Yves Bégin

Dendroecological evidence of lake-level changes during the last three centuries in subarctic Québec

Abstract Dendroecological analysis of black spruces (Picea mariana (Mill.) BSP.) near the shore of Clearwater Lake in the eastern Canadian subarctic yielded information on water level over the past three centuries. Tree positions, growth froms, layering, tree ring patterns, and ice scars provide direct evidence of a major rise in lake level. From the 17th century to the present, trees on the lake shore were progressively submerged by the increasing water level, which reached a maximum in this century. Higher levels began around the mid-18th century when shoreline spruce trees started to lean because extensive wave erosion caused landward shore displacement. High mortality of leaning spruce and abundant ice scar formation occurred during this century. The rise in water level in inferred to have been caused by snowier winters occurring parallel to climatic warming.

Tree-ring response of Populus tremula L. and Quercus robur L. to recent spring floods of the Saône River, France

The effects of floods on the radial growth of Populus tremula L. and Quercus robur L. in past decades were studied on a forested island in the Saone River (France). The study considered two stands ...

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.

CLIMATE AND PICEA MARIANA SEED MATURATION RELATIONSHIPS: A MULTI‐SCALE PERSPECTIVE

One of the most important ways by which northern forests will respond to anticipated climate change is through variations in seed maturation. In this study, the relationship between growing degree-days (DD) >5°C and seed maturity was evaluated at three spatial scales. At the continental scale, the development of female gametophytes and embryos was evaluated as a function of the heat sums obtained from 11 sites distributed across the Canadian range of black spruce. At the regional scale, cone size and the percentage of germinated seeds formed in 1998, 1999, and 2000 were analyzed from seven sites situated along a latitudinal gradient in northern Quebec. At the local scale, cones were collected along perilacustral and insular transects according to their exposure to large water bodies, and from 10 islands located within a 2835-km2 hydroelectric reservoir. Our results confirm the 800–940 DD thermal sum threshold necessary for the complete maturation of black spruce embryos at several populations distributed across the total range of the species. Along the regional south-to-north climatic gradient, the percentage of germination can be predicted by a sigmoid function of a thermal sum (y = 2.8 + 25.1/[1 + e(x−896.6)/84.6]; r2 = 0.85, P < 0.0001) that attains a plateau at around 800–940 DD. Once the 800–940 DD threshold is attained, variations in the percentage of seed germination are mainly associated with inter-tree differences and local site factors (thickness of organic matter, tree density, tree height, tree age, and fetch). In the springtime, cold enclaves are created by the presence of the hydroelectric reservoir. The seed germination percentages in these enclaves varied from 0.6% ± 0.7% to 14.9% ± 19.1% (mean ± SD) according to the site (compared to 22.7% ± 15.1% for a site not exposed to the reservoir), which was equivalent to the germination percentages for sites at latitudes 1–3° farther north. These data suggest that the potential for black spruce regeneration increases strongly beyond the 800 DD isotherm, which evokes the possibility that subarctic open forests may become more dense under the current anticipated climate changes.

1300-year tree-ring width and density series based on living, dead and subfossil black spruce at tree-line in Subarctic Quebec, Canada

Living, dead and subfossil trees of black spiruce (Picea mariina [Mill.] BSP were used to build a 1300-year chronology based on ring width and wood density. All density sariables (maximcum, minimum, earlywood and latewood densities) among the three types of trees were simlilar, whereas ring width was significantly Higher in liviing trees thani in deacd and subfossil trees. Correlition of the indexed series from liviing and dead trees and from dead and subfossil trees that grew during the samiie periods were higher for Maximum density (r = 0.70, 0.63) and mean latewood density (r = 0.65, 0.66) than for Minimum density (r = 0.16. 0.35 and ring with (r=0.15, 0.49). respctively. Maximum density and mean latewood density were significantly correlated with all temperature variables: mean annual (January-December) and growing season (May September) temperatures, sum of degree days and frost-free days. Accordingly, Maximum and latewood density in tree-rings of spruce stem at tree-linec can be conisidered as a function Of summner-temnperature distributions and different types of trees can be combined for the reconstruction of long-term climatic trends due to theii synchronous variations.

A quantitative definition of light rings in black spruce (Picea mariana) at the arctic treeline in northern Québec, Canada.

