Several long-term studies at the Sevilleta LTER measure net primary production (NPP) across ecosystems and treatments. Net primary production is a fundamental ecological variable that quantifies rates of carbon consumption and fixation. Estimates of NPP are important in understanding energy flow at a community level as well as spatial and temporal responses to a range of ecological processes. Above-ground net primary production (ANPP) is the change in plant biomass, including loss to death and decomposition, over a given period of time. To measure this change, vegetation variables, including species composition and the cover and height of individuals, are sampled up to three times yearly (winter, spring, and fall) at permanent plots within a study site. The weight data presented here is obtained by harvesting a series of covers for species observed during plot sampling. These species are always harvested from habitat comparable to the plots in which they were recorded. This data is then used to make volumetric measurements of species and build regressions correlating biomass and volume. From these calculations, seasonal biomass and seasonal and annual NPP are determined.
Generating Cover Range for Harvest Samples:
Prior to making harvest collections, a range of cover values and the number of observations is generated for each plant species recorded at a site from the NPP data. A perl script is located in pc_field/palmtop/npp. This script will generate a list of species with the minimum and maximum cover and number of observations from all excel files in the folder. From these values a range of covers to be harvested will be produced.
To produce a range for a species, first decide the number of samples you will be collecting for that species. 15 samples will be collected for dominant or frequently observed species and 10 samples will be collected for most other species. Next decide if you will be making this range based on counts or cover measurements. The majority of species will be based on cover measurements. Only extremely small plants will be based on count measurements.
For ranges based on cover measurements, start by writing down the cover value one size-class below the smallest cover observed at a site. For example, if the smallest cover observed at a site was 0.5 write down 0.25. If smallest cover value observed is 0.01 do not go below this value. Next write down the cover value one size-class above the largest cover value recorded at a site. These two numbers represent the largest and smallest covers that will be harvested at a site. Within the bounds of these two numbers fill in cover values. Ultimately you will need to end up with either 10 or 15 cover values depending on the observed frequency of the species. Try and keep the numbers as balanced as possible. Smaller covers will have more variability than larger covers and thus it is best to collect duplicates of the smaller cover sizes. Also, as covers increase the size-class increments between covers should also increase. When generating these ranges keep in mind that the purpose of harvest samples is to produce a statistical linear regression equation for each species at each site from which biomass will be estimated. A similar process is used to obtain a range of values for samples that will be based on counts.
Enter the species ranges, date and site into a Sevilleta LTER NPP Harvest Data book. Once this is complete, label collection bags. In general, harvests with covers between 0.01 and 0.5 are collected into coin envelopes and larger covers are placed in paper bags. Label each bag or envelope with the date, site, plant species, sample number, and cover value. Leave a blank spot for height.
Collecting Harvest Samples:
Collect plants from an area that is close to but not actually on the trapping webs or NPP plots at each site. Avoid collecting plants near the roadside since these plants may have morphologies that vary from plants on the NPP quadrats. From 1999 to 2003, the site of collection corresponds to the site where the data were collected (see above for site codes). In 2004, several new sites were added that had been burned by a prescribed fire in 2003. Separate weight data was needed for the burned and unburned plants at these sites. Therefore, after 2004 a new variable "type" was added. And, a new site "L" was added. "L" refers to all the Lower sites and does not include P or J. The new variable "type" has four codes. "B" refers to harvested plants that were burned in the 2003 fire, and "U" refers to plants that were not burned in recent history. However many plants grow similarly in both treatments, especially annual plants and some perennial forbs. These plants are designated type "F" and can be harvested from either burned or unburned areas. Type "A" is the default and applies to all pre-2004 data as well as the Cerro Montosa site P. In Fall of 2010, type "B" refers to samples that were harvested from area of the Sevilleta that were burned by a wildfire in August 2009. The one exception was for creosote that was still harvested from the 2003 prescribed burn area.
