Dominique Arseneault

Estimating Lichen Biomass and Caribou Grazing on the Wintering Grounds of Northern Quebec: An Application of Fire History and Landsat Data

1. Landsat Thematic Mapper (TM) remote sensing imagery, previously published fire history data (Payette et al. 1989), and field observations were used to examine the influence of caribou grazing in part of their lichen-dominated wintering grounds in northern Quebec, Canada. Lichen biomass and percentage ground cover were measured in 1989 and 1992 over a large area (88 150km 2 ) stratified into five postfire successional stages. 2. Lichen-dominated vegetation covered 55%, moss- or shrub-dominated vegetation 25%, and water bodies 20% of the study area. Lichen biomass increased with postfire stand age, from 530 kg ha -1 in young stands ( 90 years). The cumulative amount of lichen cover removed by caribou before 1989 averaged 10% over the study area; by 1992 this value had risen to 21%. Between 1989 and 1992, ground cover of lichens decreased from 55 to 42%. Lichen removal was concentrated in stands > 50 years old, where it occurred at a rate of about 5% per year. No significant change in lichen cover was observed in younger stands. 3. The number of caribou grazing in the study area between 1989 and 1992 exceeded the carrying capacity estimated from the annual increment in lichen biomass ( 1% year -1 ); lichens were reduced by both consumption and collateral damage. Lichen cover can be progressively reduced by winter grazing of caribou in large lichen-dominated continental areas in much the same way as in insular ranges. 4. The combination of remote sensed and fire history data may be a helpful tool for managing large herds of wild caribou.

RECONSTRUCTION OF MILLENNIAL FOREST DYNAMICS FROM TREE REMAINS IN A SUBARCTIC TREE LINE PEATLAND

Tree ring and growth form sequences of 319 black spruce (Picea mariana (Mill.) BSP.) stems buried in a treeless peatland at the arctic tree line of northern Quebec were used to reconstruct the development of a woodland in response to climate change and fires during the last 2500 years. A high frequency of diagnostic tree rings (light rings and narrow rings) allowed the cross-dating of 142 individuals and the construction of a master chronology spanning AD 690-1591. Three floating chronologies covering 964 ( 178 BC- AD 785), 349 (-587-239 BC) and 210 (-1274-1065 BC) years were also developed. The buried stems were classified as arborescent (75%), small fragments of unknown origin (13%), stumps (11%), and portions of stunted stems (1%). The high frequency of arborescent individuals indicates that the buried spruces were the remains of a former forest. The forest bordered the peatland between 600 BC and AD 1568, and successfully regenerated after two fires around 350 BC and 10 BC. Winter-damaged trees dominated during periods of suppressed growth at AD 760-860 and AD 1025-1400, whereas undamaged trees were more frequent during periods of rapid growth around AD 700-750, 860-1000, 1400-1450, and 1500-1570. Fast growth between AD 860-1000, along with the concurrent establish- ment of symmetrical trees, suggests well-defined boundaries for the Medieval Warm Period in northeastern Canada. The forest abruptly shifted to an open krummholz (stunted spruce) after the last fire in AD 1568, indicating that a climatic threshold inhibiting postfire re- generation was crossed between the second and the last fires. The maintenance of this old growth forest over 1500 yr, in the absence of external disturbances except climate change, was probably due to the buffering effect of aggregated trees on wind-drifted conditions at the snowpack line. With the exclusion of the forest influence on microclimate and local growth conditions, the AD 1568 fire caused the forest vegetation to shift to krummholz. The amplitude of these ecosystem changes at tree line does not mirror that of climate change. Hence, it is concluded that climate and vegetation reconstructions from proxy indicators cannot portray full ecological impact, because vegetation change at tree line is nonlinear relative to climate change.

