Dendroclimatology
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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 first obtaining small-diameter radial cores or complete disk 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-ring data.
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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-ring 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. stomatal functions).
Dendroisotopic reconstruction of hydroclimatic conditions over the past 200 years in the James Bay hydropower region
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Issues of water supply are among the most critical challenges for Canadian society in adapting to climate change. In Quebec, for example, 93% of power production by Hydro-Québec (HQ) 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. HQ 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.
The principal objective of this activity is to quantitatively reconstruct, over the last two centuries, hydroclimatic variables required for hydrologic forecast models using dendroisotopic analysis as a basis for improved prediction of water supply in the main area of hydropower production in Québec.
Methods include the following steps:
- Reconnaissance of the region and selection of old growth sites saved from fire and in a variety of drainage situations; 4 dendroisotopic sites overall for the project.
- Description and dendrochronological sampling of these sites.
- Preparation of samples and dendrochronological analyses; 20 trees per site.
- Analyses of isotopic ratios for carbon and oxygen in the tree-ring cellulose; 4 trees per site – 520 analyses for each isotope (Delta-Lab; GSC-Quebec). Analysis was conducted biannually for the years 1800-1940, and annually for the years 1941-2004.
- Data treatment and statistical analysis; comparison of isotopic data with other natural climate indicators available for the region.
- Comparison of isotopic values to hydroclimatic data (response functions) to determine the hydroclimatic variables that influence the isotopic signatures as well as other tree-ring parameters.
- Production of hydroclimatic reconstructions using multiple regression models (transfer functions) and validation with independent climate series.
Isotopic results are available for two sites in the upstream sector of the La Grande River basin. Statistical tests show that variations in both isotopic ratios (13C and d18O) are mainly controlled by summer maximum temperatures but also reflect a summer climatic index integrating temperature and precipitation effects. However, detailed analysis has shown that 13C variations are highly governed by factors related to water regime (e.g. water supply, vapour pressure deficit).
Multiple regression models coupled with a bootstrap verification procedure were applied to isotopic data to reconstruct targeted hydro-climatic variables over the last 200 years. Finally, the reliability of the reconstructed series was established by comparing it to published data, and it was confirmed that dendroisotopes could potentially be very useful proxies of historical hydroclimatic conditions.;