Each study site is 1 km by 0.5 km in area. Four replicate experimental
blocks were randomly located at each study site depending on whether
the site was grazed. The blocks are all oriented on a site in an X/Y
coordinate system, with the access road to each site forming the X
axis. The compass orientation at the Jornada grassland site is to the
north, and Jornada creosotebush site is oriented to the south.
Treatments within each block include one unfenced control plot
(Treatment: C; control); one plot fenced with barbed wire, hardware
cloth, and poultry wire to exclude rodents, rabbits, and cattle
(Treatment: R; rodent); one plot fenced only with barbed and poultry
wire to exclude cattle and rabbits (Treatment: L; lagomorph); and one
plot fenced with barbed wire to exclude cattle (Treatment B; bovine).
Cattle exclosure plots with barbed wire occur only at the black grama
grassland site where cattle were present, for a total of 4 treatment
plots at each of the four grassland site blocks. There are only 3
treatment plots at each of the creosotebush site blocks. The
treatments were randomly assigned to each of the three or four
possible plots in each block independently, and their arrangements
differ from block to block. Each of the plots in a replicate block is
separated by 20 meters. Each plot measures 36 meters by 36 meters.
Thirty-six sampling points are positioned at 5.8-meter intervals on a
systematically located 6 by 6 point grid within each plot. A permanent
one-meter by one-meter vegetation measurement quadrat is located at
each of the 36 points. All cover values are measured from the
vegetation measurement frame, which is 1 meter by 1 meter, and
partitioned into a grid of 100, 10 cm by 10 cm squares.
Leaf litter cover (in %) was measured on the soil surface of the
quadrat using the 10 cm by 10 cm squares as a reference. For cover
values less than 5, increments of 1.0 were used. For cover values
greater than 5, increments of 5.0 were used. Leaf litter includes all
detached dead plant material on the soil surface, including woody
branches. Only leaf litter cover that is in the open and not within
clumps of grass, etc. Some leaf litter cover has distinctive margins
and is easy to define and measure. However, much leaf litter consists
of many diffuse small patches that are separated by bare soil, and
distributed throughout the quadrat. For such diffuse cover, we
determined the actual cover in one typical 10 by 10 cm square (e.g.,
0.3), then counted the number of squares with diffuse cover (e.g., 5),
and multiplied the number of squares by the actual cover for a typical
square (e.g., 0.3 X 5 = 1.5, then rounded to 1.0 or 2.0, or if the
value had been greater than 5, rounded to the nearest increment of 5.0
) for the total litter cover. All litter cover is pooled into one
observation, and no height is measured.
Twice a year in the spring and fall from 1995-2005, percent leaf
litter in each quadrat was measured. After 2005, sampling frequency
changed to once every 5 years in the fall. This study is ongoing.
Further details on measurements and data preparation can be found in
the SMES vegetation quadrat protocol included with this package
("SMESvegquadprotocol.pdf").
