Saturday, 15 July 2006

Assessing post-fire Soil Change: First Year Results from a Long-term Monitoring Project Following Ecosystem Recovery in Mesa Verde National Park, CO USA.

Colin Robins1, Michael Howell2, and Patrick Drohan1. (1) Univ of Nevada, Las Vegas, Dept of Geoscience, 4505 Maryland Pkwy Box 4010, Las Vegas, NV 89154-4010, (2) Univ of Nevada, Las Vegas, Dept of Geoscience, 4505 Maryland Pkwy Box 4010, Las Vegas, NV 89154-4010

Mesa Verde National Park, Colorado, USA is a prime location to study fire effects on pedogenesis and forest ecology, given its extensive land-management and fire history and the present-day need to preserve unique archaeological resources within the region. In August of 2000, approximately 5,000 acres of largely piņon-juniper woodland along the southwestern border of Mesa Verde National Park, and in the Ute Mountain, Ute Reservation burned (The Pony Fire). Due to the reported severity of the fire, national attention focused on the park area and concerns were raised about possible fire disturbance to archeological ruins and a potential future failure in ecosystem recovery. Soils were sampled in 2004 as part of a long-term monitoring study focusing on fire severity and the park's recovery. Soils consisted of an Aridic Paleustalf (Morefield series 3-6% slopes) on a transect extending approximately 0.5 km through burned and unburned areas of Wetherhill Mesa. The few areas of burned logs or stumps were not sampled. Study area bedrock and parent material consists of Late Cretaceous sandstone of the Cliffhouse Formation. Mean annual precipitation in the area is 43.2 cm and mean annual temperature is 9.4 ēC. Slopes across the transect range from 3-6%. Within the burned and unburned areas, soils were sampled at 5 pseudo-random locations on the transect with 3 holes excavated in the A horizon at each location (0-5 cm, 5-10 cm, and 10-15 cm). Soils underwent physical (particle size analysis, sand-fraction assessment), chemical (available P, pH, exchangeable Ca, Mg, Na, K, Fe, and Mn, %organic matter, and Total %N) and mineralogical analysis (x-ray diffraction and optical color analysis of sand fractions) in order to assess changes due to the fire. Analyses suggest that soils sampled in this study area of the Pony fire were not affected by a high-intensity burn. Statistically higher fine-sand fractions were found in burned vs. unburned surface horizons. Statistical increases in pH and concentrations of Na and Fe were also found in burned sites. No significant difference was discernable in color (rubification), mineralogy, nor additional chemical properties between burned and unburned sites. Inconsistencies between hypothesized and measured soil analytical results underscore the need to better delineate relationships between fire characteristics and fire-induced changes in soil physical and chemical properties. Future sampling will be expanded throughout the park in coming years to assess the spatial variability of burn severity on soils, and to track change over time during the park's recovery. Analysis of soil transects across distinct and varied geomorphic surfaces is also critical to extend fire-severity assessments across larger areas. Long-term monitoring studies, such as this one, are needed to better constrain soil resilience to landscape and ecological disturbances and to expand data collection to a more diverse array of burn conditions such as burned snags, shrubs, and bare ground sites. Analysis of data between snag, burned shrub, and bare soil sites may also provide better understanding of local variability within fire-disturbed landscapes. Fire-assessment studies should also address additional chemical parameters and analyze samples from successive 5-yr sample intervals. Such efforts may allow more precise description of the relative magnitude of fire intensity versus soil resilience. Long-term monitoring of post-fire soil changes, and careful design of soil-ecological monitoring studies have important implications for the better understanding of long-term forest succession, pedogenetic, and geomorphologic processes under piņon-juniper woodland.

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