A Postfire Shift From Lichen‐Spruce to Lichen‐Tundra Vegetation at Tree Line

The environmental changes associated with a fire-induced shift from old- growth lichen-spruce krummholz to lichen-tundra vegetation have been evaluated at a tree line site in northern Qu&bec. Tree ring and growth form patterns of black spruce (Picea mariana) remains lying on the ground in a lichen-tundra community were used to recon- struct, within a 4600-M2 quadrat, the structure of a conifer stand at the time of the burn (-AD 1750). The prefire spruces were the last members of a long regenerative sequence leading to maintenance of a lichen-spruce stand formed after a burn z 1700 yr BP. At this time postfire recruitment was most likely facilitated by favorable climatic conditions. Before the 1750 fire event the krummholz was predominantly maintained by layering, due to severe climatic conditions at least since the beginning of the Little Ice Age (-AD 1580). The site was deforested by the 1750 fire, because of the limited regenerative potential of stunted spruce. Postfire spruce exclusion has been responsible for major environmental changes associated with a thinner snow cover. Along the border of the deforested site snow cover depth was controlled by living spruces, whereas postfire shrubs (mostly dwarf birch, Betula glandulosa) of the lichen-tundra stand were unable to trap drifting snow. A signif- icant decrease in thickness of the snow cover following deforestation was deduced from a comparative analysis of stem morphology of prefire spruces and present snow conditions. Inception of gelifluction lobes and mudboils was also caused by a change in the soil thermal regime associated with a reduced snow cover. Our results confirm the hypothesis that several lichen stands of the forest-tundra are postfire communities succeeding from de- graded conifer stands during cold periods of the late Holocene.

Resistance of the boreal forest to high burn rates

Significance Climate change is expected to drastically increase both fire size and the frequency of large-fire years in the North American boreal forest, with consequent effects on forest ecosystems and carbon stocks. However, the influence of fire size and time since previous fire on fire activity is poorly understood because of incomplete records of past fire overlaps. Here we reconstruct the length of overlapping fires during the last 200 years along a 190-km transect and provide direct field evidence that extreme burn rates can be sustained by occasional droughts triggering immense fires. Fire occurrence in the most fire-prone regions of the North American boreal forest is, however, already fuel-limited and will resist further climate change because of overabundant young forest stands. Boreal ecosystems and their large carbon stocks are strongly shaped by extensive wildfires. Coupling climate projections with records of area burned during the last 3 decades across the North American boreal zone suggests that area burned will increase by 30–500% by the end of the 21st century, with a cascading effect on ecosystem dynamics and on the boreal carbon balance. Fire size and the frequency of large-fire years are both expected to increase. However, how fire size and time since previous fire will influence future burn rates is poorly understood, mostly because of incomplete records of past fire overlaps. Here, we reconstruct the length of overlapping fires along a 190-km-long transect during the last 200 y in one of the most fire-prone boreal regions of North America to document how fire size and time since previous fire will influence future fire recurrence. We provide direct field evidence that extreme burn rates can be sustained by a few occasional droughts triggering immense fires. However, we also show that the most fire-prone areas of the North American boreal forest are resistant to high burn rates because of overabundant young forest stands, thereby creating a fuel-mediated negative feedback on fire activity. These findings will help refine projections of fire effect on boreal ecosystems and their large carbon stocks.

LANDSCAPE CHANGE FOLLOWING DEFORESTATION AT THE ARCTIC TREE LINE IN QUÉBEC, CANADA

This study examines the influence of fire-induced deforestation in well- drained conifer sites on the development of adjacent, surviving spruce stands in peatlands at the arctic tree line (northern Quebec). Tree-ring and growth-form patterns of cross-dated black spruce (Picea mariana) remains in peat were used to analyze stand responses in two contrasting topographical settings: a moderately protected valley-bottom peatland, and an exposed hilltop peatland. Dense, old-growth spruce populations occupied the studied peat- lands between the 11th and 16th centuries, with stunted and erect spruce dominating the hilltop and the valley sites, respectively. Spruce continued to colonize the peatlands as long as the adjacent well-drained sites were occupied by forest trees able to attenuate the dam- aging effect of winter snow-drifting conditions. The surviving conifer stands exhibited similar growth responses following deforestation of the well-drained sites by wildfire in AD 1567-1568. These responses included dieback of supranival (above snow) stems and death of trees due to drowning in permafrost-induced ponds. The postfire degradation and disappearance of the conifer stands from the peatlands were the ultimate stage of a positive feedback process triggered by a modification of the snow regime at the landscape scale. Deforestation of the well-drained sites led to reduced snow accumulation and enhanced snow-drift exposure, thus inducing permafrost aggradation and drainage impediment in the nearby peatland sites. While the hilltop conifer stand responded immediately to forest exclusion in the nearby sites, ecosystem changes in the valley peatland were delayed until AD 1580-1590, at the beginning of the Little Ice Age. Our results emphasize the sensitivity and connectedness of adjoining ecosystems to fire and frost disturbances in a changing subarctic landscape mosaic.

