Transect study background
Starting in 1982, a 135 ha portion of a 1500 ha, internally
drained, moderately-to-highly grazed watershed was exclosed from
grazing by domestic livestock. Within this exclosure, three
permanent, parallel transects (2.7 km in length) were established
by the Jornada Basin LTER, extending from the middle of the
College Playa up into the foot of Mt. Summerford at the Chihuahuan
Desert Rangeland Research Center (CDRRC) in the Jornada Basin. The
study site has a history of moderate to heavy livestock grazing
for 100 years before 1982. The Control transect (C) is located on
the west, the Treatment transect (T) is on the east side of the
Control transect, and the Alternate Control (X) is to the east of
the Treatment transect. The three transects pass through several
soil and vegetation zones as they ascend from playa to mountain.
Each transect is 30 meters wide with a 45-meter buffer zone
between each transect. The Treatment transect was treated annually
with NH4NO3 in a concentration equal to 10g N/m2 until 1987. On 17
July 1986 the Control Transect was aerially fertilized by mistake
with granular NH4NO3 instead of the Treatment Transect. As a
result of this mistake, the Alternate Control Transect was
established prior to the fall measurements of 1986 and the
Treatment Transect was subsequently fertilized by hand with a team
of 12 persons using mechanical spreaders.
To measure the effects of nitrogen fertilizer on plant
communities, the average percent coverage of each plant species
was calculated along plant intercept lines perpendicular to the
permanent, 2.7 km transects. There are 91 station markers arranged
at 30 meter intervals along each permanent transect, and each has
a plant intercept line that extends 15 meters on either side of
the transect. These data are available in dataset
knb-lter-jrn.210119001.
Litterfall trap methods
The litterfall traps were placed in the creosotebush zone on the
upper bajada betwen the College Playa and Mt. Summerford. On the
LTER-I Control transect, litterfall traps were placed between
Station C61 and C62. On the LTER-I Treatment transect, litterfall
traps were between Station T60 and T62. There were 10 creosote
bushes on each transect that were each surrounded with four 30 x
30 cm baskets constructed of aluminum window screening. Litter
fall traps were located midway under the canopy with a basket at
each of the cardinal directions. Litter baskets were mounted under
each shrub and suspended 3 to 4 inches above the ground from 3/8th
inch rebar.
The litter in the baskets was collected monthly using a CarVac run
off of a marine 12 volt deep-charge battery. The 4 baskets spaced
equally around each shrub were collected as one sample. Litter was
oven-dried at 50 degrees Celsius and total weight recorded. For
the period 07/11/1985 through 11/01/1985 the litter from all 10
shrubs was combined, mixed thoroughly, and approximately 10 grams
subsampled and then sorted into leaf, stem, and seed fractions and
weighed. Beginning in 12/03/1985 a 5 gram subsample from each
shrub was sorted into leaf, stem, and seed fractions and their
oven-dried weights recorded. For any sample less than standardized
subsample weight, all of the sample was sorted into leaf, stem,
and seed fractions. Field collections were done by John Anderson,
Phil Harrigan, and Fadi Anboubi.
Creosote shrub size and litter collection data.
Shrubs at the litter traps were measured by Phil Harrigan and Fadi
Anboubi on 6/21/88. The 'JRN389001_creosize.csv' file contains
Larrea tridentada shrub size measurements for 10 shrubs on the
control transect and 10 shrubs on the treatment transect which
were used to collect creosote litterfall from 11 July 1985 to 2
April 1988.