Locate individual plants that correspond to the cover values written on a bag. Measure the height of the plant to the nearest cm and write this height on the bag. Then with a pair of shears clip the foliage and place it in the bag. It is important to collect all of the foliage and to clip it off as near to the base as possible. Do not include any root material in the sample.
If you cannot find a natural clump of foliage of a particular cover value you may take a sub-sample from a larger plant. This should be done only when absolutely necessary. The most important thing to remember is that foliage harvests need to have a similar morphology to plants measured on the quadrats. If a sub-sample must be taken try to collect a sample that seems representative of a cover that would occur naturally on a quadrat.
After a sample has been collected fold the bag so that no portion of the sample can fall out. When all the samples of a species have been collected, indicate that the collection of that species is complete in the Harvest Data book. Place the samples into a large plastic garbage bag and try to keep them in a cool location until you are able to store them.
Sorting Harvest Samples:
Empty the contents of a single bag onto a large tray or newspaper. Pick through the foliage and separate live stems, leaves and inflorescence from dead material. For Fall, consider any yellow or green material as live material and any brown or gray material as dead. Store the dead material in a separate bag. For Spring, consider only green material as live and discard yellow, brown and gray material. When you have finished sorting live from dead, place the live material back in the original bag and discard the dead material, except for Fall samples. Bags containing sorted material are then placed in a drying oven located at the Sevilleta field station. Dry the bags for 4 days at 50 degrees C or until completely dry. It usually takes 6-10 days for cacti to dry in the oven and the samples should be dried separately from other samples. Before placing cacti in the oven perforate them several times with a knife to promote the escape of moisture. Remove the bags with the samples from the oven when they are completely dry. The plants should be very brittle. Store the plants in a dry location until weighed.
Weighing Harvest Samples:
Try to weigh the samples as soon after drying as possible so that the do not re-absorb any moisture. Sort the bags by species and site so that these can be weighed at the same sitting. Remove the sample from the bag and place it into a tarred tray. Make sure that no portion of the plant remains in the bag. Weigh the sample to the nearest centigram. Record the date of collection, site (L, the lower flats or P, the pinyon-juniper), the type of collection (A, unburned or B, burned) plant species, sample number, cover value, height and weight into the Sevilleta LTER NPP Harvest Mass Data book. Set the sample aside. When all samples of a particular species and site have been weighed, combine the dry material and retain 50g for a voucher specimen. Discard any excess material. If there is not enough dry material to make up 50g just retain what is there. Place the voucher specimen into a paper bag labeled voucher with the date of collection, site (L or P), type (A or B), plant species, and approximate weight of the sample.
Notes: In 1999 and 2000, all species from all sites were collected. In 2001and seasons 1 and 2 of 2002, only those species that had not yet been encountered were harvested. In Fall of 2002, the regressions were studied and many species with irregular regressions were harvested again. In winter of 2003, much of the data was lost and has not been recovered. From spring of 2003 to the present, problem species (bad or inconsistant regressions), new species, and dominant species at each site have been harvested each season.
Employee History for Sevilleta Field Crew
Mike Friggens, 1999-September 2001;Karen Wetherill, February 7, 2000-August 2009; Terri Koontz, February 2000-August 2003, August 2006-August 2010; Shana Penington, February 2000-August 2000; Heather Simpson, August 2000-August 2002; Chris Roberts, September 2001-August 2002; Caleb Hickman, September 9, 2002-November 15, 2004; Seth Munson, September 9, 2002-June 2004; Maya Kapoor, August 9, 2003- January 21, 2005, April 2010-Present; Tessa Edelen, August 15, 2004-August 15, 2005; Charity Hall, January 31, 2005-January 3, 2006; Yang Xia, January 31, 2005-Present; Michell Thomey, September 3, 2005-August 2008; Jay McLeod, January 2006-August 2006; Amaris Swann, August 25, 2008-January 2013; John Mulhouse, August 21, 2009-June 2013; Stephanie Baker, October 2010-present; Megan McClung, April 2013-present; Chandra Tucker, April 2014-present