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NPP Grassland: Montecillo, Mexico, 1984-1994, R1
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Revision date: April 22, 2015

Summary:

This data set provides three data files in text format (.txt). One file contains monthly biomass measurements and net primary productivity (NPP) estimates made between June 1984 and December 1994 on an ungrazed saline grassland (MNT1) that was accidently burned in February 1986 at the Colegio de Postgraduados field station site, Montecillo, Mexico. The second file contains a shorter time series of monthly biomass measurements and NPP estimates on an adjacent saline grassland (MNT2) that was burned in May 1989, probably by local farmers following normal burning practices. Both files also contain above-and below-ground dead matter decomposition rates. There are data gaps in both files. The third file contains monthly and annual climate data for the period 1963-1989 from the Chapingo meteorological station located five km northeast for the study area.

Annual NPP at MNT1 was calculated for 1985-1987 to determine impacts of the unexpected fire. Above-ground NPP (NPP) averaged 669.2 g/m2/yr despite post-fire decline and recovery and variable rainfall over the period. Below-ground NPP (BNPP) was higher, averaging 1,007 g/m2/yr, but with a similar post-fire decline and recovery pattern. Total NPP reflected the yearly variation in ANPP and BNPP, averaging 1,676 g/m2/yr over the three year period. Monthly ANPP and BNPP values for MNT2 were lower overall but were not analyzed to determine annual trends.

Revision Notes: Only the documentation for this data set has been modified. The data files have been checked for accuracy and are identical to those originally published in 1996.


burning Montecillo


Figure 1. Burning of the Montecillo grassland site, Mexico. (Fire probably started by local farmers following normal practice every 4-5 years. Photograph taken May 1989 by Dr. E. Garcia Moya, Colegio de Postgraduados, Chapingo).

Additional Documentation

The NPP data collection contains field measurements of biomass, estimated NPP, and climate data for terrestrial grassland, tropical forest, temperate forest, boreal forest, and tundra sites worldwide. Data were compiled from the published literature for intensively studied and well-documented individual field sites and from a number of previously compiled multi-site, multi-biome data sets of georeferenced NPP estimates. The principal compilation effort (Olson et al., 2001) was sponsored by the NASA Terrestrial Ecology Program. For more information, please visit the NPP web site at http://daac.ornl.gov/NPP/npp_home.shtml.

 

Data Citation:

Cite this data set as follows:

Garcia-Moya, E. 2015. NPP Grassland: Montecillo, Mexico, 1984-1994, R1. Data set. Available on-line [http://daac.ornl.gov] from Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, USA. http://dx.doi.org/10.3334/ORNLDAAC/413

This data set was originally published as:

Garcia-Moya, E. 1996. NPP Grassland: Montecillo, Mexico, 1984-1994. Data set. Available on-line [http://daac.ornl.gov] from Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A.

 

Table of Contents:

 

1. Data Set Overview:

Project: Net Primary Productivity (NPP)

This work was carried out under the United Nations Environment Programme (UNEP) project on "Primary Productivity and Photosynthesis of Semi-Natural Ecosystems of the Tropics and Sub-tropics." The study commenced in 1984, and forms part of an ongoing project through support from UNEP and many other sources. Five sites were chosen for the project: a dry savanna in Nairobi National Park, Kenya; a monsoon savanna in the Klong Hoi Khong field site, Thailand; a saline grassland in the Montecillo field site, Mexico; a floodplain grassland on the island of Marchantaria (near Manaus), Brazil; and a bamboo stand in Miao Shan, China.

Measurements of above-ground live phytomass and mortmass at Montecillo MNT1 were made on the 15th day of each month nearly continuously over the 1984-1994 time series (apart from February and March 1986 when data were lost due to the unplanned fire and in Jan-Sept 1991 for unexplained reasons). The time series for below-ground biomass measurements (live and dead) at MNT1 is more discontinuous during the time series with gaps from June 1988 through December 1992. ANPP estimates are available for most months 1984-1987 and 1991-1994 (except for gaps as previously noted for above-ground biomass). Gaps in the BNPP time series follow the pattern noted for below-ground biomass measurements plus missing data for Jun-May 1988. Monthly decomposition rates were determined in 1991-1994 for above-ground dead matter and in 1993-1994 for below-ground dead matter.

