Thursday, 13 July 2006 - 2:25 PM
71-4

Post-fire Pedogenic Response: Wildfire Effects on the Soil Environment in a 60-Year-Old Biosequence.

Robert C. Graham1, Louise M. Egerton-Warburton2, Paul F. Hendrix3, Peter J. Shouse4, Jodi L. Johnson-Maynard5, Sylvie A. Quideau6, Paul D. Sternberg7, Jack A. Jobes4, and Joan M. Breiner7. (1) Univ of California, Riverside, CA 92521-0424, (2) Chicago Botanic Garden, Glencoe, IL 60022, (3) Institute of Ecology, Univ. Of Georgia, Athens, GA 30602, (4) George E. Brown, Jr. Salinity Laboratory, USDA-ARS, Riverside, CA 92507, (5) Univ. of Idaho, Moscow, ID 83844-2339, (6) Univ. of Alberta, Edmonton, AB T6G 2E3, Canada, (7) University of California, Riverside, CA 92521-0424

A soil biosequence was established in 1946 at the large lysimeter installation on the San Dimas Experimental Forest (SDEF) in southern California. Since 1987 we have studied pedogenesis and various aspects of the soil environment as a function of the near-monoculture vegetation types, which consist of chamise and scrub oak (chaparral species) and Coulter pine. The individual biosequence plots have 5.3 by 5.3 m horizontal dimensions. Annual precipitation ranges from 292 to 1224 mm annually, with a mean of 678 mm. Most of the precipitation falls as rain between December and March. The mean annual air temperature is 14.4C, with summer temperatures frequently exceeding 38C and winter temperatures rarely below -4. In September 2002 the biosequence, along with the rest of the SDEF, was burned in a severe wildfire. We have taken advantage of this situation to study changes in the soil environments both immediately after the fire and as the different plant species of the biosequence recover. The wildfire killed all aboveground parts of the vegetation, leaving charred stems and trunks. Litter layers were combusted to white ash and charred material that covered the soil surface. Despite nearly complete loss of cover, runoff and erosion were minimal after the fire, probably because biotically produced soil structure and macropores persisted and maintained high infiltration rates. The loss of plant cover, and attendant transpirational activity, resulted in a warmer, moister soil environment throughout the year, though this effect has diminished as the vegetation has reestablished. Within the first week after the fire, chemical conditions in the charred litter layers and the top few centimeters of soil were markedly altered compared to pre-fire conditions. The pH and concentrations of C, N, and water soluble and exchangeable base cations all increased. During the winter after the fire, concentrations of base cations in soil solutions also increased several fold. Combustion of organic matter mineralized the constituent cations and greatly increased their solubility, producing a flush of soluble base cations during the first winter rainy season. Nevertheless, six months after the fire (and after the rainy season) the chemical composition of the mineral soil was not significantly different from pre-fire conditions with respect to pH, water soluble and exchangeable base cations, C, and N. The biological environment was considerably altered by the fire, beginning with the initial destruction of the aboveground components. Plant life responded quickly to winter rains and within six months after the fire scrub oak and chamise resprouted and pine seedlings had germinated. Surface active macrofauna increased in the post-fire environment, though seasonal patterns of abundance remained the same as pre-fire, with highest densities in the summer. Taxonomic richness of ant populations was unchanged, though some shifts in composition did occur. The predominant ant genera were found in higher concentrations on the chamise plot compared to oak or pine. The fire had profound effects on the fungal communities as well. Dark septate endophytes increased in abundance across the biosequence after the fire, and arbuscular mycorrhizal (AM) hyphae significantly increased under oak and pine, where they are usually minor or absent. Large declines in ectomycorrhizal fungi under oak and pine, and AM fungi under chamise, were observed. The similarity of post-fire mycorrhizal hyphal abundances throughout the upper 35 cm of the soils of the biosequence reflects a thermal impact on the microbiota throughout that depth. Post-fire fungal communities of the different components of the biosequence (pine, oak, chamise plots) were distinctly different than the pre-fire communities and were more similar to each other than were pre-fire communities. Data from six months after the fire show that the fungal communities are evolving as the soil-plant environments change. Overall, many of the impacts of wildfire on these chaparral and pine plots appear to be of short duration. Biological recovery is rapid and, within six months, many soil processes and properties are either not significantly different from the pre-fire conditions or are rapidly returning toward them.


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