SEPTEMBER/OCTOBER 2005, VOL. 81, No. 5 ��� THE FORESTRY CHRONICLE 675 Climate change impacts on drought-prone forests in western Canada by E.H. (Ted) Hogg1 and Pierre Y. Bernier2 ABSTRACT From a climate change perspective, much of the recent international focus on forests has been on their role in taking up carbon dioxide (CO2) from the atmosphere. The question of climate change impacts on forest productivity is also emerg- ing as a critical issue, especially in drought-prone regions such as the western Canadian interior. Because of the complex- ity of interacting factors, there is uncertainty even in predicting the direction of change in the productivity of Canada���s forests as a whole over the next century. In the most climatically vulnerable regions, however, successful adaptation may require more innovative approaches to forest management, coupled with an enhanced capacity for early detection of large-scale changes in forest productivity, dieback and regeneration. Key words: climate change, boreal forest, productivity, drought, impacts, adaptation R��SUM�� Dans le cadre des changements climatiques, la majeure partie de l���attention internationale r��cente sur les for��ts a port�� sur leurs r��les d���emmagasiner le dioxyde de carbone (CO2) en provenance de l���atmosph��re. La question des effets des changements climatiques sur la productivit�� foresti��re constitue ��galement un enjeu majeur, surtout dans les r��gions sujettes �� la s��cheresse comme la partie int��rieure de l���Ouest canadien. �� cause de la complexit�� des facteurs en interac- tion, il existe une incertitude m��me dans la pr��diction de la direction des changements dans la productivit�� des for��ts canadiennes dans son ensemble au cours du prochain si��cle. Dans le cas des r��gions les plus susceptibles en terme clima- tique, une adaptation r��ussie pourrait n��cessiter des approches plus innovatrices en mati��re d���am��nagement forestier, associ��es �� une capacit�� accrue de d��tection h��tive des changements �� grandes ��chelles dans la productivit�� foresti��res, le d��p��rissement et la r��g��n��ration. Mots cl��s : changements climatiques, for��t bor��ale, productivit��, s��cheresse, effets, adaptation 1Natural Resources Canada, Canadian Forest Service, Northern Forestry Centre, 5320���122 Street, Edmonton, Alberta T6H 3S5. E-mail: thogg@nrcan.gc.ca (corresponding author) 2Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, C.P. 3800, Ste-Foy, Quebec G1V 4C7. E-mail: pbernier@rncan.gc.ca E.H. (Ted) Hogg Pierre Y. Bernier underway. According to the US National Climatic Data Center (2005), the five warmest years on record to date were 1998, 2002, 2003, 2004 and 2001, when average global land temperatures were about 0.8��� 1.0o C above the long-term normal since 1880. The rate of warming has been even greater in northern, continental regions such as the western Canadian boreal forest, where temperatures have increased nearly 2o C since the late 1940s (Environment Canada 2004). This warming trend has led to a rapid thawing of permafrost (Camill 2005), along with a recent tendency for shorter peri- ods of winter road access into remote, forested areas. One of the major challenges for the future management and conservation of forests in this region is that continued warming is expected to lead to drier conditions and more severe droughts in the future. Such changes would likely lead to an increase in the area of forests affected by natural distur- bances, notably fire and insects (Volney and Hirsch 2005). Equally important, however, is the potential for direct impacts of climate change on forest productivity, health and regeneration. This issue is emerging as a critical concern, especially across western Canada where record dry conditions Introduction The earth���s climate is expected to warm by an average of 1.4o to 5.8o C during this century, mainly because of human- caused increases in the levels of CO2 and other greenhouse gases in the atmosphere (Houghton et al. 2001). There is growing evidence that significant global warming is already

during 2001���2003 have led to widespread forest dieback and mortality (Hogg et al. 2005). Several reviews have been con- ducted on the topic of climate change impacts on the forest sector at the global scale (e.g., McCarthy et al. 2001) and with- in specific regions of North America (e.g., Papadopol 2000, Shugart et al. 2003). The Canada Country Study provided regional and national reviews on the potential impacts of cli- mate change and variability on several sectors of Canadian society, including the forest sector (e.g., Saporta et al. 1998). The present paper focuses more specifically on the climate change impacts that may be already be leading to large-scale changes in the western Canadian interior. The objectives of this paper are 1) to provide an overview of the global science issues relating to forests and climate change, 2) to examine