PARENT SESSION
Symposium #13: Fire suppression impacts in crown fire ecosystems.
Sponsored by ESA Vegetation Section
Organized by: J. E. Keeley and E. A. Johnson.
Wednesday, August 8, 2001. 8:00 AM to 12:00 PM. Lecture Hall
A facultative crown fire regime tuned down by culture? 700 years of change in boreal Sweden.
GRANSTRÖM, ANDERS1, NIKLASSON, MATS2, 1 2
ABSTRACT- It is frequently stated that the North European boreal forests burn in surface fires, and that large-scale stand replacing fires, comparable to those in boreal North America, do not occur. This assumption is based primarily on the widespread occurrence of forest with multiply-scarred Pinus sylvestris, testifying to sub-lethal fires in the past. There are, however, reasons to be cautious in generalizing from the dendro-record. First, the record is highly biased towards pine-dominated sites. Second, most data published so far cover only the last few hundred years, a period when anthropogenic fire has been dominant over lightning-ignited fires. Here we argue that stand-replacing fires may have been regionally and locally more prevalent in a "natural" fire regime, than the records seem to indicate. There are at least three mechanisms whereby anthropogenic fires could influence the fire regime in the direction of lower fire intensity. 1) Anthropogenic fires, whether accidental or not, will on average tend to burn during less severe weather conditions than lightning fires. 2) If anthropogenic fire shortens the fire cycle (as it probably most often does), fuel loads will potentially be smaller. 3) In wilful burning by man, fire intensity can be reduced by controlling the ignition pattern. Of these three mechanisms, the effect of the first is probably the most significant, whereas the second might have an effect in particular situations, and the third yet has to be demonstrated for historic times. A long-term spatial analysis in northern Sweden, reaching beyond the period of large anthropogenic influence, suggest that fires during a few select years indeed were stand-replacing over very large tracts. With increasing anthropogenic influence over the fire regime, the typical pattern became one of sub-lethal fire with occasional, small-scale stand killing. Analysis of a few recent burns also show that the necessary requirements for crown fire behaviour (with regard to fuels and weather) occur surprisingly frequently. This suggest a much more dynamic and variable fire regime in the past than what is today assumed by land managers, and that man can have ameliorated the fire regime, intentionally or not, several hundred years prior to the industrial forest management era.
KEY WORDS: fire, management, boreal