At Montecillo MNT2, measurements of above- and below-ground biomass were made on the 15th day of each month after the grassland was burned in May 1989 (except for below-ground biomass during 1992). Relative decomposition rates of above-ground dead matter were determined in 1991-1994. Below-ground decomposition rates were determined similarly (except for a gap in 1992). Monthly ANPP and BNPP estimates are available for 1991 and 1993-1994.

Above-ground biomass at the study sites exhibited unimodal patterns in each year of the study. Peak values occurred during, or immediately after, the rainy season (May to October) and lowest values on the drier, colder part of the year. Above-ground dead vegetation accumulated throughout the study period at MNT1, until it was eliminated by the fire in February 1986. However, it quickly recovered. [The study at MNT2 started after the fire.] Below-ground biomass did not exhibit any strongly defined trends, although higher values tended to occur in winter. Overall, below-ground biomass was about three times higher than above-ground. Below-ground dead vegetation was not depressed by the fire in 1986. Above- and below-ground decomposition rates were highly correlated (P<0.05) with precipitation and air temperature.

Annual NPP at MNT1 was calculated for 1985-1987 to determine impacts of the unexpected fire in February 1986. ANPP values averaged 669.2 g/m2/yr despite post-fire decline and recovery and variable rainfall over the period. BNPP was higher, averaging 1,007 g/m2/yr, but with a similar post-fire decline and recovery pattern. Total NPP reflected the yearly variation in ANPP and BNPP, with 1985 being the most productive (1,994 g/m2/yr). NPP fell to 1,422 in 1986 after the fire and then rose to 1,611 g/m2/yr in 1987 during the recovery phase. NPP values for MNT1 reported in other published accounts differ due to different measurement and calculation methods (Table 1).

Monthly ANPP and BNPP values for MNT2 were lower overall but were not analyzed to determine annual trends.

Table 1. ANPP, BNPP, and TNPP values for Montecillo MNT1 plots reported by various published data sources

Source Units ANPP BNPP TNPP
Garcia-Moya and Montanez Castro (1992) g/m2/yr 669 1,007 1,676
Scurlock and Olson (2013) 1,063 678 1,741
Scurlock et al. (2002)1 609 1,174 1,783
GPPDI Class A Site #67 in Olson et al. (2013a)2 gC/m2/yr 130 97 227
EMDI Class A site #67 in Olson et al. (2013b)2

Note: 1Determined by calculation method #5 described in the publication. 2Calculation method not described.

 

2. Data Description:

Spatial Coverage

Site: Montecillo, Mexico

Site Boundaries:(All latitude and longitude given in decimal degrees)

Spatial Coverage

Site: Montecillo, Mexico

Site Boundaries: (All latitude and longitude given in decimal degrees)

Site (Region) Westernmost Longitude Easternmost Longitude Northernmost Latitude Southernmost Latitude Elevation (m)
Montecillo, Mexico -98.9 -98.9 19.5 19.5 2,240

Spatial Resolution

1-hectare exclosure with 0.25-m2 quadrats in a randomized block design

Temporal Coverage

1984/06/15-1994/12/15 for the saline grassland burned in February 1986 (MNT1). 1989/06/15-1994/12/15 for the saline grassland burned in May 1989 (MNT2). There are gaps in both time series. Climate data are available from 1963-1989.

Temporal Resolution

Monthly measurements made on the 15th day of each month

Data File Information

Table 2. Data file descriptions

FILE NAME TEMPORAL COVERAGE FILE CONTENTS
mnt1_npp.txt 1984/06/15-1994/12/15 Monthly biomass, NPP, and decomposition rates for above- and below-ground dead matter for the saline grassland burned in February 1986 at Montecillo, Mexico
mnt2_npp.txt 1989/06/15-1994/12/15 Monthly biomass, NPP, and decomposition rates for above- and below-ground dead matter for the saline grassland burned in May 1989 at Montecillo, Mexico
mnt_cli.txt1963/01/01-1989/12/31 Climate data from the Chapingo weather station 5-km from the grassland study site

NPP Data. The results of this study are provided in two text files (.txt format), one file for each treatment (Table 3). The variable values are delimited by semicolons. The first 18 lines are metadata; data records begin on line 19. Missing data are denoted by the values -999.9 and -999.999. All biomass units are expressed in g/m2 (dry matter weight). NPP units are expressed as g/m2 per sampling period (dry matter weight). Decomposition rates are expressed as g/g/month.

