|
Forest stand structure, species composition, and succession are products of any number of combinations of biotic and abiotic agents. In the interior Douglas-fir plant associations of the United States and Canada, fire has had a prevalent
influence, though the effects have been both spatially and temporally variable. The natural range of variability of fire, its frequency, intensity, seasonality and extent, are collectively known as a fire regime.
The goal of the ongoing research project at Pothole Creek is to determine the stand development processes on the site. Since fire was
a common disturbance agent in these forests, understanding the historical fire regime is essential for any analysis of current stand structure and past growth.
To examine the fire history at Pothole Creek, a master tree-ring chronology was constructed and was used to cross-date samples collected from fire-scarred wood. Annual tree ring width varies in response to changes in climate parameters, and this
variation is particularly marked for trees growing under marginal conditions. At Pothole Creek, the growth of trees on dry rocky ridges is limited by soil moisture during the growing season which in turn is limited by precipitation. Consequently,
these trees produce narrower rings in dry years and wider rings in wet years. We used these trees to develop a master chronology, which is a time series of tree-ring widths in which the common climate signal has been maximized. To cross-date
fire-scarred wood samples, patterns of wide and narrow rings on the samples were matched with those on the master chronology to assign the correct calendar year to every tree ring and every fire scar. This cross-dating can identify missing and false
rings that would lead to inaccurate fire dates.
Evidence was found of fires occurring as long ago as 1693 and as recently as 1967. The mean fire-return interval was 13 years, which is within the expected range for interior Douglas-fir forests. The minimum and maximum return intervals were 1
year and 46 years, respectively, indicating varying periods of both low-intensity/high-frequency fire and higher-intensity/low-frequency fire. This high disturbance frequency promotes plant species with adaptations to fire, such as Douglas-fir,
lodgepole pine, and pinegrass (Calamagrostis rubescens), and an open
stand structure with sparse, patchy regeneration. However, there was no evidence
that the Pothole Creek Study Area site has burned within the last 40 years. The
most recent moderate intensity fire was in 1967, 1 year after harvest and
perhaps the result of post-harvest clearing operations. This is reflected in the
current stand structure, in which the regeneration clumps are numerous and
dense, and lodgepole pine regeneration is absent although the site is surrounded
by pine seed sources.
Taken from:
Gary, R. and E. Riccius. 1999. Historical fire regime for the Pothole Creek
Interior Douglas-fir research site. B.C. Min. For. Res. Br., Victoria, B.C.
Work Pap. 38.
|
