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Elizabeth LevittAdvisor: Monica Turner MS 2006: SOURCES OF VARIATION IN SOIL NITROGEN AVAILABILITY AMONG POST-FIRE LODGEPOLE PINE STANDS IN YELLOWSTONE NATIONAL PARK 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. |
