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Wildlife, Animals, and Plants |
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FIRE CASE STUDIES
CASE NAME:Impacts of herbaceous fuel, weather, and fire temperature in single winter fires on honey mesquite REFERENCE:Ansley, R. J.; Joney, D. L.; Tunnell, T. R.; [and others]. 1998 [10] FIRE CASE STUDY AUTHORSHIP:Steinberg, Peter. 2001. SEASON/SEVERITY CLASSIFICATION:Winter/low to medium STUDY LOCATION:The study was conducted on 3 pastures on the Rolling Plains of north-central Texas. One pasture studied was the Ninemile Pasture at 1,247 feet (381 m). Two other study pastures were on the Y Experimental Ranch at approximately 1,640 feet (500 m). PREFIRE VEGETATIVE COMMUNITY:Both sites on the Rolling Plains were had a honey mesquite overstory. At the Ninemile site, mesquite were 6.6 and 13.1 feet (2- 4 m) high and had stem basal diameters ranging from 2 to 4.7 inches (5- 12 cm). At the Y ranch, stems were 6.6 to 9.9 feet (2-3 m) high and had stems between 1.2 and 3.1 inches(3-8 cm) in basal diameter. The understory of the Ninemile site is dominated by a mixture of cool- and warm- season grasses including Texas wintergrass (Nassella leucotricha), Texas bluegrass (Poa arachnifera), Japanese brome (Bromus japonicus), buffalograss (Buchloe dactyloides), and sand dropseed (Sporobolus cryptandrus). The Y Ranch grasses include buffalo grass (Buchloe dactyloides), tobosagrass (Pleuraphis mutica), and sideoats grama (Bouteloua curtipendula). TARGET SPECIES PHENOLOGICAL STATE:Fires occurred during late January to mid-March when honey mesquite was physiologically inactive and leafless. Honey mesquites on the site grew in multi-stemmed clumps as the result of top-kill by herbicide treatments (herbicide prescriptions were not given). On the Ninemile site herbicide top-kill occurred approximately 20 years prior to prescribed fire. Honey mesquite on the Y experimental ranch had been treated with herbicides approximately 12 years prior to prescribed fire. SITE DESCRIPTION:Soils are Mollisols and Vertisols derived from sandstone that are fine textured, mixed, and up to 13.1 feet (4 m) deep. Mean annual rainfall at the Ninemile site is 26.6 inches (665 mm), and mean annual rainfall at the Y ranch site is 18 inches (450 mm). FIRE DESCRIPTION:Weather data are presented below in the "Fire Effects" section of this fire case study. Fire peak temperatures and durations (means and 1 standard error) over 100 °C (FTD 100) and 200 °C (FTD 200) at 3 heights, including both sites, are presented below:
Fire temperature duration patterns were most strongly related to fine fuel loads. The relationship between fire peak temperatures, FTD100, and FTD200 and fine fuel amount, moisture, air temperature, relative humidity, and wind were analyzed with regression analyses; r2 values are presented here:
*= significant at p<0.01. FIRE EFFECTS ON TARGET SPECIES:Moisture content of honey mesquite stems was high and there was little dead mesquite wood to increase fire severity. Most damage to stems was by scorching rather than by combustion. Top-kill (including all sites) ranged from 11% to 94%, and percent foliage remaining on non-top-killed trees ranged from 4% to 100%. Where fire did not top-kill, it often burned the lower part of the canopy giving the appearance of a browse line. No more than 1% of honey mesquite on any site were root-killed; top-killed trees all resprouted from the base. Of the fire temperature parameters, the strongest relation was between top-kill and FTD100 between 3.25 and 9.75 feet (1-3 m), though there was also a clear effect of FTD200 on top-kill between 39 and 117 inches (1-3 m) and FTD200 between 4 and 12 inches (10-30 cm). Top-kill was influenced more by the conditions within the canopy than those at ground level even though fire severity (peak temperature and duration) was lower at canopy height. The authors concluded that top-kill occurs by convective heating rather than by girdling stem bases. Branches are thought to be less resistant because their bark is thinner and less dense. Of the fuel and weather characteristics, air temperature, fine fuel moisture, and fine fuel amount all influenced top-kill. The finding that air temperature had a dramatic effect on top-kill is in contrast to the findings of Britton and Wright [30] who found little correlation between top-kill and air temperature. Relative humidity was a factor in top-kill and foliage loss only on the Y Ranch pastures; Ninemile was thought to be less affected by the relative humidity because fine fuel there was more green. Conditions required for root-kill are much different than those required for top-kill. High temperatures at the ground level influence root-kill more than top-kill. Presented below are r2 values for regressions between fuel, weather, fire temperature, and fire temperature duration (FTD) variables and mesquite responses.
**= significant at p<0.01 FIRE MANAGEMENT IMPLICATIONS:Ansley and others [10] mention that in most of Texas where mesquite is a management issue, significant fire-induced whole-plant mortality is unreasonable to expect. Factors, such as air temperature and humidity, that affect fire temperature peak and duration at canopy level are important considerations in prescribed fire planning. In this study top-kill was more influenced by these factors than by fine fuel amounts. Deciding which day to conduct burns can be as important in managing mesquite as preparation made a year in advance such as grazing reduction to increase fine fuel. Root kill did not occur much here, likely because mesic climate and deep alluvial soils allowed extensive root development. Similarly, Wright [190] found greater root kill from single winter fires on upland compared to lowland sites. The authors also caution that maximizing fire intensity to increase root-kill and top-kill may produce thorny, dense stands of multi-stemmed plants similar to stands produced by herbicidal and mechanical brush control methods. The authors state that "an alternative to this approach with fire may be needed for long-term sustainable mesquite management in certain areas." The alternative approach suggested was that prescribed fires be manipulated to reduce foliage without top-kill, thus facilitating savanna development.
Related categories for SPECIES: Prosopis glandulosa | Honey Mesquite |
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