Hidden Canyon ecohydrology experiments

Hidden Canyon is the site of experiments and measurements designed to elucidate the importance of snowpacks and snowpack variability on the cycling of water through subalpine forest ecosystems. Snowpack meltwater is the dominant source of water availabe for soil moisture recharge and plant water use during the year, but the amount and timing of this water resource is set by some interaction between snowpack size, snowmelt speed and timing, infiltration of meltwater into soil, and plant transpiration. These aspects of snowpack dynamics and ecosystem response have high spatial and interannual variability.

Some research objectives

These experiments use artificial perturbations of the snowmelt cycle along with natural spatial and interannual variability to explore:

  • The influence of dust deposition on forest snowpack dynamics.
  • Seasonal, interannual, and spatial patterns of snow accumulation and melt in a subalpine forest.
  • Seasonal patterns in soil moisture availability and water availability for trees under varied snowpacks and snowmelt cycles.
  • The response of soil organisms to varied snowpack dynamics.

Hypotheses

  • Enhanced dust deposition on a forest snowpack surface leads to an accelerated snowmelt cycle, higher rates of sublimation water loss, and reductions in ecosystem water availability.
  • Early snowmelt (whether from dust or natural variability) in an area will lead to more rapid depletion of soil moisture and longer periods of low tree xylem water potentials during the following growing season.
  • Seasonal patterns in soil moisture and xylem water potentials will be similar in low snowpack and early melt scenarios.

Experimental design

The main experimental treatment for this experiment is the addition of dust to the snowpack surface with the intention of accelerating the snowmelt cycle. We expect the responses in the hypotheses above to occur as a result. The experiment's plot and sensor designs are desribed in:

Design changelog

  • Only plots 1 and 2 were operational from Nov 2009, to Oct 15, 2010. Plot 1 was a control and plot 2 a treatment plot. See the snowmel design pagefor a detailed layout. Each plot had 4 soil moisture profiles and two soil temperature profiles during this time.
  • Xylem water potential and soil respiration) was measured during the 2010 growing season. Measurements were made on 9 large trees in each treatment. We found little difference between control and treatment and are considering switching to smaller trees for 2011.
  • Moisture and temperature profiles 5 (in lower plot 3) and 7 (lower plot 4) were installed and functional on October 15, 2010.
  • Plots 3-6 were set up and measured for the 2011 snowmelt season. Currently plots 2, 4, and 5 are dust additions and 1,3, and 6 are controls.

Methods

Measurement details and logs

SWE and melt rate at Hidden Canyon

Snowpack depth and SWE are measured prior to each application of dust. Measurements are made using a Federal Snow Sampler. 10 or more locations are measured in Control and dust addition plots, and these measurement locations are roughly the same (within 2m) at each measurement date.

Melt timing

Presence or abscence of snow at each 10m grid point in the HC forest (80 x 80m grid) was recorded at each spring visit during the snowmelt season. The proximity of snow was also noted. This gives an approximate date of snow disappearance for each grid point, and these dates may be related to canopy density, slope position, or other spatially linked influences on snowpack energy balance. This method is still being developed.

Regional SWE and melt rate

Weather, climate, and snowpack/snowmelt data from nearby SNOTEL sites or other stations in the region can provide important context for the Hidden Canyon experiments.

Soil water and plant xylem pressure

Xylem water potentials of are measured using a pressure the pressure bomb method using a PMS instrument. All measurements are made on Abies lasiocarpa twigs from trees in a particular size class (large trees in 2010, probably saplings in the future). Twigs are excised from south-facing (downhill) branches at a consistent height above the ground. These twigs are stripped of phloem at the cut end and measured in the chamber within 10 minutes of being cut. Midday measurements are made between 1 and 5pm, and predawn measurements are made between 4 and 7am on consecutive days. Days immediately following rain events are avoided.

Snow surface measurements

Several other measurements are relevant to snowmelt and ecosystem water availability, and they can demonstrate the influence of added dust on these ecosystem processes. They include sublimation, ambient dust loading, and albedo measurements of the snow surface. Not all of these have well defined measurement methods yet, but development of these methods is important.

Other logs

  • Met measurements: meteorological measurements at Hidden Canyon towers.
  • Soil profiles: soil temperature and soil moisture data from the control and treatment plots.