Table 3. Column headings in NPP file.

COLUMN HEADING DEFINITION UNITS
Site Site where data were gathered (code refers to site identification) text
Treatmt Long term management of site (code refers to treatment described in metadata in data file)
Year Year in which data were collected yyyy
Mn Month in which data were collected mm
Dy Day on which data were collected dd
Tyear Date in decimal year (year plus the Julian date divided by 365) numeric
AGbiomass Above-ground biomass g/m2
AGtotdead Above-ground dead biomass
AGtotmatter Total above-ground biomass (live + dead + litter)
BGbiomass Below-ground live biomass
BGdead Below-ground dead biomass
BGtotmatter Total below-ground biomass (live + dead)
ANPP Above-ground net primary production g/m2 per sampling period
BNPP Below-ground net primary production
AGr Relative rate of decomposition for above-ground dead matter g/m2/month
BGr Relative rate of decomposition for below-ground dead matter

Sample NPP data record for saline grassland burned in February 1986 <mnt1_npp.txt>

Site;Treatmt;Year;Mn;Dy;Tyear;AGbiomass;AGtotdead;AGtotmatter;BGbiomass;BGdead;BGtotmatter;ANPP;BNPP; [units g/m2] AGr;BGr

mnt;1_lngtrm;1984;06;15;1984.460; 130.9; 338.1; 469.0; 427.2; 505.3; 932.5;-999.9;-999.9;-999.999;-999.999
mnt;1_lngtrm;1984;07;15;1984.540; 265.7; 428.9; 694.6; 292.2; 813.9;1106.2; 415.9; 576.6;-999.999;-999.999
mnt;1_lngtrm;1984;08;15;1984.620; 400.7; 368.7; 769.3; 100.8; 322.8; 423.5; 115.1;-641.7;-999.999;-999.999
mnt;1_lngtrm;1984;09;15;1984.710; 363.5; 272.9; 636.3; 297.0; 846.7;1143.7; -38.4; 842.6;-999.999;-999.999
...

Sample NPP data record for saline grassland burned in May 1989 <mnt2_npp.txt>

Site;Treatmt;Year;Mn;Dy;Tyear;AGbiomass;AGtotdead;AGtotmatter;BGbiomass;BGdead;BGtotmatter;ANPP;BNPP;  AGr;BGr

mnt;2_burn89;1989;06;15;1989.450; 9.6; 19.8; 29.4; 301.8; 627.8; 929.6;-999.9;-999.9;-999.999;-999.999
mnt;2_burn89;1989;07;15;1989.540; 86.9; 11.0; 97.9; 576.9; 782.8;1359.7;-999.9;-999.9;-999.999;-999.999
mnt;2_burn89;1989;08;15;1989.620; 188.2; 23.1; 211.2; 329.0; 568.6; 897.6;-999.9;-999.9;-999.999;-999.999
mnt;2_burn89;1989;09;15;1989.710; 197.5; 41.8; 239.2; 467.9; 678.4;1146.3;-999.9;-999.9;-999.999;-999.999
...

Climate Data. Climate data are provided in one text file (.txt format). The first 18 lines are metadata; data records begin on line 19. The variable values are delimited by semicolons. There are no missing values.

