<emphasis role="strong">Project Overview</emphasis>
<emphasis role="strong">Permanent chart quadrats were established as
an observational study to demonstrate the efficacy of science-based
grazing management for recovering and maintaining rangeland
productivity. More recently, the quadrats are used to investigate the
patterns and mechanisms of vegetation dynamics in response to static
variations in soils and landforms and temporal variations in climate
across the Jornada Experimental Range.</emphasis>
122 permanent quadrats are distributed across the Jornada Experimental
Range. Sometimes quadrats were organized in linear series at the time
of establishment (e.g. A1, A2, A3, etc.), with members of a series
placed at ½ or 1-mile intervals radiating out from a livestock
watering point. Quadrats were established between 1915 and 1934.
Initially, a subset quadrats were located within fenced exclosures to
prevent grazing by cattle, though some fences broke down over time and
ceased to effectively exclude cattle (see
Jornada_quadrat_pastures.csv).
<emphasis role="strong">Vegetation Sampling</emphasis>
Quadrats are 1m x 1m. Quadrats were mapped annually from 1915-1947.
From 1947-1979 a subset of the quadrats was sampled at irregular
intervals. In 1995, quadrats were relocated and sampling resumed at 5-
or 6-year intervals. Ongoing sampling (1995-present) occurs between
September and March. Historical sampling (pre-1995) was typically
conducted September-March as well, although there are exceptions. To
account for the fact that a single sampling event often spanned
several months, we include in the data table a column for
“project_year” as well as actual month and year of the sample, so that
data can be grouped appropriately. For example, the 1937 sampling
event data was collected from October 1937 to March 1938, and so these
data are labeled with project_year = 1937. In a small number of
instances during the early years of the project, quadrats were sampled
twice within a year. Information on dates each quadrat was sampled is
contained in Jornada_quadrat_sampling_dates.csv.
Vegetation data collection consists of chart quadrat maps, in which
the locations and shapes of all living plants located within the
quadrat were mapped onto data sheets. Perennial grass species were
mapped as polygons representing the boundary of each individual grass
bunch at a point 1 inch above the soil surface (i.e. basal area);
shrubs were mapped as polygons representing the canopy cover of each
individual; and perennial forbs/subshrubs were mapped as point
locations only. As annual grasses and forbs were not initially a
primary focus of this study, it is unclear whether these species were
recorded consistently 1915-1979. From 1995-2016 all annual species
located within quadrats were identified, counted, and recorded as a
tally on data sheets (but not mapped). Information on which species
belong to each category (perennial grass, shrub, etc.) is contained in
Jornada_quadrat_species_list.csv. Chart maps are presented here as
shapefiles, with geolocation data removed to protect the integrity of
the study site. GPS locations can be obtained by request from the data
manager.
<emphasis role="strong">Soil Sampling</emphasis>
Knowledge of soil properties is crucial for understanding vegetation
dynamics in this system. Previous studies using this data have found
that cover of perennial grass species were significantly correlated to
soil texture and topography, both during drought and non-drought
conditions (Yao et al. 2006). Soil sampling at the permanent quadrat
locations, including soil particle size analysis and soil depth
measurement, was mostly done in Fall 2001, with a small number of
samples collected in 2020. Soil cores for particle size analysis were
taken from 1-4 locations around each quadrat at bare (non-vegetated)
areas. The number of soil cores varied based on availability of bare
patches at each quadrat. At each location cores were taken at two
depths: 0-5cm below surface and 5-20cm below surface. In 2001, the
0-5cm core had a diameter of 6.1cm, and the 5-20cm core had a diameter
of 4.6cm. In 2020, a 5.8cm-diameter core was used for both depths. If
multiple cores were taken from a quadrat, samples from each depth were
combined for analysis resulting in a single particle size analysis per
quadrat and depth layer. Soil samples were air dried at room
temperature, then hand sieved to 2mm to remove large inorganic
material and the >2mm fraction. Particle size analysis by the
hydrometer method was performed on a 60g subsample. If the sand sample
left over from the hydrometer analysis (>53µm) was sufficiently
large (>15g), sand fractionation analysis was performed. The sand
sample was washed through a 53µm sieve, dried at 105C for 24 hours to
remove moisture, then separated into five sand size classes (53-106µm,
106-250µm, 250-500µm, 500-1000µm, and 1000-2000µm) in an ultrasonic
sifter for 5 minutes, then weighed to determine percent sand
fractionation. Sand fractionation was not performed on the samples
collected in 2020.
Depth to petrocalcic horizon was measured outside the bounds of the
quadrat. In 2001, this was done at all four corners, close to the
quadrat, by pounding a steel rod into the ground until it met
resistance. If the rod reached its maximum depth of 125cm without
encountering the petrocalcic horizon at all four corners, the quadrat
received a “0” in the “pc_horizon_125cm” column and no depths are
reported. In some cases, the rod met significant resistance but there
was no evidence of white petrocalcic horizon on the tip, indicating
the rod may have hit rock instead. In these cases, or if the rod
reached its maximum depth of 125cm without encountering the
petrocalcic horizon on at least one but not all of the four
measurements, the data is marked with a flag in the “notes_pc” column
to indicate the measurements may be underestimates. In 2020, depth to
petrocalcic horizon was measured at points 2.7m away from quadrat
corners using a soil auger. At quadrats L3, L4, and Y7 soils are known
to be deep and so only one measurement was taken to confirm that there
was no petrocalcic horizon shallower than 125cm. If the soil was too
rocky or gravelly, depth measurements were not possible.
