Quantifying Plant Water Use in the Native Coteau Rangelands of North Dakota

Methods of Field Measurements


The field measurement sites are located at the CGREC grazing intensity trials (Patton, et al., 1998; Kirby, et al., 1999). We selected six silty sites on three different grazing intensities for the field measurements: (1) moderately grazed pasture with and without drought treatment (sites 5D and 5C, respectively); (2) extremely grazed with and without drought treatment (sites 11D and 11C, respectively); and (3) two non-grazed control sites (6X and 11SX). Most of the ecophysiological measurements were conducted on these sites this year and will be continued in the next few years. Results will support the current plant water use study as well as the rangeland ecosystem modeling study.

  1. The Li-6200 Portable Photosynthesis System was used for measuring leaf gas exchange. The measurement was taken in full sunlight on three days during the growing season (in June, August, and September) for selected plants in each treatment. Of the many parameters measured by this equipment, stomatal conductance and water use efficiency (WUE) are especially useful in the current study. Depending on the richness of plant species on a particular grazing intensity treatment, about 10-16 representative plants (representing different plant functional types) were selected for stomatal conductance measurement. This same set of measurements will be used to estimate photosynthetic parameters used in the ecosystem model.
  2. Leaf area index was measured approximately at the same place as the gas exchange measurement. Representative plant communities were selected for measurement, (1) Kentucky bluegrass communities on ungrazed, moderate and extreme grazed treatments, (2) buckbrush communities on moderate and extreme treatments, (3) stiff goldenrod communities on moderate and extreme treatments. Leaf area index was estimated by measuring the total leaf area of each plant species inside a ¼ m2 frame. This includes spreading the leaves onto contact paper, scanning the leaf image into a computer and calculating leaf area by the Sigma Scan Pro 5.0 software. The measured leaf area index of different functional types was used to weight the stomatal conductance so that an averaged leaf conductance from a particular community can be obtained. This averaged value can be used in the revised energy-combination model (Shuttleworth and Wallace, 1985) to compute evapotranspiration.
  3. We collected soil samples from three sites: ungrazed, moderate and extreme grazed and sent the samples to the laboratory for testing: a) Soil texture. Samples were obtained at eight depths (6 in. intervals for first 12 in, and 12 in. intervals for the lower depth). For each sample, “sand” portion is sub-divided into five sub-classes; “silt” into four sub-classes; and “clay” into two sub-classes. Data was measured at NDSU soil and water environmental lab using the Pipet Particle Size Analysis with organic matter, carbonates and soluble salts removed. Total organic matter of each soil sample was determined (Table 1). b) Soil water characteristic curve. Samples from the same eight soil depths of the ungrazed treatment were used to determine the water potential versus water content relationship. For moderate and extreme treatments, only two samples were used in the analysis (one from 0-6 in.; the other from 30-36 in. These two layers have a large difference in organic matter content and soil bulk density presumably due to difference in root activity). In measuring soil water characteristic curves, organic matter, carbonates and soluble salts were not removed (Table 2). c) Saturated soil hydraulic conductivity. A hydraulic conductivity test was run on two samples (one from 0-6 in.; one from 30-36 in.) from each of the plots (ungrazed, moderately and extremely grazed). Both soil water characteristic curve and conductivity tests were done at the NDSU Soil Lab.
  4. The drought treatment will begin in 2002; however, for the year 2001 “drought” should be the same as “control”.

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