Data Package Metadata   View Summary

Snow Manipulation: To determine the quantity of snow to add or remove to achieve the target change in date of snowmelt (date when experimental plots were completely free of snow), we measured SWE at each of the six experimental sites prior to snow manipulation. We calculated reference plot SWE using a federal snow sampling tube (Rickly Hydrological Company, Columbus, OH, USA) in 10 locations per plot. The mass of snow samples was calculated as the difference between the mass of the empty and full snow sampling tube in kg. SWE was calculated by multiplying the mass of snow by average density of snow (35.27 kg inch-1) and dividing by 2.54 cm inch-1 to determine SWE in cm (Hardy et al. 2001). At each of the six 'early snowmelt' plots, a separate snow pit was excavated immediately adjacent to each plot. In this area outside the experimental plots, a snow pit was excavated to ground level, and a 25 cm X 25 cm column of snow was removed using an avalanche shovel and snow saw. The snow that was removed was then marked at 10 cm depth intervals, and each 10 cm depth X 25 cm X 25 cm block was weighed separately using a hanging scale. We then calculated how much snow needed to be removed to halve the SWE of each plot to the nearest 0.5 cm, with the goal of accelerating snowmelt by one week. Snow was then manually removed from each of the 'early snowmelt' plots until the target depth was uniformly achieved to obtain 50% SWE across each plot. The snow that was removed was placed a minimum of 1-2 m away from the plots.

For the 'late snowmelt' plots, we created a 1-m wide buffer strip on the outside perimeter of these six plots to minimize plot disturbance. Snow was then manually removed from the 2-meter-wide strip beyond the 1-m plot perimeter and evenly added to the six 10 X 10 m2 'late snowmelt' plots to double the SWE with the goal of delaying the onset of snowmelt by at least one week. After snow removal (for 'snowmelt acceleration' plots) and addition (for 'snowmelt delay' plots) was completed in each of the 12 treatment plots, a federal snow sampling tube was used to sample and weigh snow samples from ten randomized locations throughout each of the 12 treatment plots to confirm we effectively doubled or halved SWE.

Soil Frost and Snow AUC: Soil frost was measured using soil frost tubes inserted 50 cm into the soil according to methods described in Ricard et al. (1976). Soil frost tubes contain transparent tubing with water and methylene blue dye, which is displaced from solution when water freezes. Frost depth is measured as the length of the dye-free ice below a pre-marked ground line. Snow depth was measured manually using a meter stick within one meter of each frost tube. The depth and duration of snow and soil frost were integrated over the entire winter and snowmelt period, respectively, to calculate the area under the curve (AUC) parameter. AUC was measured in 'cm days,' with daily snow and soil frost totals calculated by linear interpolation and represents the cumulative total of snow and soil frost depth over time (Duran et al. 2014; Reinmann et al. 2019). We report snow and soil frost AUC throughout each winter (Nov 19, 2021 - April 25, 2022 and Nov 15, 2022 -April 25, 2023) and during the post snow manipulation treatment period only (March 8 to April 25, 2022 and March 6 to April 25, 2023).

Root Biomass: We installed eight root ingrowth and eight exclusion cores in each of the 18 plots (288 cores total) to evaluate root growth, soil solution N, and lab-based N cycling rates over time. Root ingrowth and exclusion cores were constructed using rigid plastic nylon mesh and heavy fishing line to be 15 cm tall and 5 cm diameter. All cores had 2 mm openings to allow fine roots to grow into the cores. Root exclusion cores were covered with ultra-fine mesh (0.05 mm openings) to exclude fine roots, but allow fungal hyphae, water, and air to pass through. At the time of field installation but prior to addition of soil to ingrowth and exclusion cores, we placed one ion-exchange resin bag at the bottom of each core. See below for details about construction of ion exchange resin bags. We added 230 grams of homogenized soil from each of the six field sites just above each ion exchange resin bag in all ingrowth and exclusion cores at each site (n = 24 ingrowth and 24 exclusion cores per site; 288 total across 18 plots) to match the bulk density of these sites. To create the homogenized soil at each of the six field sites independently, soil pits were excavated to 15 cm depth (including both organic and mineral horizon soil) and soil was homogenized in the field using an 8 mm sieve before adding 230 grams root-free soil to each core. All ingrowth and exclusion cores were installed in October 2021. Half of the cores were harvested from each plot in June 2022 and the remaining half incubated for an additional year and were harvested in July 2023.

