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I'm sorry my colleague, Peter Brown, is not in town for this
discussion. He has done quite a bit of fire history work in this area
by using dendrochronology to date fire scars on trees. For an overview
of his work, see http://www.rmtrr.org/ and click on fire history.
Just to summarize a bit from some of his studies (these cover the Black
Hills, eastern WY, Colorado), it appears that fire intervals in
ponderosa to mixed ponderosa/Douglas-fir average about 20-25 years,
although in certain stands, it can be much longer. Also, within the
time span of the fire history chronologies (most go back into the 1500s
or before), intervals between fires can be quite variable, and shifts in
timing and frequency are apparent as well. Since the late 19th-early
20th century, there has been a cessation of surface fire coinciding with
livestock grazing and fire suppression efforts. In a study in the
Cheesman area, there is evidence that there have been both low-intensity
surface and stand-replacing fires.
Fire scars found for the same year in trees across a very broad area
(and some of the same fire scar years are found in trees from Colorado
to New Mexico and Arizona) does not indicate that there was one huge
fire. What this does suggest is that multiple ignitions during dry
summers resulted in many fires across the region. In some areas, like
the Southwest, the biggest fire years seem to occur during dry years
that follow wet years. During the wet year, grasses and shrubs
flourish, providing fuel in the following wet year. I'm not sure how
much that is the case here in the Front Range. In the SW, fire
frequency seems to be closely tied to ENSO, (which is more important to
climate down there than here) with the big fire years during dry La Nina
years that follow wet El Ninos.
That's about the limit of my understanding and I may be overgeneralizing
a bit, but not too much, I hope.
Connie
-- Connie Woodhouse NOAA Paleoclimatology Program National Geophysical Data Center 325 Broadway E/GC Boulder, CO 80303 ph: (303)497-6297 fax: (303)497-6513 email: woodhous@ngdc.noaa.gov See http://basin.org/forum/fire-flood.html for more information on this list.