Sample Climate Data Record

Site;Temp;Parm; Jan; Feb; Mar; Apr; May; Jun; Jul; Aug; Sep; Oct; Nov; Dec; Year

mnt;mean;prec; 10.7; 6.9; 13.9; 28.5; 46.0;107.4;118.6;105.8; 86.7; 34.9; 10.1; 10.2; 580.0
mnt;mean;tmax; 22.1; 23.4; 25.5; 26.7; 26.9; 24.8; 23.5; 23.5; 23.2; 23.5; 23.1; 22.2; 27.7
mnt;mean;tmin; 1.3; 2.4; 4.5; 6.6; 8.5; 10.0; 9.5; 9.2; 9.1; 6.4; 3.4; 2.3; 0.9
mnt;numb;prec; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0
mnt;numb;tmax; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0
mnt;numb;tmin; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0; 27.0
mnt;stdv;prec; 15.8; 7.5; 15.1; 23.6; 21.7; 44.4; 40.5; 44.7; 35.0; 18.3; 13.7; 21.0; 106.6
mnt;stdv;tmax; 1.4; 1.2; 1.5; 1.6; 1.3; 1.2; 0.8; 0.8; 1.1; 1.1; 0.9; 1.0; 1.1
mnt;stdv;tmin; 1.0; 1.3; 1.3; 1.0; 1.0; 0.9; 1.2; 1.1; 1.0; 1.5; 1.4; 1.7; 0.9
mnt;1963;prec; 0.0; 0.0; 25.0; 27.0; 55.0; 99.0;179.0;114.0;109.0; 25.0; 2.0; 12.0; 647.0
mnt;1963;tmax; 24.4; 23.8; 27.4; 29.7; 27.2; 25.9; 23.6; 24.2; 22.9; 21.5; 23.8; 22.2; 29.7
mnt;1963;tmin; 0.2; 0.4; 5.5; 6.7; 8.4; 10.8; 9.5; 9.2; 8.9; 7.1; 3.4; 2.2; 0.2
mnt;1964;prec; 25.0; 0.0; 11.0; 98.0; 99.0;191.0; 99.0; 65.0;105.0; 32.0; 50.0; 69.0; 844.0
mnt;1964;tmax; 22.8; 26.0; 27.4; 29.5; 27.2; 24.6; 24.7; 25.3; 24.5; 23.2; 23.6; 22.1; 29.5
mnt;1964;tmin; 2.6; 2.2; 4.1; 6.5; 8.3; 9.8; 8.9; 7.0; 9.1; 3.9; 3.6; 1.9; 1.9


Where,
Temp (temporal) - specific year or long-term statistic:
  mean = mean based on all years
  numb = number of years
  stdv = standard deviation based on all years
Parm (parameter):
  prec = precipitation for month or year (mm)
  tmax = mean maximum temperature for month or year (C)
  tmin = mean minimum temperature for month or year (C)

 

3. Data Application and Derivation:

Primary production and the factors that influence it have been studied in various grassland ecosystems around the world. However, few studies have been made in high altitude, saline grasslands. Even fewer studies have included measurements of below-ground biomass and decomposition rates of dead materials, both of which are important aspects of primary production. No figures for the productivity of D. spicata in the tropics were available prior to this study. The primary objective of this study was to assess NPP of a tropical saline grassland using techniques that would account for changes in biomass, dead material, and decomposition over several years.

This work was carried out under the United Nations Environment Programme (UNEP) project on "Primary P and Photosynthesis of Semi-Natural Ecosystems of the Tropics and Sub-tropics." The data collected by this study contribute  to the understanding of the structures and production capacities of the several types of grassland ecosystems.

Grassland biomass dynamics data for Montecillo are provided for comparison with models and estimation of NPP. Climate data are provided for use in driving ecosystem/NPP models.

 

4. Quality Assessment:

NPP of grasslands is subject to a number of different methods of estimation from biomass data, some of which may be inappropriate for particular biome types. Methodology of estimation/calculation needs to be taken into account, as well as methodology of measurement, when making comparisons between different regions. Errors in biomass measurement may also occur between different study sites.

Saline communities have been the subject of previous productivity studies; however, these largely concerned temperate communities, with the exception of mangroves. The closest comparisons therefore to the high altitude, saline grasslands studied here are with saline communities of warm temperate regions. No figures for the productivity of D. spicata in the tropics were available prior to this study. However, D. spicata in the Gulf of California is reported to produce up to 600 g/m2/yr (Mudie, 1974). Bellis and Gaither (1985) reported shoot productivity of 250-1,000 g/m2/yr for the species, with below-ground increments of some 10 times these values.

