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Climate records for Fairbanks (UAF Experiment Station) from 1904 to 2019 were acquired from the National Oceanic and Atmospheric Administration (https://www.ncdc.noaa.gov/cdo-web/). Additional data were obtained for the Nenana station (about 70 km southwest of Fairbanks), to fill in small data gaps (particularly precipitation/snow depth ruler measurements) in the Fairbanks record. We attributed the data with fields for summer (May-September) and winter periods (October-April) and hydrologic year (October-September) and calculated mean air temperature, precipitation, and snow depth by seasonal period (average of daily values) and year. The broad summer and winter periods were of interest because warmer and wetter summers increase soil heat input and warmer and snowier winters reduce soil heat loss.

Plant cover was determined by point-sampling at 100 points (including repetitive “hits” for all layers) distributed along 5 equally spaced rows (4-m long, 20 points per row) across the 10-m long plot. The plots were examined for additional species and a cover of 0.1% was assigned to all species not captured in the point sampling. Taxonomy follows that of the Flora of Alaska provisional checklist (https://floraofalaska.org/provisional-checklist/).

Water levels and temperatures of near-surface groundwater at two parallel, 15-km-long fens (three sites each) and a small lake (Figure 2) were measured hourly from summer 2011 through early fall 2014 using non-vented pressure transducers (HOBO U20 Fresh Water Level Data Logger 13 feet, U20-001-04, Onset Hobo Company, Bourne, Massachusetts). They were deployed inside a 4-cm-diameter perforated PVC pipe situated at least 10 m from the fen margin. The sensors rested on top of a 1-cm-diameter steel rod with a 3-cm-diameter cap. The rod was pounded into the sediments below the floating mat with a slide hammer to a depth greater than the typical seasonal freeze-thaw depth in the area (usually 1.5 m) to avoid jacking from seasonal frost. The sensors were between 1 and 1.5 m below the water level in the fall and secured to the top of the pipe with a slacked line for retrieval. Barometric pressure measurements were made using an additional sensor hung from a tree at the Lake, Fen 1_Mid, and Fen 2_Upper sites (Figure 2). Elevations of the sensors were determined using a high-resolution, differential global positioning system (DGPS). Based on the measurement error for the z coordinate direction with the DGPS and the pressure transducer (+/-0.3 cm of water) we estimate the vertical water level elevation error to be +/- 4 cm. Wells were accessed via helicopter (summer) or snow machine (winter) for downloading and maintenance.

For data processing and analysis, we used HoboWare (Onset Computer Corporation, Bourne, Massachusetts) for initial processing and barometric pressure correction and the data were compiled into an Excel database. To allow comparison of water-level trends among sites, the long-term mean water surface elevation (WSE) of each site (ranging from 128.48 m to 162.58 m amsl) was subtracted from the hourly readings to provide a normalized stage (above and below average reference height). Mean daily water levels and temperatures were calculated and plotted to examine temporal trends.