the main climatic factors affecting for- est growth and ecosystem functioning in the western Canadian interior, 3) to apply a simple model for identifying the most climatically sensitive areas, and 4) to present some preliminary forest management options that might be con- sidered for adapting to the projected climatic changes in this region over the next few decades. Forests And Climate Change: The Major Science Issues Two major science issues can be identified under the general topic of forests and climate change. The first issue concerns the role of forests in global cycles of carbon, water and ener- gy. This includes the question of how much carbon dioxide (CO2) is absorbed or released by the world���s forests, including the effects of forest management and land use changes on the earth���s carbon cycle, which can ultimately influence the rate of long-term global warming. Internationally, much of the for- est-related climate change research to date has been focused on this general question (e.g., Kurz and Apps 1999, Caspersen et al. 2000) because of its relevance to the attainment of CO2 emission targets set out under the Kyoto Protocol (Watson et al. 2000). A related question is the effect of forests on region- al weather and climate. In northern areas, the presence of coniferous forests on snow-covered landscapes leads to warmer air temperatures during late winter and spring, as they absorb much more of the sun���s energy compared to the surrounding white, treeless areas. In fact, this albedo effect could significantly decrease the benefit of afforestation of treeless lands for climate change mitigation (Betts 2000). On the other hand, deciduous forests such as aspen can act as ���giant green humidifiers��� after spring leaf-out, as they release large amounts of water vapour into the atmosphere, cooling the air and triggering more summer rainfall in continental areas (Hogg et al. 2000). Increasingly, these effects are being included in the newer versions of global climate models, because they could be just as important as rising CO2 levels in determining the rate of future climate change in response to changes in vegetation and land use (Pielke et al. 1998). The second major science issue, that forms the main focus of the present paper, is the potential impact of climate change on forest growth and ecosystem functioning, including natu- ral disturbances such as forest fire, insects and diseases. Despite scientific and technological advances in simulation modelling and remote sensing, great uncertainty remains even in predicting the direction of change in the potential productivity of the world���s northern forests. Part of this uncertainty arises from the difficulty in determining how the effects of positive and negative factors will balance in a given geographic area, as illustrated diagrammatically in Fig. 1. On the one hand, climatic warming may be expected to benefit the productivity of forests, especially in cold climatic regions where the length of the growing season is limited by low temperatures. For example, warmer springs lead to earli- er leaf-out of deciduous trees such as aspen, which leads to increased CO2 uptake and productivity (Barr et al. 2004). Further benefits may be expected owing to the role of CO2 in the photosynthesis of trees and other plants: as atmospheric CO2 concentrations increase, forest growth will tend to increase due to ���CO2 fertilization,���if other factors are not lim- iting. It has also been suggested that large-scale forest produc- tivity has been significantly enhanced by human-induced increases in nitrogen (N) deposition (Chen et al. 2000), although such an increase has not been detected in the analy- sis of plot re-measurement data in the United States (Caspersen et al. 2000). The inclusion of these positive effects in global models has led to projections of increasing forest productivity and carbon sequestration over the next century, especially in northern forests (e.g., Cao and Woodward 1998). In a survey of 11 leading ecologists, most believed that stand- ing biomass of northern forests was likely to increase under future climate change with a doubling of CO2 levels (Morgan et al. 2001). A global analysis of remote sensing measure- ments from 1981 to 1999 suggested that the global warming trend during this period had led to an overall ���greening��� of most northern areas, especially in Eurasia (Zhou et al. 2001). On the other hand, however, there are a number of poten- tially negative factors, whose impacts are often difficult to predict. For example, recent results from an outdoor field experiment in the northern U.S. showed that the expected benefits of elevated CO2 on aspen growth were negated when 676 SEPTEMBRE/OCTOBRE 2005, VOL. 81, No. 5 ��� THE FORESTRY CHRONICLE Fig. 1. Diagram summarizing major factors likely to affect produc- tivity and biomass of northern forests under global climate change.