Regional tree growth and inferred summer climate in the Winnipeg River basin, Canada, since AD 1783 Scott St. George a,b,c, ⁎ , David M. Meko b , Michael N. Evans b,c a GSC Northern Canada, Geological Survey of Canada, Ottawa, Ontario, Canada b Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ 85721, USA c Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA Received 22 March 2007 Available online 3 July 2008 Abstract A network of 54 ring-width chronologies is used to estimate changes in summer climate within the Winnipeg River basin, Canada, since AD 1783. The basin drains parts of northwestern Ontario, northern Minnesota and southeastern Manitoba, and is a key area for hydroelectric power production. Most chronologies were developed from Pinus resinosa and P. strobus, with a limited number of Thuja occidentalis, Picea glauca and Pinus banksiana. The dominant pattern of regional tree growth can be recovered using only the nine longest chronologies, and is not affected by the method used to remove variability related to age or stand dynamics from individual trees. Tree growth is significantly, but weakly, correlated with both temperature (negatively) and precipitation (positively) during summer. Simulated ring-width chronologies produced by a process model of tree-ring growth exhibit similar relationships with summer climate. High and low growth across the region is associated with cool/wet and warm/dry summers, respectively; this relationship is supported by comparisons with archival records from early 19th century fur- trading posts. The tree-ring record indicates that summer droughts were more persistent in the 19th and late 18th century, but there is no evidence that drought was more extreme prior to the onset of direct monitoring. Crown Copyright © 2008 University of Washington. All rights reserved. Keywords: Canada; Dendrochronology; Northwestern Ontario; Vaganov–Shashkin model; Winnipeg River Introduction The principle of site selection is one of the main tenets of dendroclimatology, and states that tree-ring samples should be collected from sites with characteristics that are likely to maxi- mize the desired environmental signal (Fritts, 1976; Schwein- gruber et al., 1990). Researchers have demonstrated repeatedly that tree-ring records from sites near the limits of the ecological range of a species can be strongly related to a single climate factor. The most prominent examples of this phenomenon are the relationship between trees at the altitudinal or latitudinal treeline and surface temperature during the growing season (e.g., Jacoby et al., 1996; Luckman and Wilson, 2005) and the strong moisture signal recovered from trees in semi-arid environments (e.g., Cook and Krusic, 2004; Woodhouse et al., 2006). In contrast, ring-width records obtained from trees growing near the center of their range usually exhibit a more complex relationship with climate (Hughes, 2002). This complexity does not mitigate the need to obtain information about past climate conditions in these areas. The Winnipeg River in south-central Canada is the largest single source of water used to generate hydroelectricity in the Canadian Prairies. Water managers are interested in under- standing how the hydrology of this and other watersheds in the region has behaved during the last millennium, and have sup- ported a number of independent research projects using several types of environmental proxies (Beriault and Sauchyn, 2006; Laird and Cumming, 2008). This study uses a multi-species network of trees at sites within the Winnipeg River region to examine the nature of the environmental signals recorded in regional tree-ring data. A combination of principal-component and response-function analyses, and forward modeling are used Available online at www.sciencedirect.com Quaternary Research 70 (2008) 158 – 172 www.elsevier.com/locate/yqres ⁎ Corresponding author. Laboratory of Tree-Ring Research, University of Arizona, Tucson, Arizona 85721, USA. Fax: +1 520 621 8229. E-mail address: sstgeorg@nrcan.gc.ca (S. St. George). 0033-5894/$ - see front matter. Crown Copyright © 2008 University of Washington. All rights reserved. doi:10.1016/j.yqres.2008.04.009