Oak woodlands and savannas cover approximately 80,300 km2 in mountains and high valleys of the southwestern United States and northern Mexico. Fire, which normally occurred as a result of lightning during the late spring and early summer, was the most important natural disturbance before European settlement. However, over a century of settlement has altered the dynamics of these ecosystems, resulting in less frequent fires.
Private and agency land managers support the re-introduction of fire into the oak ecosystems but are uncertain about the best prescriptions to restore and enhance ecosystem health. Many managers would like to duplicate natural warm season burning, but others prefer low intensity, cool season burning in the early spring or winter.
The Cascabel watershed study was initiated in 1999 by Rocky Mountain Research Station scientists as part of the Southwestern Borderlands Ecosystem Management Project. The study is a collaborative, interdisciplinary project to determine the effects of cool season and warm season prescribed burning on an oak-savanna ecosystem common to the southwestern United States and northern Mexico. The study also provides baseline hydrological information about watersheds in the region, which can help with monitoring change over time. This information was requested by the Coronado National Forest and its cooperators to aid in sound land management decisions and increase general knowledge about the ecology of southwestern oak-savanna watersheds.
The RMRS Southwest Watershed Science Team is a major partner on the study by collaborating with sedimentation and fire effects research and assisting with general technical and research support. Other cooperators include the Coronado National Forest, USDA Natural Resources Conservation Service, the Animas Foundation, Diamond A Ranch, the Malpai Borderlands Group, the University of Arizona, Northern Arizona University, and The Nature Conservancy.
The Cascabel watersheds prescribed fire study is focused on oak savanna situated between deserts and grasslands and higher elevation oak-pine woodlands of the southwestern United States and northwestern Mexico. The study uses a paired-watershed approach to understand the effects of seasonal prescribed burning on hydrology and sedimentation dynamics, soil nutrient dynamics, overstory and herbaceous vegetation, and selected wildlife species. Research is occurring on 12 gauged watersheds ranging in size from 8 to 24 hectares, all located on the eastern side of the Peloncillo Mountains in southwestern New Mexico.
In 2001, sets of two Parshall flumes were installed in each drainage, as were sediment dams, basins, and channel cross sections. Two full weather stations were set up with seven tipping bucket rain gauges. Measurements of fire effects, soil erosion and deposition, overstory trees, herbaceous vegetation, and wildlife use were collected in each watershed at permanent sampling points arranged on transects perpendicular to the stream channel. There were 35 to 45 points on each watershed depending on its size and shape for a total of 421 sampling points.
Paired watersheds were divided into two groups of three. Each set included a cool-season (November-April) prescribed fire treatment, warm-season (May-October) prescribed fire treatment, and a control treatment. The cool-season watersheds were burned by crews from the Douglas Ranger District of the Coronado National Forest in March 2008 and three of the warm-season watersheds were burned on May 20, 2008. However, the fire escaped on May 21 and burned the remaining unburned watershed, the control watersheds, and about 1,620 hectares in adjacent areas. It was named the Whitmire Fire.
Researchers modified the study design to appropriately account for effects of the Whitmire Fire on five of the watersheds. Indicators of fire effects included soil bum severity, water repellency, and mortality of overstory trees.
Combustion temperatures (intensity) were high and variable between fuel types, with greatest burn temperatures in oak, bear grass, and manzanita fuels and lowest temperatures in litter and light grass.
Soil burn severity and soil-water repellency were generally low to moderate following a cool-season prescribed burn in oak savannas of the Southwest.
High combustion temperatures did not necessarily result in a high severity bum, especially with short residence times for prescribed burns.
The production of early-growing and late-growing grasses was significantly greater after the prescribed burns and Whitmire Fire than before. Depending on the fire event, there was a five to seven-fold increase in the production of early-growing grass species. Increases in late growing species were also significant but of a smaller magnitude.
Fire effects on the minor forb component were inconsistent. The cool-season prescribed burn did not affect forb production, but the warm-season burn and the wildfire did result in increases. Late-season forbs were not affected by the fires.
Total herbaceous production, which includes grasses and forbs, increased relative to pre-treatment condition because of the dominance of grasses in the understory. The increased production has been attributed to the increased soil water availability related to reductions in tree overstory and litter and duff layers and to increased soil nutrients.
The growth of shrubs generally was not affected by the burning events, except for the late-season growth of shrubs on the watersheds impacted by the wildfire. The wildfire produced an increase in warm season shrub production mainly because of tree sprouts.
The prescribed burns and the wildfire did not affect the use of the watersheds by two keystone species: Coues white-tailed deer (Odocoileus virginianus couesi) and desert cottontail (Sylvilagus audubonii).
Prescribed burning treatments and wildfire on the Cascabel Watersheds had relatively little effect on bird species richness, species diversities, or evenness on the watersheds.
Ffolliott, P.F., G.J. Gottfried, and S.R. Woods. Occurrence and production of agave on the Cascabel watersheds following three burning events. Hydrology and Water Resources in Arizona and the Southwest 40:7-10.
Ffolliott, P.F., C.L. Stropki, G.J. Gottfried, D.G. Neary. 2006. Initial estimate of soil erosion on the Cascabel watersheds in the oak savannas of the Malpai Borderlands region. Hydrology and Water Resources in Arizona and the Southwest 35:51-52.
Neary, D.G., and G.J. Gottfried. 2005. Geomorphology of small watersheds in an oak encinal in the Peloncillo Mountains. Hydrology and Water Resources in Arizona and the Southwest 34:65-71.
Gottfried, G.J., D.G. Neary, and R.J. Bemis. 2003. Assessing the impacts of prescribed burning on soil and water resources of oak savannas in the southwestern United States. Pp. 115-122 in J.P.M. Chamie (ed.). Proceedings of the International Arid Lands Consortium Conference and Workshop, Assessing Capabilities of Soil and Water Resources in Drylands: The Role of Information Retrieval and Dissemination Technologies; 20-25 October 2002; Tucson, AZ. University of Arizona, Office of Arid Lands Studies, Tucson.
Gottfried, G.J., D.G. Neary, and R.J. Bemis. 2002. Assessing the hydrologic impacts of cool season and warm season prescribed burning on the oak savannas of the southwestern borderlands. Pp. 56-57 in H.L. Halvorson and B.S. Gebow (eds.). Proceedings of the 4th Conference on Research and Resource Management in the Southwestern Deserts; 15-17 May 2002; Tucson, AZ.
Gottfried, G.J., D.G. Neary, and R.J. Bemis. 2000. Watershed characteristics of oak savannas in the southwestern borderlands. Hydrology and Water Resources in Arizona and the Southwest 30:21-28.
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