We established two experimental study designs to examine the effects of in situ manipulation of environmental conditions on nutrient composition and production of the goose forage species Carex subspathacea. The first design (temperature-thaw) involved temperature and timing of spring snow thaw manipulations paired with weekly vegetation sampling in which new plots were selected annually. The basic goal of the temperature-thaw manipulations was to simulate historic conditions (i.e., later thaw, colder summers) and future conditions (i.e., earlier thaw, warmer summers) to compare with current unmanipulated conditions. In the second design (multi-year) we manipulated temperature and maintained the established plots for 3 years, but restricted vegetation sampling to 3 occasions per year. The temperature-thaw design allowed for detailed analysis of temperature and spring thaw effects on forage characteristics within a given year whereas the multi-year design allowed us to assess the potential cumulative effects of temperature manipulations and restricted grazing.
The temperature-thaw study was a blocked design consisting of 3 temperature treatments (exclosed, greenhouse, shadehouse) crossed with 3 spring thaw treatments (advanced, natural, delayed) for a total of 9 plots at each site. Grazing effects were added to this design by establishing plots that were identified but otherwise unmanipulated and were available to be grazed by geese. Exclosed plots were fenced with 2 cm plastic mesh deer netting attached to a structure (120 x 120 x 60 cm) constructed of 1.25 cm pvc pipe; these structures were designed to exclude grazing without altering temperature or precipitation regimes. Greenhouse plots were surrounded with semi-clear fiberglass Open-Top-Chamber greenhouses (OTCs) that have been used previously to increase ambient temperatures without impeding normal precipitation input. Our OTCs were conical in shape, measuring 68 cm in height with a diameter of 200 cm at the base and 120 cm at the top. Shadehouse plots were shaded with 10% red shade cloth suspended 30 cm above ground by a structure constructed from 2 cm steel pipe with a footprint of 365 x 380 cm. Red shade cloth reduces the spectrum of blue, green, and yellow light and increases the red and far-red light spectrum, simulating solar radiation typical of an overcast day. Thus, this approach was intended to decrease solar input and heating without restricting precipitation input. Spring thaw treatments were conducted by manipulating snow depth associated with plots during late spring, approximately 2 weeks prior to breakup and before considerable snowmelt had occurred on the landscape. For advanced spring thaw plots, we removed snow from a 5 x 5 m area to promote soil thaw and expose the vegetated surface to sunlight. For delayed spring thaw plots, we piled snow to a height of 1 m over a 5 x 5 m area in order to slow the rate of soil thaw and limit sunlight penetration. Snow depths were unmanipulated in natural spring thaw plots.
We used data loggers (HOBO U-12, Onset©) to record soil and air temperature in all plots on 15-minute intervals. Soil temperature sensors were positioned at a depth of 6 cm below the soil surface; air temperature sensors were located inside vented solar radiation shields and positioned at a height of 10 cm above the soil surface. Temperature sensors and temperature treatment structures were put in place when snowmelt had exposed surface vegetation, but prior to substantial soil thaw. Large-scale flooding associated with spring breakup delayed the placement of treatment structures and temperature sensors for some sites.
Plots measured 120 x 120 cm and were sub-divided into sub-plots measuring 20 x 20 cm; these sections were separated from each other by a 6 cm buffer. On approximately 7-day intervals, beginning at the onset of the growing season, we randomly selected a section from each plot for vegetation sampling. We removed the 20 x 20 cm section to a depth of 15 cm. All above-ground biomass was then clipped from the turve using scissors, rinsed in fresh water, and transported to a field camp where samples were dried at 50°C for 48 hours and stored for later processing. Turves were returned to the substrate within their respective plots after vegetation sampling. Following completion of the field season, we sorted vegetation samples to separate live from dead material, dried samples to a constant mass at 50° C, then weighed samples on a digital scale to the nearest 0.001 g. A sub-sample (0.5-1.0 g) of live material from each sample was analyzed for percent nitrogen using a C-N analyzer at the University of Alaska Anchorage stable isotope laboratory. We replicated the experiment at 2 sites in 2011, and at 3 sites in 2012-2013 with new sites and plots being selected each year.
For the multi-year design, we established 6 sites, each consisting of four plots (one plot for each temperature treatment) that were maintained over the 3-year study period. Temperature sensors and treatment structures were put in place when snowmelt had exposed surface vegetation, but prior to substantial soil thaw. Vegetation sampling methods for multi-year plots were the same as described above, except that multi-year plots were sampled only 3 times per year.