After harvest, soil and roots were removed from ingrowth and exclusion cores in the laboratory at Boston University. To quantify root biomass, all fine roots (2 mm diameter or less) were hand-picked using a combination of a 2 mm brass sieve for larger fragments and DI water to remove smaller root fragments. After rinsing soil from root surfaces, roots were weighed wet, then dried for a minimum of 48 hours at 50 degrees C and reweighed. Root biomass per unit volume was calculated using dry mass of roots. Root biomass was significantly higher in root ingrowth than exclusion cores after both one (2022 harvest) and two-year (2023 harvest) incubations (p < 0.01; Fig. 4A), indicating the exclusion mesh worked as intended.

Soil Solution N: Soil solution NH4+ and NO3- were measured using the ion exchange resin bags installed at the bottom of each root ingrowth and exclusion core (Templer et al. 2005; Giblin et al. 1994). Resin bags were made by filling air and water permeable nylon bags with 10 g mixed anion and cation (50:50) exchange resin (Monosphere MR-3 beads), which were rinsed with Millipore water prior to filling. One resin bag incubated at the bottom of each root ingrowth and exclusion core in the field for 9-10 or 21-22 months. After cores were harvested in June 2022 or 2023, each ion exchange resin bag was removed, transported to the lab, weighed, and extracted three times with 50 mL (150 mL total) of 2M potassium chloride solution (KCl). Samples were shaken in 2M KCl solution for three rounds of 30 minutes (90 minutes total) and filtered using Whatman #1 filter paper. Resin extract solutions were analyzed for NH4+ and NO3- concentrations using the microplate method (see below for details). Soil solution N results from ion exchange resin bags are reported after one year of incubation only (2022 harvest). Cumulative NH4+ and NO3- values per gram resin during the second year of incubation (2023 harvest) were lower than after one year, indicating exchange sites on the resin bags likely saturated between the first and second year of incubations and are therefore not suitable for reporting.

Soil N Cycling: Rates of net mineralization were measured in the laboratory on fresh soil taken from each root ingrowth and root exclusion core within 48 hours of harvest in June 2022 or 2023. Fresh soil from each core was sieved using a 2 mm brass sieve. A ten-gram subsample of sieved soil was extracted with 60 mL of 2M KCl, shaken for one hour, and a second 10 g subsample of sieved soil incubated in the laboratory for thirty days before extraction with 2 M KCl. Gravimetric soil moisture was determined by weighing a separate 10 g subsample of freshly sieved soil, drying the subsample at 50oC for 72 hours, and recording the dry weight. The difference between the initial and final NH4+ and NO3- concentrations were divided by the duration of the sample laboratory incubation and by percent soil moisture to calculate net ammonification and nitrification, respectively. Net mineralization was further calculated as the sum of net ammonification and nitrification over time.

Quantification of Ammonium and Nitrate: Potassium chloride (KCl) extract solution from resin bags (soil solution samples) or soil samples (N cycling samples) were pipetted into 96-well plates with ammonium nitrate (NH4+ analysis) or potassium nitrate (NO3- analysis) standards and external quality controls (ERA Environmental #985, Ammonium as 1000 mg N L-1; ERA Environmental #991, Nitrate as 1000 mg N L-1). Standards and quality control solutions were made with either 2M KCl in resin extract solution (for resin bag samples) or 2M KCl (for N cycling samples) to match the background matrix for sample solutions for analysis. To quantify NH4+ in solution, citrate, sodium nitroprusside, and sodium hypochlorite reagents were added in succession to sample extracts, standards, and quality controls. Samples incubated for 30 minutes and were analyzed on a light absorbance microplate reader (Versa Max tunable microplate reader), with the color intensity of each sample was read at 667 nm wavelength (Ali & Lovatt 1995). The range of detection for NH4+ in solution was 0.1 - 10 mg N L-1. To quantify NO3- in solution, vanadium chloride reagent was added to samples and incubated for 16 hours, then analyzed for color intensity at 540 nm wavelength (Doane & Horwath, 2003). The range of detection for NO3- in solution was 0.025 - 10 mg N L-1.

Foliar N and 15N: Sunlit foliage from the central A. saccharum tree in each plot was collected using the shotgun sampling method. Briefly, upper canopy branches with sunlit leaves were shot down using a 12-gauge shotgun loaded with 3-inch, shot size 3 non-toxic pellets in early August of 2022 and 2023. Approximately 20 leaves were collected from each tree, with as few branches shot down as possible to minimize disturbance (~1-4 branches per tree). Leaves were collected (n = 10 for foliar chemistry), air dried, ground using a Wiley mill, and analyzed using an isotope ratio mass spectrometer for C, N, and 15N content. Samples were sent to the Cornell Stable Isotope Laboratory for analysis. Subsamples of sieved soil from each root ingrowth and exclusion core were oven dried at 50C and analyzed for C, N, and 15N on an isotope ratio mass spectrometer. Soil and foliar 15N values were calculated as the 15N:14N ratio in a sample divided by the 15N:14N ratio of atmospheric N2, subtracting 1 and multiplying by 1000.