Site description and experimental design
This study was carried out in Carbon in Permafrost Experimental Heating
Research (CiPEHR) site, established in September 2008. This site is a
typical moist acidic tundra located in the northern foothills of
Interior Alaska (63°52’59”N, 149°13’32”W), near the Denali National Park
and Preserve of Eight Mile Lake. The annual mean temperature of the year
1976-2009 was -1.0 °C, with monthly mean temperature ranging from -16 °C
(December) to 15 °C (July). The annual mean precipitation of 1976-2009
was 378 mm. The soil type is
gelisol, with a 45-65 cm thick organic horizon covered by a mixture of
glacial till and loess. The active layer thickness is 50-60 cm. Dominant
plant species included the tussock-forming sedge Eriophorum
vaginatum (Cyperaceae) and the small deciduous shrub Vaccinium
uliginosum (Ericaceae). Other plants included Carex bigelowii(Cyperaceae), bryophytes Pleurozium schreberi (Hylocomiaceae),Sphagnum spp. (Sphagnaceae), and lichens Cladonia spp.
(Cladoniaceae).
The experimental soil warming
study was initiated in October 2008.
A
total of 6 snow fences (1.5 m tall × 8 m long) were established and
arranged into 3 blocks, which was ~ 100 m apart to
mitigate small-scale spatial variability. For each snow fence, the
warmed plot was located at the leeward side with 5 m distance from the
fence, and the paired control plot was located at the windward side with
8-14 m distance from the fence.
Snow
fences increased soil temperature via trapping insulating layers of snow
in the leeward side. For example, snow fences led to a threefold
increase in snow depth of warmed plots in 2009 (1.35 m in warmed plots
versus 0.38 m in control plots) (Natali et al., 2011).
Different from traditional snow
fence experiments, both snow and snow fences were removed before
snowmelt to ensure that soil hydrology, light conditions, and plant
phenology were not affected by extra snow.
On
March 8-15 of 2010, extra snow in warmed plots was shoveled to ensure
comparable snow cover depth between warmed and control plots. Snow
fences were removed at the time of snow removal to avoid shading of
plots during the growing season (Natali, Eag, & Rubin, 2012; Natali et
al., 2011). Snow removal allowed this experiment to focus on the effects
of soil warming.