DOI: 10.18739/a2t14tr08
title: Environmental and vegetation control on active layer and soil temperature in an Arctic tundra ecosystem in Alaska

description: The sites of this research are near Utqiaġvik, Alaska (formerly known as Barrow), which is the largest town in the Alaskan North Slope Borough. Our team has and continues to maintain several micrometeorological and eddy covariance towers in Utqiaġvik over the last decades (Zona et al. 2016). This research was conducted near two of these eddy covariance towers, which were in operation from 2005-2009 (Fig. 1b, the US-Ben (North), and US-Bec (Central) sites, established during the Biocomplexity Experiment, see Zona et al. 2009; Zona et al. 2012). Environmental drivers (such as air temperature, and photosynthetic active radiation (PAR)) measured from an additional currently operational tower (US-Bes), in close proximity to the US-Ben and US-Bes sites, were also included in this study. The locations of the US-Bes, US-Bec, and US-Ben are: 71.2809N, -156.5965W; 71.28316N, -156.60342W; and 71.28628N, -156.60424W respectively (Zona et al. 2009). These sites are located in a drain lake basin ecosystem and a mixed polygon wet sedges ecosystem characterized by mosses, lichens, and graminoids with patches of water and partially to fully submerged patches of vegetation (Davidson et al. 2016). Given the proximity of these sites (US-Ben and US-Bec being within 662-meters and 356-meters of US-Bes respectively), we assume that the air temperature and PAR collected in US-Bes were representative of the US-Ben and US-Bec sites. Access to US-Bes, US-Bec, and US-Ben sites was facilitated by the establishment of boardwalks during the Biocomplexity experiment in summer 2005 (Zona et al. 2009). These boardwalks allowed sampling across the sites while limiting disturbance. Data was collected every two meters in both US-Ben and US-Bec across 124-meters transects that parallel historical water table data collection (Zona et al. 2012), for a total of 62 plots in US-Ben and 62 in US-Bec. The data collection was performed at 2-meter intervals along two 124-meter transects at US-Bec and US-Ben (Fig. 1b). In each of these plots we recorded the thickness of the total moss layer and the thickness of the green living moss layer. A previous in-depth vegetation analysis from our team showed that the general region where this data was collected was highly homogenous in its vegetative structure being composed primarily of graminoids as the dominant vascular plant group and mosses such as Sphagnum sp. and Drepanocladus sp. (Davidson et al. 2016). We also collected data on the dominant moss genus at every point and found Sphagnum sp. and Drepanocladus sp. to be the dominant genera at our sites with most of the plots being Sphagnum sp. (n = 55) or some combination of predominantly Sphagnum sp. and Drepanocladus sp. (n = 46) and a limited number of plots with only Drepanocladus sp. (n = 21). The moss layer thickness and genus identification were recorded in sections of approximately 25-square cm (5 cm x 5 cm) in each of the 2-meter plots only once (i.e., the first week of July in 2021 for all plots and a subset of these plots on the first week of July in 2022) during the peak season (i.e., the first week of July to the second week of August 2021) to reduce the disturbance to the moss layer. These sections were carefully removed using a serrated knife trying to limit damage to surrounding vegetation. Afterwards, the thickness of the entire moss mat was measured with a ruler, and the samples were reinserted into the hole created by sampling. In each of the 62 plots we also recorded moss and soil temperature every cm from 1-cm below the surface until 20-cm below ground on a weekly basis using type T thermocouples connected to a CR3000 datalogger (Campbell Scientific, Logan, UT, USA). These temperature profiles allowed us to determine how the presence and thickness of the moss layer affected the thermal difference across the moss and soil layers. These 21 thermocouples were attached to a fiberglass probe, which facilitated insertion in the moss layer and soil. Each point was measured for approximately 3 minutes as the temperature readings stabilized within the first couple of minutes. We also collected soil water content (percent water) weekly in the first 5-cm of the moss or soil layer using a FieldScout TDR300 (Spectrum Technologies, Aurora, IL, USA) and 5-cm rods (Beringer et al. 2005; Hayashi et al. 2007; Hrbáček et al. 2020). The FieldScout was calibrated using local water samples to account for nutrients which may influence conductivity. Thaw depth and water table levels (cm) were also collected weekly in each of the sampling plots using a metal and wooden probe respectively with markings indicating intervals of 1-cm depths. Water table measurements were collected inside PVC pipes (with holes every 1 cm) previously installed along the transects (Zona et al. 2009; Zona et al. 2012) in each of the sampling locations. This data collection was repeated for a second field season in a subset of the plots (n = 20 in Summer 2022 vs. n = 124 in Summer 2021) to reduce disturbance to the site, but both samples were compared for consistency in the thickness of the moss layers across different field seasons. This comparison showed good agreement between the measured moss thickness in both seasons (R2 = 91.82%, p-value less than 0.001, 2022 Moss Thickness (cm) = 1.01967 * 2021 Moss Thickness (cm) + 0.228272). Environmental variables collected by the eddy covariance US-Bes tower, included PAR, air temperature, local surface and subsurface soil temperature, relative humidity, wind speed, and net radiation. Elevation above sea level was collected in each of the sampling plots at US-Ben and US-Bec as reported in Zona et al. (2012). These measurements described the microtopography of each of the sampling plots and let us test its influence on the environmental conditions, vegetation, and active layer development.