Levitt, E. A. 2006. Sources of variation in soil nitrogen availability among post-fire lodgepole pine stands in Yellowstone National Park. MS Thesis, University of Wisconsin, Madison.

Understanding natural variation in ecosystem processes is an important research goal in ecology. In particular, spatial variability in processes such as nutrient cycling across landscapes is not well understood. The 1988 fires in Yellowstone National Park produced a landscape mosaic of early successional forests that vary widely in stand densities. In this study, I asked whether the differences in stand characteristics observed in 17-yr old post-fire lodgepole pine (Pinus contorta) were associated with differences in inorganic soil nitrogen availability. During summer 2005, I sampled 25 stands that burned in the 1988 fires. Free resin bags were used as an index of nitrogen availability during the growing season, distributed randomly in each stand in June 2005, retrieved in September 2005 and extracted for nitrate (NO3-) and ammonium (NH4+). Observed stand characteristics included vegetation, forest floor cover, litter quality and quantity and soils. Overall, vegetation explained 58% and litter explained 19% of the variability in soil N availability among 25 stands. Of these attributes, graminoid cover (grasses and sedges) explained 51%, while pine sapling density explained 7% of the variation in the model. Litter quantity and quality explained 14% and 5% of the variation, respectively. Pine sapling density was negatively related to N availability (R2=0.25; p=0.01). Within-stand heterogeneity of inorganic N availability as measured by coefficients of variation (CV) ranged from 47 to 284% among stands, and CVs were largely attributed to litter mass (partial R2=0.47, p<0.001). The results of this study showed that postfire differences in vegetation structure was associated with  patterns of N availability almost 20 years after the 1988 fires in YNP.