Home
Project
Overview
Project
Constellation
People
Nebraska
Sandhills
Educational Outreach
 
Job Opportunities

Grassland Destabilization Experiment (GDEX)

click for larger view

Background

Range management research documents that plant species composition varies with topography, management and climate in upland Sand Hills grasslands. Studies through the drought cycle of the 1930’s also noted changes in the relative abundance of dominant C4 grasses, but surprisingly little loss of grass cover in the Sand Hills. However, few, if any, studies record the cycle of grassland loss, dune destabilization, and grassland recovery that the geologic record indicates happened at least once in the Holocene.

No Sand Hills studies have examined the impact of upland destabilization and recovery on the processes critical to this proposal: the coupled budgets of energy and water that drive both ground water recharge and canopy – atmosphere interactions. Studies elsewhere confirm that ET (latent heat flux) is the dominant sink for radiation energy in grasslands, if soil moisture is non-limiting. However, temporary (e.g. fire) or long-term removal of green leaf area by tillage or shrub invasion can dramatically decrease ET and alter the ratio of sensible to latent heat flux (i.e., the Bowen ratio). Estimates of potential evapotranspiration (ET) for upland Sand Hills grasslands are relatively high (between 3.4 and 4.2mm/day), similar to our summer wet meadow ET measurements from GSL. Actual ET must be considerably less than potential ET, however, becomes potential ET exceeds precipitation on an annual basis. Yet, the Sand Hills are a region of significant ground water recharge. How does a moisture-limited grassland “lose” so much water?

Several studies suggest that large intra- and interannual fluctuations in energy and water balance drive ground water recharge in semi-arid landscapes, but the complexities of runoff and soil hydraulic conductivity make predictions of recharge difficult. Although micrometeorological techniques (e.g., eddy covariance or Bowen ratio towers) have greatly advanced our measurement of water and energy fluxes between plant canopies and the atmosphere, the complexity of soil moisture dynamics has hindered comparable progress belowground. We suggest that the relative uniformity of upland dune sand throughout the Sand Hills simplifies belowground aspects of the basic water balance equation (St = St-1 + P – ET – R – D): runoff (R) is minimal, drainage (D) is tractable, and soil moisture (St) is readily measurable. Questions about the validity of micrometeorological tower methods in rolling topography, the cost of 10 installations, and the size of experimental plots needed for tower footprints (e.g., 25 ha) make the use of eddy covariance and Bowen ratio towers less attractive in our proposed upland experiment.


Research Questions

  1. How do evapotranspiration (ET), soil moisture (S), and drainage (D) change as the state of the uplands changes from continuous grass cover to bare sand?

  2. How does the energy balance (e.g., albedo, net radiation, sensible, latent and soil heat fluxes) respond as uplands change from continuous grass to bare sand?

  3. Do the intact litter layer and soil structure that persist during severe short-term disturbance (loss of the plant canopy for one growing season) significantly modify the effects of bare
    sand on water and energy balance? Do they allow rapid recovery of grassland functioning?

  4. How are the dynamics of soil moisture and drainage from the rooting zone coupled to the dynamics of water in the vadose zone and the water table?

Research Protocol

For this experiment, we will create 10 plots, each 120m x 120m (1.44 ha), in upland Sandhills grasslands that have been seasonally grazed for > 50 yrs at UNL's Barta Brothers Ranch. The site occurs on gently rolling dunes (5-15% slope, total relief <20m) with surface textures classified as sands and loamy sands (85-90% sand, soil organic matter 0.7 - 0.8% to 15cm depth). Vegetative cover ranges from 75-90% (i.e., bare sand <25%) and is dominated by C4 grasses (Andropogon hallii, Calamovilfa longifolia, Schizachyrium scoparium). Low woody shrubs (Amorpha canescens), C3 grasses, forbs, and cacti are common. Annual precipitation (40 yr mean) is 575mm, with 80% falling from April - September. Pre-treatment analyses of the area will be done in late May to early June of 2004 using the hyperspectral AISA airborne imaging spectrometer at 1-2 m resolution.

Small Swale Displaying Topological Variance at BBR

Because of spatial heterogeneity (e.g., slope, aspect, moist swales), a large number of possible plots will be screened and candidate plots containing a significant proportion of moist swales will be excluded. Ten plots for which the range and relative abundance of topographic conditions and vegetative cover are similar will be chosen and treatments randomly assigned. Prior to treatment, vegetation and soils data will be collected and each plot surveyed in order to construct a very fine resolution digital elevation model (DEM). Twenty permanent survey points per plot will be established (2m deep rebar posts permanently installed, surveyed and marked), at which aboveground plant biomass (clipped samples sorted to live and litter), LAI, % vegetative cover, and root biomass will be measured. Pre-treatment soils data in each plot will include several deep cores (up to 4m with a Giddings hydraulic corer) to identify horizons and soil structure that could influence moisture holding and drainage characteristics. At each survey point, 50cm deep soil samples will be collected and analyzed for total soil C and N, bulk density, and texture. The hyperspectral imagery combined with the ground data will enable us to construct a high-resolution pre-treatment "picture" of each plot. Treatments represent a range of states for uplands as well as transitions among the states.

Treatments

Grazed Control Plots - part of BBR's rotational grazing study. P9 and P10 are in different quarter section pastures on an 8 pasture rotational unit, so they are never grazed at the same time. Plot 9 contains SESM 1 (low) and ET4 (high, not on the GDEX data wet site). A number of measurements are made in the exclosure at the top of Plot 9 (referred to as Mt Billesbach). For simplicity you could refer to these as “grazed plots”.

Ungrazed Control Plots – A cattle exclosure around the plots was constructed in May 2004. Data collection started on the plots and SESM stations were constructed during 2004. Plot 2 contains SESM 3 (high) and SESM 7 (low), while Plot 4 contains SESM 4 (high) and SESM 8 (low). These plots will remain ungrazed, but have received no herbicide or physical disturbance. For simplicity, you could refer to these as “control plots”.

Long-Term (Press) Disturbance - A cattle exclosure around the plots was constructed in May 2004. Pre-treatment data collection started on the plots and SESM stations were constructed during 2004. The plots were killed with herbicide (mainly roundup) in mid-May 2005. No physical disturbance of the plots occurred. These plots have been kept dead with periodic herbicide treatment in 2005 and 2006, and will remain dead for at least 5 years. Intensive measurements are being in made in the plot to document how and when the sand surface destabilizes. Plot 1 contains SESM 2 (high) and SESM 6 (low). Plot 8 contains SESM 5 (high) and SESM 9 (low).

Short-Term (Pulse) Disturbance - A cattle exclosure around the plots was constructed in May 2004. Pre-treatment data collection started on the plots during 2004. These plots do not contain SESM stations. The plots were killed with herbicide (mainly roundup) in mid-May 2005. No physical disturbance of the plots occurred. These plots have been kept dead with periodic herbicide treatment in 2005, but received no herbicide treatment in the spring of 2006. During 2006, a dramatic growth of annual weeds (mainly lambs quarters) has occurred. In 2007, recovery of the vegetation will continue. Plot 3 and Plot 6.

Aggressive Bare Sand - A cattle exclosure around the plots was constructed in May 2004. The plots were herbicided (roundup) in May 2004, followed by shallow disking and raking (with an ATV) to remove dead vegetation. The plots do not contain SESM stations, and no pre-treatment data were collected. These plots have been kept dead with periodic herbicide treatment in 2005 and 2006, and will remain dead through 2007.

Plots

This section coming soon! To include photographs showing how each plot progresses after treatment.