Site descriptions and experimental design
To examine plant community and soil type controls on belowground C fluxes in temperate forests, we worked in six eastern U.S. temperate forests within the Smithsonian Forest Global Earth Observatory (ForestGEO) network (Anderson-Teixeira et al. 2015): Harvard forest (HF; 42° 32’ N, 72° 11’ W) in North-Central Massachusetts, USA; Lilly-Dickey Woods (LDW; 39° 14’ N, 86° 13’ W) in South-Central Indiana, USA; the Smithsonian Conservation Biology Institute (SCBI; 38° 54’ N, 78° 9’ W) in Northern Virginia, USA; the Smithsonian Environmental Research Center (SERC; 38° 53’ N, 76° 34’ W) on the Chesapeake Bay in Maryland, USA; Tyson Research Center (TRC; 38° 31’ N, 90° 33’ W) in Eastern Missouri, USA; and Wabikon Lake Forest (WLF; 45° 33’ N, 88° 48’ W) in Northern Wisconsin, USA. At each of the six sites, we established nine 20m × 20m plots spanning a gradient of ECM tree dominance (by basal area) for a total of 54 plots. At each site, the three plots with the lowest ECM tree dominance and the three plots with the highest ECM tree dominance were designated as AM and ECM ‘end-member’ plots. The remaining three plots per site were designated as AM/ECM ‘mixed’ plots.
The sites vary in climate, soil type, and plant species composition but each host a diversity of AM and ECM associated canopy tree species. For the most dominant tree species at each site, see Table 1. Soils are predominantly Oxyaquic Dystrudepts at HF, Typic Dystrudepts and Typic Hapludults at LDW, Typic Hapludalfs at SCBI, Typic or Aquic Hapludults at SERC, Typic Hapludalfs and Typic Paleudalfs at TRC, and Typic and Alfic Haplorthods at WLF. For site-specific soil properties, see Table 2.