1FEBRUARY 2009 S T . G E O R G E E T A L . 689 The Tree-Ring Record of Drought on the Canadian Prairiesa,b SCOTT ST.GEORGE,c DAVID M. MEKO,d MARTIN-PHILIPPE GIRARDIN,e GLEN M. MACDONALD,f ERIK NIELSEN,g,l GREG T. PEDERSON,h DAVID J. SAUCHYN,i JACQUES C. TARDIF,j AND EMMA WATSONk c Geological Survey of Canada, Natural Resources Canada, Ottawa, Ontario, Canada d Laboratory of Tree-Ring Research, The University of Arizona, Tucson, Arizona e Canadian Forest Service, Natural Resources Canada, Ottawa, Ontario, Canada f Department of Geography, University of California, Los Angeles, Los Angeles, California g Manitoba Geological Survey, Winnipeg, Manitoba, Canada h Big Sky Institute, Montana State University, Bozeman, Montana i Prairie Adaptation Research Collaborative, and Department of Geography, University of Regina, Regina, Saskatchewan, Canada j Centre for Forest Interdisciplinary Research, University of Winnipeg, Winnipeg, Manitoba, Canada k Toronto, Ontario, Canada (Manuscript received 22 January 2008, in final form 15 July 2008) ABSTRACT Ring-width data from 138 sites in the Canadian Prairie Provinces and adjacent regions are used to estimate summer drought severity during the past several hundred years. The network was divided into five regional groups based on geography, tree species, and length of record: the eastern Rockies, northern Saskatchewan, central Manitoba, southern Manitoba, and northwestern Ontario. Regional tree-ring records are primarily related to summer moisture and drought conditions, and are less responsive to droughts caused by deficits in winter precipitation. These summer-sensitive data are not linearly related to major modes of climate variability, including ENSO and the Pacific decadal oscillation (PDO), which primarily affect the climate of western Canada during winter. Extended drought records inferred from tree rings indicate that drought on the Canadian Prairies has exhibited considerable spatial heterogeneity over the last several centuries. For northern Saskatchewan and northwestern Ontario, intervals of persistently low tree growth during the twentieth century were just as long as or longer than low-growth intervals in the eighteenth or nineteenth centuries. Longer records from southern Alberta suggest that the most intense dry spell in that area during the last 500 yr occurred during the 1720s. At the eastern side of the prairies, the longest dry event is centered around 1700 and may coincide with low lake stands in Manitoba, Minnesota, and North Dakota. Although the Canadian Prairies were dry at times during the 1500s, there is no regional analog to the sixteenth-century ‘‘megadroughts’’ that affected much of the western United States and northern Mexico. 1. Introduction of trees) from individual locations (e.g., Sauchyn and Beaudoin 1998; Sauchyn et al. 2003) or multiple chro- Most prior studies using tree-ring data to understand nologies from a relatively small area [100–200-km tran- past drought in the Canadian Prairie Provinces have sects; e.g., Case and MacDonald (1995); St. George and been based on either single chronologies (composite Nielsen (2002); Girardin and Tardif (2005)]. This work time series that represent mean growth within the stand has demonstrated that data from Prairie trees can be an effective proxy for meteorological or hydrological drought, and can be used to develop annually resolved a Supplemental information related to this paper is available drought records that extend back several hundred years at the Journals Online Web site: http://dx.doi.org/10.1175/ (Watson and Luckman 2006, and references therein). 2008JCLI2441.s1. b Earth Sciences Sector Contribution Number 20070491. Despite these encouraging results, making inferences l Retired. about the drought history of the entire Prairies (or even individual regions) from a limited number of records Corresponding author address: Dr. Scott St. George, Geological remains a challenge because tree-ring data often con- Survey of Canada, 601 Booth St., Ottawa, ON K1A 0E8, Canada. tain multiple signals (of varying strengths) created by E-mail: [email protected] different environmental forcings. Synoptic-scale climate DOI: 10.1175/2008JCLI2441.1 Ó 2009 American Meteorological Society Unauthenticated | Downloaded 09/27/21 12:43 AM UTC 690 JOURNAL OF CLIMATE VOLUME 22 FIG. 1. Network of ring-width sites from the Canadian Prairies and adjacent regions. Symbols correspond to regional groups: eastern Rocky Mountains, circles; northern SK, upward trian- gles; central MB, diamonds; southern MB, squares; and northwestern ON, downward triangles. Black circles represent communities mentioned in the text. is usually a major influence on ring width, but tree sion of the North American Drought Atlas (Cook and growth can also be influenced by disturbances (affecting Krusic 2004). As noted by Cook and Krusic (2004), the both single trees and the entire stand) or the local number of moisture-sensitive ring-width chronologies in microclimate (Cook 1985). Because it is not possible Canada has, until recently, been quite modest, and these to separate these influences a