@article{KoechyWilson2005,
  author    = {K{\"o}chy, Martin and Wilson, Scott D.},
  title     = {Variation in nitrogen deposition and available soil nitrogen in a forest? Grassland ecotone in Canada},
  issn      = {0921-2973},
  year      = {2005},
  abstract  = {Regional variation in nitrogen (N) deposition increases plant productivity and decreases species diversity, but landscape- or local -scale influences on N deposition are less well-known. Using ion-exchange resin, we measured variation of N deposition and soil N availability within Elk Island National Park in the ecotone between grassland and boreal forest in western Canada. The park receives regionally high amounts of atmospheric N deposition (22 kg ha(-1) yr(- 1)). N deposition was on average higher ton clay-rich luvisols than on brunisols, and areas burned 1-15 years previously received more atmospheric N than unburned sites. We suggest that the effects of previous fires and soil type on deposition rate act through differences in canopy structure. The magnitude of these effects varied with the presence of ungulate grazers (bison, moose, elk) and vegetation type (forest, shrubland, grassland). Available soil N (ammonium and nitrate) was higher in burned than unburned sites in the absence of grazing, suggesting an effect of deposition. On grazed sites, differences between fire treatments were small, presumably because the removal of biomass by grazers reduced the effect of fire. Aspen invades native grassland in this region, and our results suggest that fire without grazing might reinforce the expansion of forest into grassland facilitated by N deposition},
  language  = {en}
}
@article{KoechyWilson2001,
  author    = {K{\"o}chy, Martin and Wilson, Scott D.},
  title     = {Nitrogen deposition and forest expansion in the northern Great Plains},
  year      = {2001},
  abstract  = {1 Atmospheric nitrogen (N) deposition has become one of the most important agents of vegetation change in densely populated regions, It may also contribute to forest expansion into grasslands at the northern edge of the North American Great Plains. 2 We measured N deposition and available soil N with ion-exchange resin over 2 years in six national parks in areas varying in population density and industrialization. N deposition was significantly higher in four parks in densely populated regions than in two remote parks. 3 Available soil N increased significantly with N deposition across all parks. 4 We measured N mass and N-15 abundance (delta N-15) in vegetation and soil in two parks: Elk Island, receiving 22 kg N ha(-1) year(-1), and Jasper, receiving 8 kg N ha(-1) year(-1). Differences between parks in tissue N concentrations were small, but forest expansion over five decades resulted in the mass of N in vegetation increasing by 74\% in Elk Island but by only 26\% in Jasper. delta N-15 in forest vegetation was significantly lower in Elk Island than in Jasper, suggesting that anthropogenic sources contribute significantly to the high rates of N entering that ecosystem. 5 We determined the rate of forest expansion within parks using six decades of aerial photographs, Parks in aspen parkland and boreal forest showed a strong positive relationship between forest expansion and N deposition. The relationships found between N deposition, available soil N and forest expansion suggest that even comparatively low rates of N deposition may accelerate the expansion of forest into temperate grasslands.},
  language  = {en}
}
@article{KoechyWilson2004,
  author    = {K{\"o}chy, Martin and Wilson, Scott D.},
  title     = {Semiarid grassland responses to short-term variation in water availability},
  issn      = {1385-0237},
  year      = {2004},
  abstract  = {Standing crop and species composition in semiarid grassland are linked to long-term patterns of water availability, but grasslands are characterized by large single-season variability in rainfall. We tested whether a single season of altered water availability influenced the proportions of grasses and shrubs in a semiarid grassland near the northern edge of the North American Great Plains. We studied stands of the clonal shrub snowberry (Symphoricarpos occidentalis) and adjacent grassland dominated by the native grasses Stipa spartea and Bouteloua gracilis. Rain was excluded and water supplied in amounts corresponding to years of low, medium, and high rainfall, producing a 2 - 4-fold range in monthly precipitation among water supply treatments. There were ten replicate plots of each water treatment in both snowberry stands and grassland. Grass standing crop increased significantly with water availability in grassland but not inside snowberry stands. Total standing crop and shrub stem density increased significantly with water supply, averaged across both communities. In contrast, water had no effect on shrub standing crop or light penetration. In summary, our finding that water has significant effects on a subset of components of grassland vegetation is consistent with long-term, correlational studies, but we also found that a single season of altered water supply had no effect on other important aspects of the ecosystem.},
  language  = {en}
}