Ecotone Study: Plant Above Ground Net Primary Productivity by Site

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   Dataset consists of the annual aboveground net primary production (ANPP) across 3
   habitats grouped by plant form and total ANPP.  The habitats are grassland, mesquite
   shrubland, and the ecotone between the 2. The plant forms are winter annual forb,
   annual forb, bi-annual forb, perennial forb, annual grass, perennial grass, shrub, and

   OBJECTIVE:  The purpose of the study is to investigate how pulses of precipitation
   translate into pulses of plant aboveground net primary productivity (NPP) and how the
   small mammal community responds to such changes also in relation to shrub gradient
   across the landscape.  Particularly we are interested in how the energy flows through
   the ecosystem in response to pulses of rain, how the small mammal community partition
   resources (in terms of C3 (forbs and shrubs) and C4 (grasses) plants) and how the
   genetic structure of some species (e.g., Dipodomys spp.) is affected by their
   population dynamics.


   1) Small mammal abundance should respond positively to precipitation and NPP.

   2) On a temporal scale, the small mammal energy use should show parallel fluxes along
   the shrub gradient.

   3) The small mammal community should consume C3 and C4 plants according to their
   availability (or NPP).

   4) At low population density, dispersal should be limited and the genetic variance will
   be distributed among populations rather than within (i.e., Fst will trend towards
   higher values).  After pulses of rain and NPP, population densities will be greater,
   dispersal prevalent, and the genetic variance of populations will be distributed within
   populations (i.e., Fst will approach zero) as dispersal homogenizes populations.

   Total aboveground annual net primary productivty is calculated for winter annual forb,
   annual forb, bi-annual forb, perennial forb, annual grass, perennial grass, shrub,
   sub-shrub, and the total of these.

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Method of Recording: Digital voice recorders

   At each site, three rectangular trapping grids (300m X 100m, 16 X 6 traps, 20m apart)
   were permanently marked in each of the three habitat types characterizing the ecotone
   (shrubland, grassland, and the transition zone between them) where plant measurements
   occurred.  Aboveground net primary productivity (NPP) is measured following a
   volumetric method developed by Huenneke et al.  (2001 and 2002).  The sampling occurrs
   at the end of the growing season each spring and fall.  On each grid the measurements
   occurr on 32 1-m2 quadrats distributed on two parallel transects (about 55 meters
   apart) of 16 quadrats each (20 meters spaced).  Two permanent plastic stakes mark the
   diagonal corners of each quadrat.  Measurements are taken within portable square
   frames, 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 at a minimum of 0.1% and the height to
   the nearest centimeter.  Cover and height are then 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, grasses,
   and forbs the spring (pre-growth) biomass was treated as the baseline for that year,
   thus annual NPP was estimated as the difference between the fall and the spring
   standing biomass.  Negative values due to estimation error were taken as zero.
   Conversely for winter annual forbs the baseline was set at zero and the standing
   biomass during the spring was treated as annual NPP.

Quality Assurance: 

Visual verification

Data was derived from field data following the procedure above mentioned.

Additional information: 

The 3 sites are located in JER Pastures 9, 12 and in CDRRC Pastures 3


twice a year (spring & fall)

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