Light rings in black spruce (Picea mariana [Mill.] BSP.) at the arctic treeline are characterized by pale-colored latewood made of a single or very few latewoodcell layers with thin-walled cells. Their widespread occurrence and their high frequency greatly facilitate the cross-dating procedure in dendrochronological studies. In this study, black spruce tree-ring density and wood structure were analyzed for light ring characteristics along with the mechanism of their formation according to ambient temperature. Light rings were quantitatively categorized into three classes based on the maximum tree-ring density using a normalized standard distribution. A light-ring chronology was established according to this classification. The results indicate that the grade of light ring was positively related to the frequency of light rings obtained from visual light-ring chronologies. The following anatomic variables were examined: number of cell layers of latewood, number of cells of the whole ring, percentage of latewood in the total ring width, and mean latewood cell-wall thickness. Among these anatomic variables, the mean latewood cell-wall thickness represents the best quantitative descriptor of a typical light ring as recognized by optical examination. The main causal factors of light rings are insufficient length of the growing season or cool summers.

Ice-push disturbances in high-Boreal and Subarctic lakeshore ecosystems since AD 1830, northern Québec, Canada.

Ice scars on lakeshore trees were surveyed on the islands of two large lakes in northern Québec: Clearwater Lake (1270 km2) and Bienville Lake (900 km2), respectively at the southern edge of the Subarctic and at the northern limit of the Boreal zones. Correspondence of ice-scar chronologies with hydrological and climatological instrumental registers indicated that shore ice pushes are due to lake floods. In the AD 1930s, a shift in the flood regimes occurred. In the high Boreal, ice-push activity was much more frequent prior to 1930 than in the Subarctic. Major regional ice pushes occurred in AD 1854, 1903, 1914, 1936, 1947, 1954,1959-60, 1970 and 1979. Prior to 1930, local major events were concentrated south of the Subarctic zone, but the situation inverted after 1930. A northward shift in the average position of the Arctic front is postulated as having been the driving force of local hydrologic regimes that allowed ice disturbances to occur, especially A in controlling the amount of snowfall. Ice scars provide proxy indicators of an increase in the frequency of snowy winters between the mid-1930s and 1980. The recent period of low levels represents an anomalous incursion in the century trend, but equivalent long episodes of low seasonal lake levels occurred prior to 1930.

A recent downward expansion of shoreline shrubs at Lake Bienville (subarctic Quebec)

Abstract The regeneration patterns of Alnus crispa (Air.) Pursh. in response to water level lowerings were studied along the shores of Lake Bienville (898 km 2 ) in subarctic Quebec. During a period of high waters in the 1970s, pruning by drifting ice was important and contributed to sprouting and crown densification of well-established shrub individuals. Low water levels prevailed between 1980 and 1985 when massive shrub colonization along the lakeshores occurred. Shrub establishment was favored on shorelines where geomorphic activity (ice pushes and wave action) was moderate or low. Well-established shrubs have undergone moderate ice-push damages since 1985, which limit the expansion of the population by seeds, but favor the densification of the stem metapopulations. Interannual and decennial fluctuations in water levels of northern basins should be studied more closely to better understand hydroclimatic regimes and causes of periods of low hydraulicity, especially in regions where the potential for hydroelectricity is high.

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.

Effects of vegetation zones and climatic changes on fire-induced atmospheric carbon emissions: a model based on paleodata

An original method is proposed for estimating past carbon emissions from fires in order to understand long-term changes in the biomass burning that, together with vegetation cover, act on the global carbon cycle and climate. The past carbon release resulting from paleo-fires during the Holocene is examined using a simple linear model between measured carbon emissions from modern fires and sedimentary charcoal records of biomass burning within boreal and cold temperate forests in eastern Canada (Quebec, Ontario). Direct carbon emissions are estimated for each ecozone for the present period and the fire anomaly per kilo annum (ka) v. present day (0 ka) deduced from charcoal series of 46 lakes and peats. Over the postglacial, the Taiga Shield ecozone does not match the pattern of fire history and carbon release of Boreal Shield, Atlantic Maritime, and Mixedwood Plains ecozones. This feature results from different air mass influences and the timing of vegetation dynamics. Our estimations show, first, that the contribution of the Mixedwood Plains and the Atlantic Maritime ecozones on the total carbon emissions by fires remains negligible compared with the Boreal Shield. Second, the Taiga Shield plays a key role by maintaining important carbon emissions, given it is today a lower contributor.