priori, even ring-width new collections fill substantial gaps in the previous chronologies developed from climate-sensitive trees can network. Using an extensive (greater than 2 000 000 include nonclimatic signals. The problem of whether or km2), relatively dense, network of chronologies allows not local proxy records can be taken as representative us to focus our analysis on signals recorded by many or for the broader region is complicated further by the all records over large areas. These common signals are inherent spatial heterogeneity of drought; at a given more likely to be related to synoptic-scale influences, time, severe drought conditions may be present over a including regional drought conditions, and should min- small area, but it does not necessarily follow that the imize contributions from local, nonclimatic noise. We entire Prairies must also be affected by drought. Al- describe the spatial and temporal variabilities of ring- though many ring-width chronologies from this region width chronologies from the Canadian prairies, and have been incorporated into the North American Drought examine the seasonality of moisture signals contained Atlas (Cook and Krusic 2004), attempts to extract re- within regional tree-ring series. We also investigate po- gional signals from networks of moisture-sensitive trees tential connections between tree growth and a suite of in Canada have either focused primarily on drought in major climate modes and discuss how the ‘‘climate win- regions adjacent to (and, in some cases, overlapping with) dow’’ (Fritts 1976) recorded by Prairie trees affects their the Canadian Prairie Provinces (Watson and Luckman ability to describe the influence of remote climate forc- 2004; Watson and Luckman 2005; Girardin et al. 2006b), ings. Finally, we examine what tree-ring data tell us about or have targeted other environmental variables (e.g., area the intensity and persistence of drought during the past burned; Girardin 2007). several hundred years as compared to drought observed In this paper, we use 138 ring-width chronologies during the period covered by instrumental records. from the Canadian Prairie Provinces and adjacent re- gions (Fig. 1) to estimate changes in regional drought 2. The Canadian Prairies severity since approximately 1500 A.D. Many of these data are relatively new; almost all records have been Most of the area in this study (approximately 478–588N developed since the mid-1990s, and nearly half were and 908–1188W) is part of the Prairie ecozone (Ecological produced after the publication of the most recent ver- Stratification Working Group 1996), a predominantly Unauthenticated | Downloaded 09/27/21 12:43 AM UTC 1FEBRUARY 2009 S T . G E O R G E E T A L . 691 mixed-grass prairie with a semiarid climate and few trees. [Quercus macrocarpa (Michx.)] from riparian forests}, The prairie ecozone also coincides with the area com- and (v) northwestern Ontario [mainly red pine (Pinus monly described as the Palliser Triangle, named after a resinosa Sol. ex Aiton) and white pine (Pinus strobus nineteenth-century British expedition that mapped the L.)]. The total network includes measurements of territory between Lake Superior and the Pacific coast nearly 5000 samples taken from almost 3000 trees. Be- and assessed its potential for agriculture, mining, and cause of a number of factors (including the scarcity of settlement (Spry 1995). Most ring-width data used in this trees in grassland ecosystems, frequent fires in the bo- studyarederivedfromtreesgrowingalongthemargins real forest, wet conditions that encourage heartrot, and of the prairie ecozone, including sites from the Montane intensive logging), the region contains relatively few Cordillera, Boreal Plains, and Boreal Shield ecozone long-lived trees. Although trees in the foothills of the (Ecological Stratification Working Group 1996). For sim- Rockies can be quite old (the oldest tree, from the Te- plicity, we refer to the broader region (shown in Fig. 1) as ton River valley in Montana, has 846 rings), the median the Canadian Prairies or Prairies. This region also encom- age of trees in the network is only 135 yr. passes the drainage basin for the Nelson–Churchill River system, the third largest watershed in North America. 4. Instrumental climate data Precipitation data for the Canadian Prairie region 3. Tree-ring data were extracted from the Climatic Research Unit (CRU) We assembled a set of regional ring-width data (Fig. TS 2.1 gridded (0.58) dataset of monthly climate ob- 1; appendix A) from the International Tree-Ring Da- servations (Mitchell and Jones 2005) and combined into tabank (information online at http://www.ncdc.noaa. annual (October–September) and seasonal (Decem- gov/paleo/treering.html) and previously published ma- ber–February, DJF; March–April, MAM; June–August, terial (Sauchyn and Beaudoin 1998; St. George and JJA and September–November, SON) totals. We also Nielsen 2002; Watson and Luckman 2002; Case and used gridded Palmer drought severity index (PDSI) MacDonald 2003; Girardin and Tardif 2005; Beriault records from the dataset developed by Dai et al.