silvicultural practices promote successive rotations of similarly composed stands. Third, the large fluctuations observed in fire frequency during the Holocene limit the use of a single fire cycle to characterize natural fire regimes. Short fire cycles generally described for boreal ecosystems do not appear to be universal; rather, shifts between short and long fire cycles have been observed. These shifts imply important changes in forest composition at the landscape and regional levels. All of these factors create a natural variability in forest composition that should be maintained by forest managers concerned with the conservation of biodiversity. One avenue is to develop silvicultural techniques that maintain a spectrum of forest compositions over the landscape.

Postglacial water-level changes of a small lake in southern Québec, Canada

Sediment stratigraphy and palaeobotanical data from five cores were used to reconstruct the postglacial water-level changes of a small lake in southern Québec. The cores were taken along two transects from the centre of the lake to the margins. Lake level was relatively high before 11 000 cal. BP, when sedimen tation was characterized by the deposition of silty gyttja (.13 000 cal. BP) and marl (13 000–11 000 cal. BP). From 11 000 to 7000 cal. BP, a sediment hiatus was indicated by the lack of two regional pollen zones between the marl and the overlying gyttja, suggesting either nondeposition or erosion of the previously deposited sedi ments due to a low lake level. The lake level was approximately 2 m lower than present on the basis of macrofossil assemblages. A rise in the water level of about 1 m occurred around 7000 cal. BP, which was interrupted by a second lowering between 6100 and 4400 cal. BP, as indicated by a hiatus in the pollen stratigra phy. During this low period, the water level was at least 2 m lower than present. The last 4400 years are characterized by a continuous rise. The lake-level fluctuations are broadly synchronous with other palaeo hydrological records in northeastern United States, Ontario, and Subarctic Québec, implying broad-scale climatic control.

Effects of fire severity and initial tree composition on understorey vegetation dynamics in a boreal landscape inferred from chronosequence and paleoecological data

Abstract Question and Location: How does soil burn severity and early post-fire tree composition affect long-term understorey vegetation dynamics in the coniferous forests of eastern Canada? Method: Vegetation dynamics were assessed using paleoecological methods and a chronosequence analysis of extant stands. The relation between environmental factors and succession was evaluated using ordination techniques on the chronosequence data. Understorey succession was studied by regression analysis on the chronosequence data and through within-site Markovian transition probabilities between successive 1-cm layers of plant macroremains from soil organic matter profiles. Results: Initial tree composition (Picea mariana and Pinus banksiana) had little effect on understorey composition. Soil burn severity (measured as the thickness of the residual forest floor humus) significantly affected temporal changes in understorey species. Following fires of high severity, stands underwent a gradual paludification with a net increase in Sphagnum and ericaceous shrubs (Ledum groenlandicum), and a decrease in feathermosses. Paludification was accelerated after low severity fires, which led to the dominance of Sphagnum less than 200 years after fire, and of L. groenlandicum shortly after fire. In situ paleoecological work confirmed results obtained with the chrono-sequence analysis. Conclusions: One vegetation gradient related to time after disturbance is insufficient to account for the full complexity of long-term changes in understorey composition following fire. Current forestry practices that protect the forest floor humus may induce a premature paludification. Abbreviations: AMS = Accelerated mass spectrometry; GCC = Global climate change; HS = High severity; LS = Low severity; TSF = Time since last fire. Nomenclature: Marie-Victorin (1995) and Montgomery (1977) for vascular plants; Anderson et al. (1990) for bryophytes and Lévesque et al. (1988) for macrofossils.

Paleoecology of a >90,000-year lacustrine sequence from Fog Lake, Baffin Island, Arctic Canada

A 137 cm sediment core from Fog Lake, a small upland basin that was not glaciated during the Late Wisconsinan, reveals the following stratigraphic succession: (I) basal diamicton, (II) laminated silts, (III) compacted dewatered gyttja, (IV) stratified minerogenic sediments, and (V) overlying gyttja. Units I—III, and most of unit IV, were deposited prior to the Holocene; unit V represents approximately the last 8000 14C years. Alternate luminescence dating techniques (TL and IRSL) place unit III in latter oxygen isotope stage (OIS) 5, ’74,000 years BP. A suite of 20 AMS 14C dates suggests that much of the post-OIS 5 paleoenvironmental record is characterized by either extremely low continuous sediment accumulation, or by one or more episodes of more rapid sedimentation. Diverse pollen and spore assemblages have concentrations that are unexpectedly high throughout the pre-Holocene sediments (’10,000 grains cm~3), by comparison to the Holocene (&5000 grains cm~3). Exceedingly low sediment accumulation rates appear to have resulted in the concentration of atmospherically derived pollen and spores. Diatom assemblages record the evolution of the lake’s chemistry and past algal production. Several abrupt changes occurred synchronously in both the terrestrial and aquatic components of the system, and these appear to represent ecological responses to pre-Holocene climatic variability. The sediments of Fog Lake provide the first integrated paleoecological record older than Holocene for a vast region of the Canadian Arctic, thereby refining our understanding of long-term arctic ecosystem ontogeny. ( 2000 Elsevier Science Ltd. All rights reserved.

