## 2 Methods  
### Study Sites  
We collected data at two sites representing typical northern boreal and arctic peatland ecosystems. The first site was at the Alaska Peatland Experiment (APEX) at the Bonanza Creek Long Term Ecological Research Station, located 30 miles west of Fairbanks, Alaska (64.82\({}^{\circ}\)N, 147.87\({}^{\circ}\)W) (Figure 1). The site is located in a sedge- and shrub-dominated rich fen on the floodplain of the Tanana River. APEX was initiated in 2005 as a long-term manipulation to study the impact of altered hydrology on peatland ecology and biogeochemistry. APEX consists of three water table manipulation treatments: a raised,a lowered, and a control treatment, each 120 m\({}^{2}\) in size (Figure 2, left panel). The lowered treatment plot is drained by a trench that borders the plot. Water from the trench is then pumped into the raised water table treatment. The water table in the control plot is not manipulated. We performed field data collection within the APEX water table manipulation treatment plots.  
The second study site was at the Spruce and Peatland Response Under Changing Environments (SPRUCE) project located at the Marcell Experimental Forest in northern Minnesota (47.30\({}^{\circ}\)N, 93.29\({}^{\circ}\)W) (Figure 1). SPRUCE was initiated in 2014 as a climate change experiment to explore the effects of increased temperatures and elevated CO\({}_{2}\) levels on ecological and biogeochemical processes in  
Figure 1: Location of the Alaska Peatland Experiment (APEX) and Spruce and Peatland Responses Under Changing Environments (SPRUCE) peatland field sites. APEX is located in central Alaska. SPRUCE is located in northern Minnesota.  
Figure 2: (**Left**) Aerial image of APEX water table manipulation treatments and the surrounding area (photo credit: [PERSON]). (**Right**) Aerial image of SPRUCE experimental chambers (photo credit: US Department of Energy, Oak Ridge National Laboratory).  
peatlands at the southern edge of the boreal region (Figure 2, right panel). The experiment is located within an ombrotophic bog dominated by _Sphagnum_ mosses and black spruce (_Pica mariana_) with an understory of ericaceous shrubs, graminoids, and forbs. The SPRUCE experiment is a regression-based model in which ecological changes are measured in response to a broad range of temperatures [63]. Temperatures and CO\({}_{2}\) levels are manipulated in 10 chambered treatment plots (Figure 2, right panel). An additional three unenclosed control plots were also tracked in this experiment. The chambers consist of octagonal enclosures 12.8 m in diameter and 7 m tall. The enclosures penetrate completely through the peat soils down to mineral material below. Peat soils average 2.5 m in depth across the site. Ambient temperatures and CO\({}_{2}\) levels are maintained in one control chamber while another control chamber has ambient temperatures and elevated CO\({}_{2}\). The remaining eight treatment chambers are maintained at +2.25, +4.5 +6.75, and +9 \({}^{\circ}\)C above ambient temperatures to simulate a range of possible warmer climate scenarios. Carbon dioxide levels are maintained at ambient levels in half of the temperature treatment chambers and elevated to between 800 and 900 parts per million in the other half of the chambers. Each chamber, as well as the unchambered control enclosures, contain three 2-m\({}^{2}\) vegetation plots, where sampling was conducted for this experiment.