Natural Elements, NRCan's Monthly Newsletter
Tree Rings Tell Story of Ancient Droughts
<Samples mounted in wood blocks>Evidence of past Prairie droughts are recorded in the pattern of wide and narrow rings.
Most people think of the notorious “Dust Bowl” of the 1930s as the worst drought in North America. But new research using tree ring data shows that earlier droughts on the Canadian Prairies were longer and more severe.
Specifically, the study found that the most intense drought in southern Alberta since A.D. 1500 occurred in the early 1700s and that the longest dry spell within the past 500 years there occurred during the 1850s and 1860s. The findings will appear in the February 1 issue of the Journal of Climate.
These discoveries are of great interest and importance since they provide a much clearer picture of long-term drought trends.
“On the Prairies, the earliest records of rain and snow measurements began in the 1880s,” explains Dr. Scott St. George, a dendrochronologist for Natural Resources Canada’s (NRCan) Geological Survey of Canada (GSC) and team leader for the project. “So records from ancient trees and other natural archives help us figure out what happened before we started keeping weather records.”
This new information is of great practical value. Tree rings provide a geological perspective on past climate change and teach us that our direct experience with historical droughts can be inadequate. Plans for the future need to consider droughts that occurred in the past because “if it happened before, it can happen again,” says Scott.
<Young man crouched over tree stump> Reading the rings of an ancient red pine in Quetico Provincial Park, northwestern Ontario.
Understanding the severity, duration and extent of past droughts helps both government and private sector agencies develop more accurate planning and practical responses for risk management. In the critical area of water management, hydro power companies, for example, can use this information to design systems that will maintain minimum energy supplies even under the most severe drought conditions.
The long-hidden story of the rhythms of drought is a fascinating one. But how is it now being told by ancient trees? That’s because the same conditions that cause droughts also affect the growth of trees, which record the history of the climate that nurtures them in their rings. Reading this history and using it to establish dates on the basis of tree-ring growth is a specialized field of scientific study — dendrochronology.
The effects of drought on tree growth are relatively straightforward. Low water levels usually mean less moisture in the soil and less water for vegetation. Trees respond to this reduced supply by forming narrow rings and also show changes in the density and chemistry of their wood.
The research team based their analysis on tree-ring data collected over the last 15 years from almost three thousand trees growing between Lake Superior and the Canadian Rockies. The oldest living tree in the group is a limber pine from northern Montana that started growing in A.D. 1163. As well as from live trees, tree-ring samples were collected from standing dead trees, buried stumps and lumber used in old buildings.
The project was funded by NRCan and several other bodies, including Manitoba Hydro, the Natural Sciences and Engineering Research Council of Canada, the Manitoba Geological Survey and the Prairie Adaptation Research Collaborative Fund.
You can find out more on the project and its findings in the online edition of the Journal of Climate article. For more information on the Geological Survey of Canada, Canada's premier agency for geoscientific information and research, visit the GSC Web site.