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Wildlife, Animals, and Plants |
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FIRE CASE STUDY
CASE NAME:Fuel reduction in Arizona ponderosa pine in southeastern Arizona REFERENCE:Harrington, M. G. 1981 [59] FIRE CASE STUDY AUTHORSHIP:Howard, Janet L. 2001. SEASON/SEVERITY CLASSIFICATION:Summer/low severity STUDY LOCATION:The study was conducted in the Santa Catalina Mountains, 10 miles (16 km) north of Tucson, Arizona. PREFIRE VEGETATIVE COMMUNITY:The study stand was predominantly Arizona pine (Pinus ponderosa var. arizonica) mixed with southwestern white pine (P. strobiformis), silverleaf oak (Quercus hypoleucoides), and Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca). Forest structure was uneven-aged stands of even-aged groups. Two distinct age classes of Arizona pine occurred on the study site: open groups with large, old-growth trees, and closed groups of dense, "doghair" sapling thickets. Arizona pine comprised 65 and 87% of the trees per acre, respectively, and 91 and 79% of the basal area, respectively. Southwestern pine was subdominant in both groups and appeared to be increasing under fire exclusion. Silverleaf oaks and Douglas-firs were more common in the closed stands. Group characteristics were:
TARGET SPECIES PHENOLOGICAL STATE:Not stated. Arizona pine is in the cone development stage in late July and early August, when burning was conducted. SITE DESCRIPTION:Study sites are located at 8,000 feet (2,400 m). Aspect is southwest, with 30 to 50% slopes. Mean annual precipitation is 30 inches (760 mm). Approximately 10% of the precipitation falls in spring; the other 90% is about equally distributed in summer, fall, and winter. Prefire fuel weights were:
Fuel moisture (%) for the 3 prescribed burns follows. Data are means and 1 standard deviation. For each maturity group, means in columns followed by different letters are significantly different (p=0.05).
FIRE DESCRIPTION:Backfires were used on the 3 sites. The 1st site was fired on 24 July, and was subject to the most rain and least number of drying days before burning compared to the other 2 sites. A 15-day rainless period had ended 17 July, followed by rains that fell until 21 July, then 3 days of clear weather. The 2nd site was fired on 3 August, 4 days after 2 brief rains. The 3rd site was fired 22 August, 4 days after 2 weeks of rain. Except for relative humidity, burning conditions were similar among the 3 sites. Fire weather and behavior were:
FIRE EFFECTS ON TARGET SPECIES:The smallest trees showed greatest mortality, increasing mean stand diameter. Greatest mortality occurred in areas that experienced greatest fuel reduction. In open groups, basal areas decreased only slightly because larger trees did not succumb to burning. Basal areas were significantly reduced in closed groups due to high mortality. Seedling and sapling mortality, respectively, was 57% and 16% on site 1; 96& and 54 % on site 2; and 84% and 43% on site 3. Postfire changes in stand structure were as follows:
Crown heights of surviving Arizona pine in closed groups were raised as a result of burning, increasing the chance of surviving the next fire. In closed groups, crown height was approximately 5 feet (1.5 m) above ground before fire. After fire, crown height was raised to 8 feet (2 m) above ground on site 1, and to 14 feet (4 m) on sites 2 and 3. Forest floor weight and depth reductions were statistically similar on sites 2 and 3, and reductions on both sites were significantly greater than reductions on site 1. Percent total forest floor fuels weights (tons/acre) and percent depth reduction of 3 forest floor layers are shown below. Data are means and 1 standard deviation. Means in columns followed by different letters are significantly different (p=0.05).
In the 3-inch size class, more fuels were consumed on open sites than on closed sites. Consumption was highly variable for these larger fuels, and statistical comparisons were not made for larger fuels. Percent weight reduction of 3-inch woody fuels was:
FIRE MANAGEMENT IMPLICATIONS:Summer prescribed burning effectively reduced fuels, and the fires were completely controllable. Individual or group crowning occurred mostly in Arizona pine thickets, where thinning may be beneficial. Harrington [59] provides the following preliminary burning prescription for fuel reduction in Arizona pine stands in the Santa Catalina Mountains:
Related categories for SPECIES: Pinus ponderosa var. arizonica | Arizona Pine |
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