FIXME

Hidden Canyon carbon cycling

Experimental manipulations of the below-canopy snowpack are taking place at the Hidden Canyon site and this is expected to influence soil water availability and temperature in the forest in both winter and the growing season. These changes in the biophysical drivers of carbon cycling are expected to influence rates of soil respiration and organic matter decomposition.

Some research objectives

  • To understand seasonal and interannual patterns of soil respiration and decomposition in Wasatch Mountain forests and how these patterns are determined by seasonal snowpacks.
  • To measure differences in soil CO~2~ fluxes and litter decomposition between dust addition (early melt) and control snowpack treatments.
  • To study the response of soil organisms to varied snowpack dynamics.
  • To understand the relative importance of winter vs. growing season periods for soil carbon cycle processes.

Hypotheses

  • Soil respiration (CO~2~ efflux) and decomposition (mass loss) are limited by moisture availability for a longer portion of the growing season in low snowpack or early snowmelt years (or areas within a landscape).
  • Areas (or years) with early snowmelt or smaller snowpack accumulations will have reduced growing season soil CO~2~ fluxes and annual litter mass loss compared the opposite case.
  • Accelerated snowmelt will lead to earlier peaks in below-snow soil CO~2~ fluxes during the transition from winter to spring.
  • A shorter snowcovered period will reduce the amount of litter decomposed in winter and the growing season.

Experimental design

The main experimental treatment for this experiment is the addition of dust to the snowpack surface with the intention of accelerating the snowmelt cycle. We expect the responses in the hypotheses above to occur as a result. The experiment's plot and sensor designs are desribed in:

Design changelog

  • Only plots 1 and 2 were operational from Nov 2009, to Oct 15, 2010. Plot 1 was a control and plot 2 a treatment plot. See the snowmelt design pagefor a detailed layout. Each plot had 4 soil moisture profiles and two soil temperature profiles during this time.
  • Soil respirationwas measured during the 2010 growing season at 10 collars in each plot (1 & 2). We found little difference between control and treatment and are considering adding more collars for 2011.
  • Moisture and temperature profiles 5 (in lower plot 3) and 7 (lower plot 4) were installed and functional on October 15, 2010.
  • Snow inlets (6 inlets in plots 1 & 2, 3 inlets in plots 3 & 4) and litterbags (6 sets in each of 6 plots) were deployed in early October 2010.
  • Plots 3-6 were set up and measured for the 2011 snowmelt season. Currently plots 2, 4, and 5 are dust additions and 1,3, and 6 are controls.

Methods

Measurement details and logs

Growing season soil respiration

Growing season soil CO~2~ fluxes are measured at collars using the Li-Cor 6400. Collars are installed in control and treatment (early-melt) plots each spring and are measured roughly every 2 weeks. Measurements are made in the during the morning hours (between 9am and noon) and measurement locations alternate between control and treatment collars so that soil temperature increases during the measurement period occur evenly between treatments.

Below-snow soil respiration

Below-snow soil CO~2~ efflux is measured in winter and spring using an inlet and tubing system. Inlets are placed on the soil surface in control and treatment plots each fall, in association with soil sensor profiles. Each inlet is tubed to a central measurement location between plots. To collect a sample, below-snow air is pumped from the inlet to the sampling assembly with a pump. Because tubing lenghts vary, the timing of pumping must be adequate to clear the tubing of stagnant air and bring the snowpack air to the sampling fitting. Samples are transfered into an evacuated Labco Exetainer with a syrnge, needle, and septa. Three samples of ambient air (above snow) are also collected.

Litterbag decomposition

Measurements of litter decomposition are measured, as litter mass loss, twice a year at Hidden Canyon. Mass loss measurements are made using litter bags recovered in spring (to measure winter decomposition) and in fall (to measure growing season decomposition). This allows a comparison of winter versus summer decomposition. Measurements are made in both the Treatment (dust addition) and Control (natural dust loading) plots, and the snow-free date is recorded for each set of bags every spring.

Other logs

  • Met measurements: meteorological measurements at Hidden Canyon towers.
  • Soil profiles: soil temperature and soil moisture data from the control and treatment plots.