Total NPP observed in this study (average 1,676 g/m2/yr for 1985-1987) was high relative to many previous studies in semi-arid tropical grasslands. Methodology differences are most likely accountable than site differences (Garcia-Moya and Montanez Castro, 1992). Outside of the tropics, most previous studies have concerned single years of study and thus provide no information on year-to-year variability in production and the correlation of this variability with climatic variables. The present study also took account of both above- and below-ground production and corrected for losses of dead vegetation through decomposition.

 

5. Data Acquisition Materials and Methods:

Site Information

Montecillo is a semi-natural saline grassland at the Colegio de Postgraduados field station (19.46 N -98.91 W), about 50-km from Mexico City. It occupies a portion of the 32,000-ha old Texcoco saline lake bed, drained in 1911. Intentional fires are common practice to improve forage quality during the dry season, preventing the establishment of woody species.

The Montecillo study area is characterized by an almost pure stand of the halophytic C4 grass Distichlis spicata var. mexicana, known locally as "zacate salado." D. spicata commonly accounts for 98% of the vegetation cover of the Texcoco lake-bed. Small quantities of Sueda torreyana and Sporobolus pyramidatus and rare occurrences of 58 other species have been recorded in this grassland. For this study, a 1-ha area of vegetation considered typical of the site was chosen and protected by an exclosure from grazing.

The climate at Montecillo is characterized under the Koppen system as being the driest of the temperate subhumid climatic group, with long and mild summers. A six month rainy period occurs from May to October, and the mean annual precipitation is 580 mm. The mean annual temperature is 15.1 degrees C. Over the course of this study, there was little difference in the annual pattern and range of maximum/minimum temperatures. The main differences between years were in amounts of precipitation and evaporation.

The site shows very little variation in relief (<10 cm). Alluvial lacustrine deposits of the Quarternary age are the main substrate. The soil is an entisol exhibiting little or no discernible horizons and saturated with water during certain seasons. The soil is also characterized by high concentrations of salts.

Table 4. Site characteristics

Description Values
mean annual precipitation 580 mm
mean monthly minimum temperature 1.3 degrees C (Jan)
mean monthly maximum temperature 26.9 degrees C (may)
vegetation type saline grassland
dominant species Distichlis spicata (C4)
historical long-term management regime alluvial plain drained 1911; fire every 4-5 years
max above-ground live biomass (typical month) 430 g/m2 (Aug)
soil type solon et / entisol
soil pH 9.0 (0-20 cm)
soil texture (sand/ silt/ clay) 0.47 / 0.35 / 0.18
soil carbon content Data not available
soil nitrogen content Data not available

Methods

General procedures for measuring production in the Montecillo grasslands are described in Garcia-Moya and Montanez Castro (1992), Long and Jones (1992), and Roberts et al. (1985) and are summarized herein.

At the Montecillo sites, 1-ha areas of vegetation considered typical of the site were fenced to control of grazing. Dry weight of both live and dead above-ground vegetation was determined monthly by clipping to ground level sets of 20 quadrats (0.25 x 1.0-m), located on a randomized block design. Live material (biomass) was separated from dead, and then separated into two components: leaves, and stems with inflorescences. The below-ground plant organs were sampled by taking 10-cm deep x 7.5-cm diameter soil cores in the center of the selected plots. Initial sample size was five cores per month. The size of the corer was increased to 15-cm x 7.5-diameter at the beginning of 1985, and eight cores were taken per month. The sample size was increased to 40 cores per month in January 1986 for the remainder of the study period. The roots were washed with running water, stained with tetrazolium chloride, sorted into live and dead components, dried, and weighed.

