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Wildlife, Animals, and Plants |
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FIRE CASE STUDY
CASE NAME:Tharp's Creek prescribed burn, Sequoia National Park
REFERENCE:Mutch, L. S.; Parsons, J. D. 1998 [233]
FIRE CASE STUDY AUTHORSHIP:Zouhar, Kris. 2001.
SEASON/SEVERITY CLASSIFICATION:fall/mixed
STUDY LOCATION:The Tharp's Creek (burned) and Log Creek (unburned) watersheds are located in the Giant Forest area of Sequoia National Park, California. Two reference stands (upper and lower) are located in each watershed.
PREFIRE VEGETATIVE COMMUNITY:White fir dominates in all stands, with an inverse J-shaped size distribution. Sugar pine is most common in Lower Tharp's and upper Log stands. Red fir (Abies magnifica) is present in significant numbers only in the Lower Log stand. Giant sequoia (Sequoiadendron giganteum), incense-cedar (Calocedrus decurrens), Jeffrey pine (Pinus jeffreyi) and California black oak (Quercus kelloggii) are only minor components of these stands. Shrub cover ranges form 2% to 20% in the four stands and is comprised mainly of greenleaf manzanita (Arctostaphylos patula), mountain whitethorn (Ceanothus cordulatus), bush chinquapin (Chrysolepsis sempervirens), and gooseberry (Ribes spp.). Litter and duff are the predominant ground cover, with scattered herbaceous vegetation.
TARGET SPECIES PHENOLOGICAL STATE:The phenological state of white fir on the site at the time of burning is not given, however, judging from the timing of the fire, it is likely that white fir seeds had been dispersed and the trees were dormant.
SITE DESCRIPTION:The study sites comprise the headwater drainages of Tharp's Creek and Log Creek. Soils of both watersheds are predominantly pachic xerumbrepts, derived from granodiorite. Slopes are moderate to steep. The aspect of Tharp's watershed ranges from south to south-east, and Log watershed aspect is primarily west to south-west and north-west. Elevations are 6875 to 7150 feet (2097-2180 m) in the Tharp's watershed and 7080 to 7775 feet (2158-2371 m) in the Log watershed. Mean annual precipitation in the area is 50 inches (1255 mm), with about half falling as snow. Mean January and July temperatures are 32 degrees Fahrenheit (0°C) and 64 degrees Fahrenheit (18°C), respectively.
FIRE DESCRIPTION:The Tharp's prescribed burn was a 35 acre (14 ha) fire ignited October 23-26, 1990. Ignition occurred primarily in early evening and into the night when relative humidity was 30-40%. Average fuel moistures for litter ad duff were 28%, for 100 hr fuels 14%, and for 1000 hr fuels 64%. Air temperatures during ignition ranged from 50 to 60 degrees Fahrenheit (10-16 °C) and winds were calm. Fire behavior ranged from a backing fire with flame lengths of 0.2 to 0.5 feet (0.05-0.15 m) and rates of spread up to 0.3 feet (0.1 m)/minute, to a strip headfire with flame lengths of 2 to 8 feet (0.6-2.4 m). Areas with heavy fuel concentrations and standing snags burned with the greatest severity. Total preburn fuel load was 94 tons/acre (210 Mg/ha) and total reduction was 85%, with the highest reduction in litter/duff (97%) and 1-hr fuels.
FIRE EFFECTS ON TARGET SPECIES:The prescribed burn in the Tharp's Creek watershed resulted in a dramatic rise in mortality rates for 5 years following the fire when compared both with the prefire mortality rates and with the unburned Log Creek watershed. Average annual mortality for all trees greater than 4.6 feet (1.4 m) tall in the Lower and Upper Tharp's Creek plots during the 5 year preburn period was 0.8% and 0.6%, respectively. Annual mortality rates in the 1st postburn year increased to 35.2% and 49.4%, and declined to 2.6% and 5.0% by the fifth postfire year. While these rates are well above prefire mortality rates, the 2.6% is within the range of annual mortality rates recorded for the Log Creek watershed for the same period. The greatest reduction in white fir numbers was in the intermediate and subcanopy classes. Populations and mortality of white fir trees >4.6 feet (1.4 m) tall in the Lower Tharp's stand:
Populations and mortality of white fir trees >4.6 feet (1.4 m) tall in the Upper Tharp's stand:
Primary factors associated with mortality in both watersheds during the prefire period and on the Log Creek watershed in general, were dwarf mistletoe, fir canker, fir engravers and stem/root failure that caused crushing by another tree. The postfire mortality in the Tharp's Creek watershed was most directly related to fire-caused crown scorch. There is no indication of a relationship between pre-existing disease or insect conditions and fire-induced mortality, since similar percentages of trees that survived the fire had dwarf mistletoe, and fire engravers associated with them. Fine root mortality may have also played a role in the death of trees several years after the fire, since the 97% reduction of litter and duff indicates a particularly severe fire that would have likely caused substantial fine root mortality. During the prefire period, the Lower Tharp's stand had a slight decline in total basal area due primarily to the death of numerous codominant and intermediate white fir trees, while the Upper Tharp's stand had an increase in basal area during this period. Both stands had a large decline in basal area after the burning due primarily to fire-related mortality. The most important change in size structure in the Log Creek watershed was in the smallest size classes. In the Tharp's Creek stands, there was a dramatic change in the size structure during the postfire period with a decline in mean number of trees/ha in most sized classes, including 75% of the trees < 50 cm dbh killed, and 25% of trees larger than 50 cm dbh killed. The different tree species present died in proportion to their frequency in the watershed. A large number of seedlings had established after the burn on Tharp's Creek and had more rapid height growth relative to seedlings in the Log watershed.
FIRE MANAGEMENT IMPLICATIONS:Suppression and higher incidence of insects and disease in higher density stands are cited as primary reasons for mortality in unburned stands. In this study, a drought during the 1987-1992 time period is thought to have contributed to the stress that led to mortality in the larger size classes in the Log Creek watershed. In white fir, the probability of mortality increased with percentage of crown scorch, and decreased with dbh. This type of information can help improve the ability of mangers to predict tree morality from prescribed burning and to plan burning conditions to meet specific mortality objectives. Differences in mortality between the Upper Tharp's stand and the Lower Tharp's stand are thought to be due to differences in fire severity between the two stands, with the more severe fire causing higher rates of initial mortality in the larger size classes. The large reductions in tee densities and basal areas in the Tharp's watershed will provide more opportunities for successful establishment of less shade-tolerant species and will help reduce fuel inputs, presumably reducing the hazard of uncontrollable wildfire in the future. However, future burns will likely be necessary to maintain reduced tree densities and low fuel accumulation rates. Mortality is expected to continue to decline over the next several years until they reach near pre-burn levels. It is also expected that the large number of seedlings that established after the burn will result in substantial amounts of ingrowth within the next 10 years.
Related categories for SPECIES: Abies concolor | White Fir |
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