Ecosystem Management in the Boreal Forest

Chapter 4 : Climate, Weather, and Forest Fires

Chapter 4Climate, Weather, and Forest Fires Martin P. Girardin, Mike D. Flannigan, Jacques C. Tardif, and Yves Bergeron  We thank the Sustainable Forest Management Network, the Fonds québécois de la recherche sur la nature et les technologies (FQRNT), and the Canadian Forest Service for funding the work done by Mr. Girardin. We also thank Pamela Cheers and Isabelle Lamarre of Natural Resources Canada for editing the manuscript. The photos on this page were graciously provided by Brian Stocks (Canadian Forest Service) and Danielle Charron. 1. Introduction..................................................................................... 81 2. Monitoring Fire Weather. ................................................................ 83 2.1. Surface Weather........................................................................ 83 2.2. Fire and the Upper Air Level...................................................... 85 2.2.1. Atmospheric Circulation. ................................................ 85 2.2.2. Vertical Structure of the Atmosphere............................. 87 2.2.3. Influence of the Oceans................................................. 89 3. Fire and Climate. .............................................................................. 91 3.1. The Recent Period (1920 to Present)......................................... 91 3.2. Fire and Climate of the Past: Contributions from Tree Rings. ..... 92 3.3. Reconstruction of Past Fire Activity............................................ 96 4. Conclusion. ....................................................................................... 98 References............................................................................................. 99 1. Introduction The past decades has seen an increasing interest in forest management based on historical or natural disturbance dynamics. The rationale is that management that favours landscape compositions and stand structures similar to those found historically should also maintain biodiversity and essential ecological functions (see Gauthier et al., chapter 1). For instance, in fire-dominated landscapes, a substitution of fire by even-aged forest management could occur without elevating the overall frequency of disturbance or affecting forest ecosystem functioning . This approach is feasible only if current and future fire activities are sufficiently low compared with pre-industrial fire activity (see Gauthier et al., chapter 3; Le Goff et al., chapter 5). In the advent of greater fire activity, other adaptation options exist to assist forest management to cope with new constraints and their consequences (see Le Goff et al., chapter 5). Reconstructing past, understanding current, and forecasting future fire-conducive climate and fire activities is thus of central importance for effective implementation of ecosystem management. The development and implementation of strategies appropriate for ecosystem management require a thorough understanding of the relationship between climate and disturbances, their features (past and present) and a better anticipation of their future development, particularly with respect to forest fires. Wildfire is a primary natural process organizing the physical and biological attributes of the forest, shaping landscape diversity and influencing biogeochemical cycles (Weber and Flannigan 1997; Bourgeau-Chavez et al. 2000). The mosaics of different vegetation types are to a large extent an expression of their respective fire regimes and many boreal tree species show adaptation to fire. Fire activity responds rapidly to changes in weather and climate in comparison to vegetation − the rate and magnitude of fire-regime-induced changes to the boreal forest landscape can greatly exceed anything expected from climate change alone (Weber and Flannigan 1997). Weather consists of short-term (minutes to days) variations in the atmosphere . Usually, weather is thought of in terms of temperature, humidity, precipitation , cloudiness, visibility, and wind (American Meteorological Society 2000). Weather influences daily fire characteristics because of its impact on fuel moisture and the effects of precipitation (particularly its frequency), relative humidity, air temperature, wind speed, and lightning (Flannigan and Harrington 1988; Flannigan and Van Wagner 1991; Harrington et al. 1991; Johnson 1992; Agee 1997; Weber and Flannigan 1997; Bergeron et al. 2001). Weather is particularly important as a control on the occurrence of forest fire; fires spread rapidly when the fuels are dry and the weather conditions are warm, dry, and windy. Despite the increasing importance of human activity as a source of fire ignition over the last few decades (Stocks et al. 2003), dry forest fuels and wind remain critical factors influencing the occurrence of large stand-replacing fires (e.g. Johnson et al. 1990; Masters 1990; Johnson 1992; Westerling et al. 2006). Nearly 81% of all burned area in Canada is caused by lightning-ignited forest fires. 82 Ecosystem Management in the Boreal Forest As distinguished from weather, climate is typically characterized in terms of suitable averages of the climate system over periods of a month or more, taking into consideration the variability in time of these averaged quantities (American Meteorological Society 2000). Climate is not constant, but rather consists of warming and cooling cycles at intervals of several decades. As climate varies, the corresponding weather variables can vary in magnitude and direction . As such, climate change can...