A belt transect was used to sample a large swath through each clearcut. Transect location was based on topographic features (e.g., ravines, depression, cliffs), knowledge of basic fire behavior, and the size of the clearcut. Swampy depressions, deep ravines, and riparian zones were avoided. Where the clearcut terrain was fairly homogeneous, I subdivided the clearcut and randomly located the transect. The width of each transect was 22.86 m (75 feet), and the length varied from 60 to 300 m, depending on the clearcut size, terrain, and amount or clarity of the fire history data. The mean area of the transects sampled within the clearcuts was 0.48 ha (S.D. 0.15 ha). Along each transect, I aged (i.e. counted tree rings) stumps and their scars and pitch rings.
All size classes and all conifer species were sampled. Additional stumps were sampled to confirm fires. Based on data from nearby fire history studies, I considered three correlating scars or at least eight regenerating trees in a cohort as evidence of a fire. Sampling for additional data was discontinued once these criteria were met. However, sampling for other fires continued until the end of the transect. Sampled stumps were chosen based on size, species, and evidence of scars. For example, Douglas-fir tend to survive, scar, and regenerate after moderate and severe fires, so Douglas-fir were of more interest than the shade-tolerant, fire-susceptible species such as western hemlock. All coniferous species were sampled, but fire-adapted species were sampled more often. Stumps outside the transect were occasionally sampled if they provided fire history information such as scars or very old regeneration dates. Usually, these were large, old, or scarred Douglas-fir whose age class was rare inside the transect. The tree rings were counted with dissecting needles and 16x magnification hand lens. Multiple counts were performed to ensure accuracy.
Other information was also collected. Along the inside edge of each transect, within a 25 foot belt, all stumps were tallied by species and diameter class. Site-level topographic information includes aspect, slope, and slope position. Tree ages were adjusted using Hall's equation for estimating age at stump height (Teensma 1987; Morrison and Swanson 1990; Connelly and Kertis 1992):
Calculated Age = 0.1582 * stump height / ring width (for inner ring width > 2 mm), and Calculated Age = 0.1582 * stump height / 2 (for inner ring width < 2 mm)
This formula was developed for Douglas-fir, which composed 82% of the sampled stumps. However, as it is a general formula (Tappeiner pers. comm.) that adds approximately 1 to 3 years to a tree's age at stump height, I applied it to all tree species. Calculated tree age was subtracted from the clearcut harvest year to obtain the calendar year of scars and pitch rings. The year of harvest was provided by RBLM and the North Umpqua Ranger District.
Notes:
Formats 1-4 are field data. Format 5 is fire event data produced by data analysis. More information about the data analysis and results can be found in the Master’s thesis:
Van Norman, Kelli J. 1998. Historical fire regime in the Little River watershed, southwestern Oregon. M.S. thesis, Oregon State University, Corvallis, Oregon.
