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: grassland,
ecotone, and shubland. 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.
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 m2. 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 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 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. 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.
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.
