Purposes regarding fire history include: (1) Reconstruct fire history and characterize fire regime over the past several centuries; (2) Analyze temporal changes in fire frequency, severity, and size over the time period of record; (3) Describe spatial variability in fire frequency and severity, and identify key environmental influences on fire pattern; and (4) Delineate portions of the study area with qualitatively different fire regimes.
Purposes regarding interactions between fire history, physical environment, stand structure include: (1) Quantitatively examine the influence of fire history on forest stand structure, especially as stand structure relates to the development of old-growth characteristics; and (2) Describe how fire history influences on stand structure vary for different environments, where topography is used as a proxy for features of the physical environment that relate to tree growth and population dynamics. Forest structure is limited to the diameter structure of trees and snags, at the scale of 4-ha sites.
Fire scar and tree origin years of primarily Douglas-fir trees were obtained from field counts of tree-rings on 4478 cut stumps in 137 clearcuts. In an effort to sample extensively over the entire study area, one randomly selected clearcut in every legal section was sampled where at least one recent (i.e., less than approximately 12 years old) clearcut was available. There are 172 legal sections across the study area, typically 1 square mile (2.79 square km), although areas vary. Mean sampling density was 0.3 sites per square km. Thirty-five sections were not sampled because they did not contain harvest units, were harvested too long ago for stump surfaces to be suitable for ring counting, or have already been harvested twice and are now on a second rotation, making evidence of pre-harvest disturbance history unavailable. Undersampled portions of the study area include the area from Lookout Mt. south to the McKenzie River, the extreme southwesternmost portion (Simmonds Ck.), and the northernmost portion adjacent to the South Santiam River.
For 47 of the sites (O sites), sampled clearcuts were old (pre-1984) and brushy, or else dangerously steep, precluding the systematic sampling of plots. At these sites, fire history was sampled "opportunistically" wherever stumps with fire evidence were found, over a two to four ha area. Sampling continued until it was apparent that a complete fire history had been sampled, which usually required from 10 to 30 stumps per site.
For the remaining 90 sites (C sites), fire history was sampled for a 4-ha square area bounded at the corners by the centers of four, intensively sampled 0.1-ha circular plots. The 4-ha area was approximately delineated by pacing, and at some sites, logistical and safety constraints prevented sampling the full area or number of plots. The actual area sampled at a site probably ranges from three to five ha. Four plots were sampled at 77 of the 90 sites, while the remaining 13 sites were sampled using from two to five plots. There was no significant difference between the number of fires detected for sites with different numbers of plots sampled, using an ANOVA analysis (F(4,85)=0.9985; p = 0.4130). Also, regression analysis shows no trend in the number of fires detected as a function of the number of trees aged at a site, when opportunistic sites were excluded (F(1,88) = 0.7787; p = 0.3799).
Within each 0.1-ha plot, all Douglas-fir stumps were sampled and stumps of other species that had fire scars. Diameters were measured at stump height and tallied by species all stumps, snags, and standing trees present. For a 0.01-ha plot at the center of each 0.1-ha plot, all stumps regardless of species were sampled. Information from the 0.01-ha plots was useful for reconstructing western hemlock origin years. For the approximately 4-ha area between the corner plots, only old stumps, or those with many scars from previously unsampled fire years, were sampled. Tree diameters for 90 4-ha sites, for which detailed fire history and environmental information were also collected, were measured and used for the stand structure development chapter (Weisberg thesis).
Fire scars and tree origin years were dated in the field by counting tree-rings under appropriate magnification (using a 3x, 10x, or 16x hand lens), after preparing the stump surface with hand tools. Scrapers were used to remove pitch and debris, a surform scraper was used to expose a new surface where necessary, and a wire brush was often used to abrade the softer spring wood, accentuating the denser, more resistant latewood. Stump surfaces were still rough after preparation, and it is likely that there were counting errors. Crossdating was not employed to correct scar and origin year estimates using correlations among ring widths and other tree-ring features (Stokes and Smiley 1968), because of logistical constraints inherent in a study whose objectives require sampling many trees at many sites to achive a desired grain and extent for developing a landscape-level representation of past fire patterns.
The following data were recorded or computed for each sampled stump: height (cm), diameter at stump height (cm), origin year, reliability of the origin year estimate, the average width of the first three rings (mm), the radius at age 40 (cm), the presence of any scar(s), the position of scar(s) relative to the slope direction (i.e., upper, side, lower), the compass bearing of scar(s) from the pith outwards, the percent circumference of the cambium killed by the scarring event(s), the radius from the pith to the scar(s) (cm), the age of the scar(s), and the reliability of the scar age estimate. Pitch rings (i.e., growth rings separated by a band of pitch) were sometimes dated, and measured similarly as scars. Field observations and previous studies have shown that pitch rings are often associated with fire years, and provide weak evidence of fire (Weisberg, unpublished m.s., Weisberg 1997). The origin year was estimated by first counting the number of tree-rings from the pith to the most recent growth ring, and subtracting this number from the harvest year. Then the average width of the first three rings was used to estimate the age of the tree at stump height, according to the formula (Morrison and Swanson 1990):
The origin year estimate was corrected by adding the estimated age of the tree at stump height. Although this formula was developed for Douglas-fir forests of a nearby study area, it is likely a coarse approximation of the number of years required for a tree to grow to stump height. Scar year estimates are therefore more accurate than origin year estimates. Origin years were usually counted once, and scar years recorded as the average of two counts.
Within each plot, I recorded diameters at stump height, by species, for all stumps of at least 5-cm diameter. Diameter at stump height or breast height, depending which was shorter, of snags of at least 1-m in height, by species was recorded. Diameter at breast height (d.b.h.), by species, for all standing trees was recorded; this information was aggregated with stump data to describe the diameter structure, by species, for each site.
Diameter measurements for each site were aggregated over all plots to calculate tree density (per ha), basal area (m2 per ha), and mean and standard deviation diameter, of each tree species present (including snags), for each site. Diameters were not corrected to breast height prior to calculating basal area, because not all stump heights were measured. Stump heights varied from 30 cm to 120 cm, but were usually between 60 and 90 cm. I believe errors in basal area calculations resulting from variable stump heights to be negligible, and randomly distributed among predictor variables. Size class histograms suggested that trees of less than 20-cm diameter were under-represented in the sample, perhaps due to being knocked down during harvest, faster decay rates of small stumps, or greater difficulty locating small stumps in brushy clearcuts. I believe trees of at least 20-cm diameter were completely sampled. Therefore, all calculations include only trees of at least 20-cm diameter. This should not significantly influence measurements of basal area, but might greatly influence density measures.
Then the average width of the first three rings was used to estimate the age of the tree at stump height, according to the formula (Morrison and Swanson 1990):
The origin year estimate was corrected by adding the estimated age of the tree at stump height.