Assessing Historical Hydro-Climatic Changes in Boreal Quebec
Activity Rationale
Hydro- Québec has been experiencing low levels in its reservoirs over recent decades. Climate change is expected to increase the frequency and severity of extreme events and affect water regimes, which represents a major concern to decision makers. This activity assesses hydrologic and climatic variability over recent centuries using tree ring analysis to facilitate improved prediction of water supply in the main area of hydropower production in Québec
Leader: Christian Bégin
The Topic
A complicated mix of factors related to climate influence tree growth such as the amount and intensity of sunlight, the level of soil moisture, and the temperatures of both the soil and the air. Throughout the growing season, all of these factors combine to affect the width of each annual growth ring, as well as the size and density of the cells within each one. Furthermore, the growth of a tree can also be influenced by the climate conditions outside the growing season and in previous years. A number of other factors, unrelated to climate conditions, also impact tree growth, including competition with other plants for nutrients, the age of the tree, and insect attacks, which must be separated out in order to isolate climate information from tree ring analysis.
Tree ring analysis involves obtaining small-diameter radial cores or complete disks from which tree ring width measurements can be taken. The patterns of tree rings can be aligned for multiple trees in an area to produce a chronology. If preserved dead trees can be obtained, it is possible to overlap a series of tree ring measurements and extend the chronology backwards in time. Statistical comparisons of tree ring chronologies and modern climate records allow for the development of equations that can be used to reconstruct historical climate conditions from tree rings (dendroclimatology).
Stable isotope analysis of tree ring samples at the GSC-Québec Delta-Lab
Isotope dendroclimatology is a relatively new area of research which involves the examination of the stable isotope (carbon, oxygen, hydrogen) composition of wood samples. Stable isotope ratios in tree rings are known to be very sensitive indicators of climate conditions because of the direct influence of hydro-climatic factors on physiologic processes which govern the isotopic fractionation occurring when basis elements (C, O, and H) are assimilated by trees (e.g. through stomatal functions).
Dendroisotopic reconstruction of hydroclimatic conditions over the past 200 years in the James Bay hydropower region
Robert-Bourassa Reservoir spillway and dam (partially visible). Behind, the westernmost part of the reservoir covering 2,835 km2 in total (photo courtesy of C. Bégin)
Issues of water supply are among the most critical challenges for Canadian society in adapting to climate change. In Québec, for example, 93% of power production by Hydro-Québec is from hydro sources and nearly half of this comes from installations in the James Bay area, along La Grande River, in northern boreal Québec. Hydro-Québec concerns for sustainable management of water supply are becoming particularly important because of the low levels in the reservoirs over recent decades combined with projected climate change which may result in precipitation changes in northern regions. The hydrological models used to predict future trends in water balance are based on hydro-meteorological records generally no longer than the last 40 years, which don't provide adequate data on long-term natural variability in the hydro-climatic system. To develop robust calibration of their models, water managers therefore need information from natural archives to extend the hydro-climatic record.
Results
Preparation of wood samples for isotopic analysis at the GSC- Québec Tree Ring Lab
Carbon and oxygen isotope analysis covering the period of 1800-2004 were produced for three old-growth black spruce stands from the upstream region of La Grande river basin (a map of sample sites is found below). The isotopic results for oxygen-18 showed strong similarities for all sample sites, indicating that the record of oxygen isotopes in tree rings is reflective of the region in which the trees are located. For carbon-13, slight differences between trees and sample sites indicate that site-specific conditions influence the partitioning of this isotope; however, similarities over the long term indicate that regional conditions also play an important role.
Statistical tests revealed that oxygen-18 values directly reflect summer temperatures, and correlate inversely with water inflow during July-November. Carbon-13 values reflect late summer maximum temperatures and link summer temperature and precipitation effects.
Oxygen-18 and carbon-13 values are sensitive to climatic variables that are linked in typical subarctic climates. Therefore, the patterns of their joint responses show a high sensitivity to climatic parameters. The correlations between combined carbon/oxygen isotopic values and hydro-climatic parameters confirm the use of dendroisotopes (the isotopes in trees) as a proxy indicator of climate, and open the possibility of using this type of analysis to reconstruct past climatic conditions.
The figure below shows the preliminary reconstruction of summer maximum temperatures (June-August) for the upstream region of La Grande river basin for the period 1800-2004. The reconstructed values (blue line) indicate that the first half of the 19th century was the coldest in the past 200 years (1824-1825 was the coldest). Climate became milder during the next 34 years (1846-1880), then colder afterward until 1936. Temperatures increased in the early 1940’s and oscillated around the mean value until 1987. The rest of the series is marked by a gradual and steady increase of summer temperatures.
Preliminary reconstruction of summer maximum temperatures based on carbon and oxygen stable isotopes ratios in tree rings.
Map of the Québec boreal and subarctic sectors showing forest sites sampled for tree ring studies. White filled stars show sites selected for dendroisotopic analysis.
Publications
Please note that subscriptions may be required to access some articles. To request a copy of publications, or for any more information, please contact Christian Bégin.
- "Check for more recent publications in GEOSCAN, the publications database of the Geological Survey of Canada and the Canada Centre for Remote Sensing.
Bégin, Y., Nicault, A., Bégin, C., Savard, M.M., Arseneault, D., Berninger, F., Guiot, J., Boreux, J.-J. et Perreault, L., 2007: Analyse dendrochronologique des variations passées du régime hydro-climatique au complexe de la grande rivière dans le Nord du Québec. La Houille Blanche, N°6 (Décembre 2007), pp. 70-77
Gingras, M., Bégin, C., Savard, M.M., Marion, J. & Smirnoff, A. (in translation): Carbon and Oxygen stable isotopes in trees rings as proxies for climate and hydrology in mid-northern Québec, Canada. To be submitted to Climate Change.
Berninger, F., Nicault, A., Savva, Y., Begin, C., Dunn, A., Savard, M.M., and Bégin, Y. (submitted): Winter climate drives growth of black spruce in large areas of North-Eastern Canada. Global Change Biology (GCB-09-0095)
Links
Trees as Indicators of Past Climate – Dr. Keith Briffa
Delta Laboratory- GSC’s Stable isotope geochemistry laboratory