Methodology

Field sites and design: Grids of permanent 1 square meter quadrats have been established in 15 sites: three sites in each of 5 community types or zones (grama grassland, creosote bush scrub, tarbush flats, mesquite dunes, playa). Grids consist of 49 quadrats arranged in a square 7 x 7 pattern, with quadrats 10 m apart. (Exception: P-COLL, the College Ranch playa, has 48 quadrats arranged in a 3 x 16 pattern.) Quadrats are permanently marked with aluminum nails inserted into the soil at two diagonal corners; quadrats are identified by nearby rebar stakes and tags (numbered 1-49, starting at the northwest corner and wrapping back and forth in a north-south direction).

Field sampling: Standing biomass is sampled three times a year: in winter (February - March), before shrubs begin spring growth; in spring (May), when shrubs and spring annuals have reached peak biomass; in fall (late summer; October), when summer annuals have reached peak biomass but before killing frosts. At each sample date, each site is visited (order of sampling may vary, according to phenological stage of sites) and the dimensions of each plant on each quadrat are measured and recorded. Recorded for each observation are: date, zone, plot, quadrat #, species (4 letter acronym), observation # (one for each measurement of that species in that quadrat), cover (percentage of quadrat covered by canopy of that individual or species), height (vertical extent of that individual or species), count (if multiple individuals with the same dimensions are present), and phenological stage (Flowering/fruiting or Vegetative). Plants rooted outside but reaching into the quadrat are measured in terms of their vertical and horizontal extent only in the rectangular volume above the quadrat. Large plants (e.g., shrubs) are broken down into individual branch systems or other sub- volumes and recorded as separate observations. Early sample periods included measurements of diameter, rather than cover, for some species; see data files from those periods. Cacti are measured for number of pads (count), average length of pad (height), and average width of pad (diameter).

From these dimensional measurements, volume can be calculated for each species in each quadrat. A regression of biomass against volume is constructed by harvesting plants near but outside of the grid of permanent quadrats. Dominant species, and others showing significant differences among sites in past regressions, are harvested from each site; for other species, samples may be taken in smaller numbers at any one site and samples from different sites pooled in the regression (see species list). Sample sizes are 15 for dominants, 10 for most other species (see species list). Range of cover values to be harvested is based on the range of cover values recorded from the permanent quadrats. Plants to be harvested are located in situations typical of those encountered in the permanent quadrats, and their cover and height values measured as for the quadrats. Volumes are harvested, returned to the laboratory, held in a cold room (4 degree Celsius), sorted (dead material discarded, unless obviously from the current season of growth), dried (55 degree Celsius), and weighed. Dominant shrubs (Larrea, Flourensia, Prosopis) are separated into leaves, stems, and reproductive material (either before or after weighing of total biomass); these components are weighed separately. Tissue samples are selected and ground for nutrient concentrations or for archival.

Harvest data are used to build regressions of biomass against volume. Quadrat data are used to estimate plant volume by species by quadrat, and then total biomass on a quadrat; positive increments in biomass over a time interval provide estimates of productivity for that quadrat. See flow chart for summary of analysis.

See NPPFIELD.PRO for detailed field sampling protocol.