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
through September).
Sites were chosen to contain 3 distinct habitat types (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 over all species to calculate total ANPP
for 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 are
available in data package knb-lter-jrn.210262001. Also available
are annual ANPP estimates by functional group in data package
knb-lter-jrn.210262004.
Due to its growth form, Yucca elata (YUEL) has been found to
produce large errors in interyear biomass estimates. This data
package contains annual ANPP estimates both with and without YUEL,
but the authors strongly recommend using the non-YUEL estimates
for most purposes.
References
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.
Huenneke, Laura F., John P. Anderson, Marta Remmenga, and William
H. Schlesinger. Desertification alters patterns of aboveground net
primary production in Chihuahuan ecosystems. Global Change Biology
8, no. 3 (2002): 247-264.
Muldavin, Esteban H., Douglas I. Moore, Scott L. Collins, Karen R.
Wetherill, and David C. Lightfoot. Aboveground net primary
production dynamics in a northern Chihuahuan Desert ecosystem.
Oecologia 155, no. 1 (2008): 123-132.
Peters, Debra PC, Jin Yao, Osvaldo E. Sala, and John P. Anderson.
Directional climate change and potential reversal of
desertification in arid and semiarid ecosystems. Global Change
Biology 18, no. 1 (2012): 151-163.