Variation in Aboveground Cover Influences Soil Nitrogen Availability at Fine Spatial Scales Following Severe Fire in Subalpine Conifer Forests
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Following fire, fine-scale variation in early successional vegetation and soil nutrients may influence development of ecosystem structure and function. We studied conifer forests burned by stand-replacing wildfire in Greater Yellowstone (Wyoming, USA) to address two questions: (1) How do the variability and spatial structure of aboveground cover and soil nitrogen availability change during the first 4 years following stand-replacing fire? (2) At fine scales (2–20 m), are postfire soil inorganic N pools and fluxes related to aboveground cover? Aboveground cover, soil N pools, and annual net N transformations were measured from 2001 to 2004 using a spatially explicit sampling design in four 0.25-ha plots that burned during summer 2000. Within-stand variability (coefficient of variation) in postfire live vegetative cover declined with time since fire, whereas variability in bare mineral soil, charred litter and fresh litter was greatest 2-3 years postfire. The soil nitrate pool was more variable than the soil ammonium pool, but annual net nitrification was less variable than annual net N mineralization. Spatial structure (quantified by semivariograms) was observed in some aboveground cover variables (for example, graminoids and fresh litter), but there was little spatial structure in soil N variables and no obvious congruence in spatial scales of autocorrelation for soil N and aboveground cover. Significant Spearman correlations (at the sample point) indicated that aboveground cover and soil N were coupled following severe fire, and the dominant influence was from aboveground cover to soil N, rather than from soil N to vegetation. Initial patterns of fire severity and re-vegetation contributed to fine-scale heterogeneity in soil N availability for at least 4 years after severe fire.
Keywordsnitrogen cycling fire ecology succession Pinus contorta Picea engelmanii Abies lasiocarpa northern Rocky Mountains spatial heterogeneity geostatistics semivariogram scale
We thank the many field and lab technicians who assisted during the 2001 to 2004 summer field seasons, and we especially thank Donna Kashian, Nicole DeCrappeo and Aaron Theil for supervision of the summer field laboratory, Kris Metzger for running many of the soil N analyses on the Lachat in Madison, and Teri Balser for use of analytical equipment. Janice Corcoran and Judy Romme provided key logistical assistance, and field support was provided by Hank Harlow and the University of Wyoming-National Park Service Research Center. The manuscript was improved by constructive comments from Jennifer Fraterrigo, Jake Griffin, John Pastor, Steve Hart and two anonymous reviewers. This study was funded by a grant from the Conservation and Environment Program of the Andrew W. Mellon Foundation, and we especially thank Bill Robertson for his support.
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