Decomposition was estimated by the use of litter bags. Initially, 10 bags of 2-mm mesh, measuring 3-cm x 2-cm, were placed in the field each month, each containing 1-g of dead matter. This was increased to 20 bags measuring 8 x 8-cm with 3-g per bag per month in February 1985 for the remainder of the investigation. Bags were located at ground level and at 5-cm depth for above- and below-ground measurements, respectively. Productivity was calculated according to the equation of Roberts et al. (1985).


quadrat sampling


Figure 2. Quadrat sampling of above-ground biomass at the Montecillo grassland site, Mexico. (Tinicio "Nicho" Palafox and other research assistants from Colegio de Postgraduados, Chapingo, are clipping grass with shears. Samples are placed in plastic bags and transported to the laboratory for drying and weighing. Photograph taken July 1984 by Dr. J.M.O. Scurlock, ORNL).

 

6. Data Access:

This data set is available through the Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC).

Data Archive:

Web Site: http://daac.ornl.gov

Contact for Data Center Access Information:

E-mail: uso@daac.ornl.gov
Telephone: +1 (865) 241-3952

7. References:

Bellis, V.J., and A.C. Gaither. 1985. Seasonality of above-ground and below-ground biomass for six salt marsh plant species. Journal of the Elisha Mitchell Scientific Society 101: 95-101.

Garcia-Moya, E., and P. Montanez Castro. 1992. Saline grassland near Mexico City, pp. 70-99. IN: Long, S.P., M.B. Jones, and M.J. Roberts (eds.). Primary Productivity of Grass Ecosystems of the Tropics and Sub-tropics. Chapman and Hall, London. 267 pp.

Long, S.P., and M.B. Jones. 1992. Introduction, aims, goals, and general methods, pp. 1-24. IN: Long, S.P., M.B. Jones, and M.J. Roberts (eds.). Primary Productivity of Grass Ecosystems of the Tropics and Sub-tropics. Chapman and Hall, London. 267 pp.

Long, S.P., E. Garcia Moya, S.K. Imbamba, A. Kamnalrut, M.T.F. Piedade, J.M.O. Scurlock, Y.K. Shen, and D.O. Hall. 1989. Primary productivity of natural grass ecosystems of the tropics: A reappraisal. Plant and Soil 115(2): 155-166. doi:10.1046/j.1365-2486.2001.00448.x

Mudie, P.J. 1974. The potential economic uses of halophytes, pp. 565-597. IN: Reimold, R.J., and W.H. Queen (eds.). Ecology of Halophytes: A Review of Their Interactions in Salt Marshes, Wetlands and Saline Soils. Academic Press, New York. 605 pp.

Olson, R.J., K.R. Johnson, D.L. Zheng, and J.M.O. Scurlock. 2001. Global and Regional Ecosystem Modeling: Databases of Model Drivers and Validation Measurements. ORNL Technical Memorandum TM-2001/196. Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A.

Olson, R.J., J.M.O. Scurlock, S.D. Prince, D.L. Zheng, and K.R. Johnson (eds.). 2013a. NPP Multi-Biome: Global Primary Production Data Initiative Products, R2. Data set. Available on-line [http://daac.ornl.gov] from the Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A.doi:10.3334/ORNLDAAC/617

Olson, R.J., J.M.O. Scurlock, S.D. Prince, D.L. Zheng, and K.R. Johnson (eds.). 2013b. NPP Multi-Biome: NPP and Driver Data for Ecosystem Model-Data Intercomparison, R2. Data set. Available on-line [http://daac.ornl.gov] from the Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A. doi:10.3334/ORNLDAAC/615

Roberts, M.J., S.P. Long, L.L. Tieszen, and C.L. Beadle. 1985. Measurement of plant biomass and net primary production of herbaceous vegetation, pp. 1-19. IN: Coombs, J., D.O. Hall, S.P. Long, and J.M.O. Scurlock (eds.). Techniques in Bioproductivity and Photosynthesis, 2nd Edition. Pergamon Press, Oxford. 200 pp.

Scurlock, J.M.O., and R.J. Olson. 2013. NPP Multi-Biome: Grassland, Boreal Forest, and Tropical Forest Sites, 1939-1996, R1. Data set. Available on-line [http://daac.ornl.gov] from Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A. doi:10.3334/ORNLDAAC/653

Scurlock, J.M.O., K. Johnson, and R.J. Olson. 2002. Estimating net primary productivity from grassland dynamics measurements. Global Change Biology 8: 736-753. doi:10.1046/j.1365-2486.2002.00512.x