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Our study sites span a 500-km climatic gradient and deliberately encompass the north-south geographic range of the of the sugar maple-dominated
(Acer saccharum Marsh.) hemlock-white pine-northern hardwood region in the Great Lakes region of North America . This enables us to
generalize our results across this wide-spread and ecologically important ecosystem. These sites are floristically and edaphically matched
(> 80% sugar maple on typic haplorthods; see Burton et al. 1991,
Pregitzer et al 2008), but they differ in climate along the north-south latitudinal gradient (View interactive map of Michigan Gradient Study Sites).
The study sites also span a gradient of atmospheric N deposition, of which
NO3--N composes ca. 60% of wet-plus-dry deposition. There are six 30-m x 30-m plots at each study site, and every plot is
surrounded on all sides by a 10-m wide treated buffer. Three plots at each site receive ambient atmospheric N deposition. The other three plots at
each site receive ambient N deposition plus 3 g NO3--N m-2 y-1, a rate approaching that expected by 2050
across large portions of North America; atmospheric N deposition in some areas of Europe already exceed the rate of our treatment. The additional
NO3- is delivered over the growing season in six equal applications (0.5 g N m-2 month-1) of solid
NaNO3 pellets, which are broadcast over the forest floor. To date, we have manipulated N deposition for 15 continuous years across the
entire geographic range of this northern hardwood ecosystem. To the best of our knowledge, this is the largest and longest running manipulation of
atmospheric N deposition in any forest ecosystem.
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