Volcano-induced regime shifts in millennial tree-ring chronologies from northeastern North America

Significance The cooling effect on the Earths climate system of sulfate aerosols injected into the stratosphere by large volcanic eruptions remains a topic of debate. While some simulation and field data show that these effects are short-term (less than about 10 years), other evidence suggests that large and successive eruptions can lead to the onset of cooling episodes that can persist over several decades when sustained by consequent sea ice/ocean feedbacks. Here, we present a new network of millennial tree-ring chronologies suitable for temperature reconstructions from northeastern North America where no similar records are available, and we show that during the last millennium, persistent shifts toward lower average temperatures in this region coincide with series of large eruptions. Dated records of ice-cap growth from Arctic Canada recently suggested that a succession of strong volcanic eruptions forced an abrupt onset of the Little Ice Age between A.D. 1275 and 1300 [Miller GH, et al. (2012) Geophys Res Lett 39(2):L02708, 10.1029/2011GL050168]. Although this idea is supported by simulation experiments with general circulation models, additional support from field data are limited. In particular, the Northern Hemisphere network of temperature-sensitive millennial tree-ring chronologies, which principally comprises Eurasian sites, suggests that the strongest eruptions only caused cooling episodes lasting less than about 10 y. Here we present a new network of millennial tree-ring chronologies from the taiga of northeastern North America, which fills a wide gap in the network of the Northern Hemispheres chronologies suitable for temperature reconstructions and supports the hypothesis that volcanoes triggered both the onset and the coldest episode of the Little Ice Age. Following the well-expressed Medieval Climate Anomaly (approximately A.D. 910–1257), which comprised the warmest decades of the last millennium, our tree-ring-based temperature reconstruction displays an abrupt regime shift toward lower average summer temperatures precisely coinciding with a series of 13th century eruptions centered around the 1257 Samalas event and closely preceding ice-cap expansion in Arctic Canada. Furthermore, the successive 1809 (unknown volcano) and 1815 (Tambora) eruptions triggered a subsequent shift to the coldest 40-y period of the last 1100 y. These results confirm that series of large eruptions may cause region-specific regime shifts in the climate system and that the climate of northeastern North America is especially sensitive to volcanic forcing.

Millennial stocks and fluxes of large woody debris in lakes of the North American taiga

Summary 1. Large woody debris (LWD) is an important cross-boundary subsidy that enhances the productivity of lake ecosystems and the stability of aquatic food webs. LWD may also be an important carbon sink because LWD pieces are preserved for centuries in the littoral zone of lakes and rivers. However, a long-term analysis of LWD stocks and fluxes in lakes, coupled with the reconstruction of past disturbances at the site level, has never been attempted. 2. Large woody debris was sampled in five lakes of the Quebec taiga. Actual LWD stocks were described and residence time of the LWD pieces was established using tree-ring and radiocarbon dating. LWD losses by decomposition and burial and other factors influencing LWD residence time were investigated using linear regressions. 3. Impacts of wildfires on LWD fluxes during the last 1400 years were reconstructed separately for the five lakes using piecewise regression models. Fire years at each site were identified from the recruitment dates of charred LWD pieces. 4. Large woody debris volume ranged between 0.92 and 1.57 m 3 per 100 m of shoreline, and extrapolating these results to the landscape scale, it was concluded that LWD littoral carbon pools represent a minimal portion of boreal carbon storage. 5. Large woody debris residence time in boreal lakes was confirmed to be very long. Tree-ring dates of 1571 LWD pieces, mainly black spruce (Picea mariana (Mill.) BSP.), spanned the last 1400 years, while LWD specimens of older floating chronologies were preserved from decomposition for up to five millennia. The most influential variables explaining the variation in LWD residence time were the degree of burial and the distance from the shore. 6. Large woody debris recruitment rates averaged 5.8 pieces per century per 100 m of shoreline. Fourteen wildfires were the primary cause for changes in the rates of tree establishment in the riparian forests and of LWD recruitment in the lakes. 7. Synthesis. Interactions between terrestrial and aquatic ecosystems in northern boreal regions are strongly influenced by wildfires whose effects can last for centuries due to the slow large woody debris decay rate. Actual LWD stocks and carbon pools are a legacy of the past fire history.

