Soil temperatures are measured at six locations on the Marcell Experimental Forest. Four sites are in peatland soils (organic) and two are at upland mineral soil sites. Two organic (Junction and Bog Lake) and one mineral (NADP-open) site have open conditions, with no overstory forest canopy. The other two peatlands are in Black Spruce (Picea mariana) stands (S2 and S3) and the other mineral site is in a mature Aspen (Populus spp.) stand (NADP-Aspen). Measurements have been taken at five sites since 1989 and at the Bog Lake peatland since 1990. The S2 peatland is an ombrotrophic bog. Three peatland sites (Junction, S3 and Bog Lake) are fens, which have some degree of exchange with groundwater in an adjacent aquifer in a deep outwash sand.
Each site has a vertical array of Type T thermocouples (twisted copper and constantan pairs) at 8 depths (5, 10, 20, 30, 40, 50, 100 and 200 cm below the surface). In the hummock and hollow microtopography of the peatlands, the sensors were installed in a hollow. Prior to 2002, the Aspen site had only 6 depths (5, 10, 20, 50, 100, and 200 cm). In the peatlands, the thermocouple wires are vertical belowground. As they approach the surface, they run horizontal just under the surface for at least 2 m (6 ft) then run along a post to a pedestal where connecters are housed in shelters.
Temperatures were measured weekly with an Omega model HH-25TC digital thermometer. Connecters were plugged into the thermometer one at a time and temperatures are read to the nearest 0.1 degree C. and recorded by hand on a field data sheet. Before October 2019, readings were usually taken on Wednesday of each week between 10AM and 2PM Central Standard Time. Since then, measurements were usually on Tuesday. If coincident with a US Federal holiday, the measurement would have been taken a day before or after the holiday.
In some years, the digital thermometer calibration was checked with a distilled water ice bath.
The data was transferred to Microsoft Excel Spreadsheets. All data entries were checked and data plots were observed to detect any spurious values.
The sites and methods are described in further detail in the publication:
Sebestyen, S.D., C. Dorrance, D.M. Olson, E.S. Verry, R.K. Kolka, A.E. Elling, and R. Kyllander (2011). Chapter 2: Long-Term Monitoring Sites and Trends at the Marcell Experimental Forest. In Randall K. Kolka, Stephen D. Sebestyen, Elon S. Verry, and Kenneth N. Brooks (Ed.). Peatland Biogeochemistry and Watershed Hydrology at the Marcell Experimental Forest (pp 15-71). CRC Press, Boca Raton, FL. https://www.fs.usda.gov/treesearch/pubs/37979.
