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Springer Old Growth Forests - Chapter 13

Chapter 13Old-Growth Forests in the Canadian Boreal: the Exception Rather than the Rule? Fire is one of the most important ecological processes in North American boreal forests (Johnson 1992; Payette 1992). Forest fire regimes, defined by fire frequency, size, intensity, seasonality | Chapter 13 Old-Growth Forests in the Canadian Boreal the Exception Rather than the Rule Yves Bergeron and Karen A. Harper 13.1 Introduction Fire is one of the most important ecological processes in North American boreal forests Johnson 1992 Payette 1992 . Forest fire regimes defined by fire frequency size intensity seasonality fire type and severity Weber and Flannigan 1997 have a significant influence on many boreal forest attributes. Fire regimes affect the distribution of species Asselin et al. 2003 Le Goff and Sirois 2004 age-class distribution of stands Bergeron et al. 2001 characteristics of wildlife habitats Thompson et al. 1998 vulnerability of forests to insect epidemics Bergeron and Leduc 1998 and net primary productivity and carbon balance Peng and Apps 2000 Wirth et al. 2002 . Our understanding of the fire regimes that burn forests throughout the Canadian boreal zone is still fragmentary making it inappropriate to generalise about fire frequency for the entire region. For example it has often been assumed that large-scale fires that produce even-aged stands are not only omnipresent but frequent in boreal forests. However it has become increasingly evident that short fire cycles apply only to parts of the boreal forest and that the regional situation is considerably more complex Bergeron et al. 2004 . Nonetheless the assumption of frequent large-scale fires has been used to justify the use of clear-cut harvesting with short rotations in most boreal forests resulting in a reduction in the proportion of older forest stands. One important consequence of the variability in fire frequency in the boreal zone is the amount of forests that can reach the status of old-growth forests between fire events. As the time needed to reach old-growth is difficult to define see Chap. 2 by Wirth et al. this volume we adopt a pragmatic definition and consider forests over 100 years after disturbance as old-growth. The post-fire cohort of trees is usually no longer dominant .