Millennial disturbance‐driven forest stand dynamics in the Eastern Canadian taiga reconstructed from subfossil logs

1. Although wildfire is the main natural disturbance factor driving changes in the North American boreal forest, understanding how the fire history of the last millennium shaped the present-day landscape diversity is a difficult task due to the lack of palaeoecological reconstructions with high spatial (few hundreds of square metres) and temporal (annual) resolutions. 2. We combined a detailed inventory of the present-day lakeshore forest of two lakes of the Eastern Canadian taiga with the dendrochronological dating of the subfossil logs that accumulated in the lit-toral zones facing these shores. Our objective was to compare the millennial impact of wildfires among stands of various structures and compositions. Past stem densities and fire years were reconstructed from log recruitment rates and dating of charred logs. 3. Multivariate analysis of the present-day lakeshore forest revealed three and two homogeneous shore segments per site (i.e. clusters). Cluster 1 at both sites exhibited denser forest, higher dead wood values and a higher percentage of balsam fir, a fire-sensitive species. 4. In total, 426 and 611 subfossil logs (mostly black spruce) were crossdated over the last ~1400 years. Their dendrochronological analysis confirmed that each lakeshore cluster, identified from the traits of the present-day forest, experienced a specific fire history over the last millennium (i.e. 0–5 fires of variable severity) that locally influenced forest composition, tree density and growth. Each fire triggered a specific forest structure trajectory characterized by a different stem density and rate of recovery. 5. Synthesis. This study provides a long-term perspective that helps explain how the present-day landscape diversity in the Eastern Canadian taiga reflects the site-specific fire history over the last millennium. Fires have caused persistent and cumulative impacts resulting in a progressive opening of the forest cover along with balsam fir exclusion. Present-day landscapes are mosaics of forest stands characterized by different times since fire and different post-fire forest structure trajectories.

Macroinvertebrates on coarse woody debris in the littoral zone of a boreal lake

Logging activity was a regular practice in the boreal forest of Quebec during the 19th century and may have had an impact on the temporal dynamics of aquatic coarse woody debris (CWD) and associated organisms. The dynam- ics of white cedar (Thuja occidentalis) CWD inputs from the riparian environment in a boreal lake in Eastern Quebec, Canada, over the past 350 years were reconstructed and differences in the macroinvertebate communities according to CWD age, season of sampling (spring, summer and autumn), depth and site were investigated. It was hypothesised that CWD macroinvertebrate community structure would change with CWD age, season and depth, but not among sites. No significant correlation was found between CWD age and macroinvertebrate densities and taxa number. The macroinverte- brate community was highly variable in space and time. Season was the main factor influencing taxa composition and the relative densities of individuals. The mean density was more than twofold greater in autumn than in spring and summer (1046, 1049 and 2335 individuals m −2 in spring, summer and autumn respectively). Density and taxa number decreased with depth, but site did not appear to influence the community. As CWD inputs increased during the log-driving period, impacts on macroinvertebrate communities were likely to be important and should be documented across the boreal zone.

Late Holocene Climate of the James Bay Area, Quebec, Canada, Reconstructed Using Fossil Beetles

We used fossil beetles recovered from three peatlands of the James Bay area (Radisson, northern Quebec, Canada), and the mutual climatic range (MCR) method, to produce a quantitative reconstruction of summer temperatures during the last 5000 yr. Our main objective was to test the hypothesis that a significant climatic cooling episode occurred during the late Holocene epoch, leading to the opening up of the boreal forest, and to the progressive replacement of black spruce (Picea mariana) forests by jack pine (Pinus banksiana) stands. Beetle assemblages in the Radisson area were very stable during the late Holocene epoch. We used a total of 22 beetle species to conduct the MCR analyses. Reconstructed mean July temperatures ranged from 14.5 to 17.5°C, which is close to the present-day (A.D. 1977–1996) temperature range found in the Radisson area. The general stability of beetle assemblages, and the MCR analyses, suggest that there was no major change in July temperatures during the last 5000 yr in the Radisson area. This study provides no evidence that the progressive replacement of black spruce forests by jack pine stands in the Radisson area during the last 2700 yr can be explained by a longterm climatic cooling episode affecting the postfire regeneration potential of trees. Consequently, an alternative hypothesis (more frequent fires favoring jack pine) should be seriously considered to explain the late Holocene changes in the forest environment of the Radisson area.