Postglacial palaeophytogeography of the eastern St. Lawrence River Watershed and the climatic signal of the pollen record

Abstract Pollen analytical studies of the sediments of over 150 lakes and bogs in southern Quebec have led to the indentification of a limited number of sequential types or temporal suites of the various pollen assemblages within each of three main stages of the postglacial development of vegetation: the Non-arboreal stage, the Afforestation stage and the Forest stage. These sequential types, within any of the three stages, show distinct geographical and chronological distributions in southern Quebec during the last 12,000 yr. When compared in a broader geographic context, they demonstrate a strong differentiation of the pollen record within and around the Great Lakes-St. Lawrence River Watershed during the last 15,000 yr B.P. Amongst the most striking gradients identified stand the late-glacial perichamplainian gradients (ca. 11,500–10,000 yr B.P.) and the early Holocene gradients. The former involve seaward and northeastward clines from closed coniferous forests to tundras, and the latter, northeastward changes in the nature and duration of the Afforestation stage. The modern pattern of even the broader vegetational gradients in southern Quebec was not attained before 3000 yr B.P. Palynological reconstruction of the various regional paleoclimates highlights the interaction of local, regional and global climatic forcings through time. Mesoscale climate modelling and palaeoclimatology should find boundary conditions in those regional reconstructions.

Origine et dynamique postglaciaire de la forêt mixte au Québec

The Mixed Forest in Quebec is basically defined by the abundance of balsam fir (Abies balsamea) in association with birches (Betula papyrifera and B. alleghaniensis). Its palynological signature and its identification in the pollen diagrams are intimately linked to the under-representation of balsam fir. The pollen spectra produced by the various vegetation units of the latitudinal gradient in Quebec are translated into a standard sequence against which the subfossil spectra are compared. The pollen representation of the dominant species on the mesic sites plays a central role in the use of this standard sequence. The sequential order and the duration of the pollen maxima of the major tree species in the pollen diagrams are examined against their present-day positions and strategies in the successional development of their populations. The interpretation of the palaeopalynological data and their examination in the light of the zonal and successional analogical models lead to the identification of periods during which, and regions in which, the balsam fir communities differed in their composition and/or their dynamics from those of the present-day Mixed Forest. The modes for the establishment of arboreal populations and for the development of the Mixed Forest vary widely throughout the territory. The departures from the models are most numerous and conspicuous during the Early Holocene, mainly because of a cold regional climate and a high incidence of natural fires. Spatial analysis of the data leads to the identification of contrasted palynostratigraphical provinces corresponding to palaeophytogeographical zones reflecting non-linear palaeoenvironmental gradients, especially for the climate at the southern edge of the Laurentian Ice Sheet between 10,000 and 7000 yr BP.

Changes in fire regime explain the Holocene rise and fall of Abies balsamea in the coniferous forests of western Québec, Canada

The coniferous boreal forest of northeastern North America is characterized by large and severe fire events and dominated by black spruce (Picea mariana ), with scattered patches of balsam fir (Abies balsamea), a species otherwise predominant in the more southern mixedwood boreal forests, characterized by smaller and less severe fire events. Because balsam fir is a late-successional species ill-adapted to fire, this study aimed at determining if the scattered balsam-fir patches found in the coniferous forest were relics of a former fire regime characterized by less frequent and/or severe conflagrations. Fire and vegetation history were assessed for a coniferous forest site through analyses of charcoal, pollen and plant macroremains preserved in lake sediments, peat and hydromorphic forest soil. Pollen and macroremains analyses show that black spruce dominated the local vegetation since deglaciation ( c. 8000 cal. yr BP). Balsam fir was abundant around the site during the warm and humid summers of the Hypsithermal (between c. 7000 and 3500 cal. yr BP), before gradually declining during the cool and dry Neoglacial, which was characterized by increased fire frequency and severity. Scattered balsam fir patches in the coniferous forest result from the fragmentation of formerly larger populations and are presently in disequilibrium with climate.

Resilience of the boreal forest in response to Holocene fire-frequency changes assessed by pollen diversity and population dynamics

The hypothesis that changes in fire frequency control the long-term dynamics of boreal forests is tested on the basis of paleodata. Sites with different wildfire histories at the regional scale should exhibit different vegetation trajectories. Mean fire intervals and vegetation reconstructions are based respectively on sedimentary charcoal and pollen from two small lakes, one in the Mixedwood boreal forests and the second in the Coniferous boreal forests. The pollen-inferred vegetation exhibits different trajectories of boreal forest dynamics after afforestation, whereas mean fire intervals have no significant or a delayed impact on the pollen data, either in terms of diversity or trajectories. These boreal forests appear resilient to changes in fire regimes, although subtle modifications can be highlighted. Vegetation compositions have converged during the last 1200 years with the decrease in mean fire intervals, owing to an increasing abundance of boreal species at the southern site (Mixedwood), whereas changes are less pronounced at the northern site (Coniferous). Although wildfire is a natural property of boreal ecosystems, this study does not support the hypothesis that changes in mean fire intervals are the key process controlling long-term vegetation transformation. Fluctuations in mean fire intervals alone do not explain the historical and current distribution of vegetation, but they may have accelerated the climatic process of borealisation, likely resulting from orbital forcing.