General study design
This experiment was conducted at three sites, each with a
grassland-to-shrubland ecotone. Two of the sites, JER Pasture 9 and
JER Pasture 12A, occur on the USDA Agricultural Research Service
Jornada Experimental Range. One site, CDRRC Pasture 3, is located on
New Mexico State University’s Chihuahuan Desert Rangeland Research
Center. Climate is arid to semi-arid, with long-term (90-year) mean
annual temperature of 15C and mean annual precipitation of 250 mm,
over half of which occurs in summer (July–September).
Sites were chosen to contain 3 distinct habitat types (or “ecotone
positions”): grassland, ecotone, and shrubland. Within each habitat
type of each site, 32 1 square meter quadrats were established in
2005, yielding 96 quadrats per site and 288 quadrats for the entire
study. Within each habitat type the 32 quadrats are arranged in two
parallel rows of 16 quadrats each, running perpendicular to the
grassland-to-shrubland gradient. Within a row, each quadrat is
approximately 20 meters apart. Distance between the two parallel rows
is about 55 meters. The rows are parallel to and between rodent
trapping lines in data package knb-lter-jrn.210262008.
Quadrats are sampled nondestructively for ocular live cover and height
of each species twice per year: once in the spring and once in the
fall after the growing season. Two permanent plastic stakes mark the
diagonal corners of each quadrat. Measurements are taken within
portable square frames placed on the part corners, with an internal
area of 1 square meter. The interior of the frame is gridded with
twine into one hundred 10 cm x 10 cm sections (each 1% of the
quadrat's area) to facilitate plant cover estimates. Plant cover is
measured for each species at a minimum of 0.1% and the height to the
nearest centimeter.
Annual Aboveground NPP estimation methods
Cover and height are used to estimate the volume of each plant. Plant
biomass is then estimated non-destructively using the regressions of
plant dimensions (i.e., plant volume) vs. live biomass derived from
harvest data previously gathered (Huenneke et al., 2001). For
perennial shrubs, sub-shrubs, and the perennial grasses Bouteloua
eriopoda, Muhlenbergia porteri, and Pleauraphis mutica, the live
spring (pre-growth) biomass is treated as the baseline for that year;
thus annual aboveground NPP is estimated as the difference between the
fall and the spring standing biomass. Any negative differences are
taken as zero. For all other species, the maximum of spring or fall
live biomass is taken to be the annual ANPP. Annual ANPP is first
assessed by species within each quadrat, then summed to functional
groups within each quadrat, then averaged over the 32 quadrats to
produce estimates for each habitat zone within each site. The plant
cover and height data used to generate these annual ANPP estimates is
available in data package knb-lter-jrn.210262001. Also available are
annual ANPP estimates by site in data package knb-lter-jrn.210262005.
Reference
Huenneke, Laura F., Dennis Clason, and Esteban Muldavin.
\"Spatial heterogeneity in Chihuahuan Desert vegetation:
implications for sampling methods in semi-arid ecosystems.\"
Journal of Arid Environments 47, no. 3 (2001): 257-270.
