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 Nitrogen Deposition Gradient Study Sites via GoogleMaps). The study sites also span a gradient of atmospheric N deposition, of which NO3-N composes ca. 50% 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 significant portions of North America; atmospheric N deposition in some areas of Europe already exceeds 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 18 continuous years across the entire latitudinal range of this northern hardwood ecosystem. To the best of our knowledge, this is the largest and longest running replicated manipulation of atmospheric N deposition in any forest ecosystem.
| Characteristic | Site A | Site B | Site C | Site D |
|---|---|---|---|---|
| Location | ||||
| Latitude (N) | 46°52' | 45°33' | 44°23' | 43°40' |
| Londitude (W) | 88°53' | 84°51' | 85°50' | 86°09' |
| Climate | ||||
| Mean annual precipitation (mm) | 873 | 871 | 888 | 812 |
| Mean annual temperature (°C) | 4.7 | 6.0 | 6.9 | 7.6 |
| Growing season length (days) | 134 | 150 | 154 | 157 |
| Wet + dry total N deposition (g N m-2 y-1) | 0.68 | 0.91 | 1.17 | 1.18 |
| Wet + dry NO3--N deposition (g N m-2 y-1) | 0.38 | 0.58 | 0.78 | 0.76 |
| Vegetation | ||||
| Overstory age (2004) | 97 | 91 | 92 | 96 |
| Overstory Biomass (Mg ha-1) | 261 | 261 | 274 | 234 |
| Acer saccharum Biomass (Mg ha-1) | 237 | 224 | 216 | 201 |
| Soil | ||||
| Soil texture, 0-10cm depth (%sand-%silt-%clay) | 75:22:3 | 89:9:2 | 89:9:2 | 87:10:3 |
| Soil texture, 10-70cm depth (%sand-%silt-%clay) | 84:11:5 | 88:7:5 | 91:6:3 | 92:5:3 |
| Coarse fragments, 0-10cm depth (%) | 4.6 | 1.8 | 0.6 | 1.7 |
| Coarse fragments, 10-70cm depth (%) | 7.3 | 7.5 | 1.9 | 3.6 |
| pH (1:1 soil:H20) | 4.8 | 5.0 | 4.5 | 4.7 |
| CEC (mmol (+) hg-1) | 3.4 | 3.8 | 2.6 | 3.0 |
| Base Saturation (%) | 71 | 96 | 73 | 80 |
