NPP Study: Quadrat field measurement data

Study number: 

11

Data set ID: 

210011002

Data Access: 

Unrestricted

Dataset status: 

Date range: 

1989-05-09 to 2010-09-24

Original investigator: 

Laura Huenneke

Data contact: 

Abstract: 

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). Attention: These data are not appropriate for estimates of percentage cover. NPP-associated percent cover measurements were developed for and are used solely to provide the best estimate of biomass production. Becuase the methodology results in measurements of overlapping subcanopy systems and canopies of adjacent individuals, NPP percent cover measurements are not an appropriate measure of actual aerial plant cover. Doing so will result in inflated numbers for the "actual" vegetative cover.

Methods: 

Methods of recording (field data sheets, instrumental, etc.):

field data sheets

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.

Quality assurance: 

Treatment of data

Data are entered into ASCII files by a PC FORTRAN data entry program (NPPQUAD.EXE); the resulting files (NPPQyys.DAT) are uploaded to the IBM mainframe. Each of these files is read in as input to a SAS program (NQBIOyys SAS, where yy is the year and s is the season) that contains regressions for biomass against plant dimensions (see harvest data and regression construction, data set documentation), updated for that season. first volume is calculated (cover*height*100), and then biomass (weight = volume*regression parameter*count) for each observation. Cover, volume, and biomass are summed for all observations of a given species in a single quadrat. Hard copies of the NQBIOyys SAS programs are archived in files with the output.

The resulting weights are sorted by species and can be summarized by site, by species, and/or by quadrat, as desired, by choosing different summary options in the NQBIOyys SAS program. Output can be the sorted data file, or can be written as separate DATA files for each site (used as input to the geostatistics program, GEOEAS).

The SAS program NQNPPyy SAS is used to merge the biomass files from individual seasons, and to calculate net primary production as the difference in total biomass between seasons. The input is two data files written by the above NQBIOyys SAS programs, with cover, volume, and biomass values for each species in each quadrat. NQNPPyy SAS merges the two data files by quadrat and species, and calculates the difference in weight between the two sample periods. If the increment is negative (e.g., a species is present at time 1 but absent at time 2), the difference is set to zero. A hard copy is printed, showing the difference is set to zero. A hard copy is printed, showing the site, quad number, species ID, weight at time 1, weight at time 2, and the increment. Then a summary is made, summing the net positive increments over all species for each quadrat -- the estimated net primary productivity for that quadrat over that interval. These values are written to a data file, NQNPPyyn DAT, where n represents the interval during the year (1 represents fall to winter, 2 represents winter to spring, 3 represents spring to fall). These productivity values can be summarized in whatever fashion is necessary (e.g., averaging over all quadrats).

Additional information: 

5 vegetation communities with 3 sites in each zone: creosotebush scrub: CALI, GRAV, SAND tarbush flats: EAST, TAYL, WEST grassland: BASN, IBPE, SUMM mesquite dunes: NORT, RABB, WELL playa: COLL, SMAL, TOBO See attached site map. C CALI NPP Creosotebush: Caliche site C GRAV NPP Creosotebush: Gravel site C SAND NPP Creosotebush: Sand site G BASN NPP Grassland: Basin site G IBPE NPP Grassland: IBP exclosure site G SUMM NPP Grassland: Summerford Mountain site M NORT NPP Mesquite: North Natural Revegetation Exclosure site M RABB NPP Mesquite: Rabbit Rain Gauge site M WELL NPP Mesquite: West Well site P COLL NPP Playa: College Playa site P SMAL NPP Playa: Small playa site (also known as Vine Mesquite Playa) P TOBO NPP Playa: Tobosa Tank site T EAST NPP Tarbush: East site T TAYL NPP Tarbush: Taylor Well site T WEST NPP Tarbush: West site

Maintenance: 

Winter, spring, and fall of each year

Long-term dataset: 

Signature dataset: 

LTER Core Area(s): 

Data category: 

LTER VI Proposal Category: 

DOI: 

10.6073/pasta/d65a72899a4294bc9e5e64158a6dbeae

Research Network: