Design, treatments, and vegetation measurements
This study was conducted on the Jornada Experimental Range (JER)
from 1938-2001 and is described in detail in Havstad et al. 1999.
The study area is located on a bajada slope where typic Paleorthid
soils have formed on an old alluvial fan. Vegetation is dominated by
creosotebush, with honey mesquite (Prosopis glandulosa) and tarbush
(Flourensia cernua) also present. Sixteen plots (21.3 m x 21.3 m)
were established in 1938 in four rows of four plots per row with a
buffer zone of 7.6 m between plots. All plots were sampled prior to
treatment applications. Each plot was divided into east and west
halves and 14 randomly located 10.65 m line intercept transects,
running north to south, were sampled for shrub canopy cover and
perennial grass basal cover in each half-plot using the line
intercept method (see Herrick et al. 2005). Grasses were measured
for basal cover, and all other species were measured for canopy
cover. The 28 transects are not marked; they are randomly relocated
for each remeasurement. Vegetation was measured using vertical line
point intercepts in 1938, 1947, 1956, 1960, 1967, 1989, 1995, and
2001 for Gravelly Ridges and 1938/9 (Parker/Dona Ana), 1947, 1960,
1967, and 2001 for the Parker Tank and Dona Ana sites.
Treatments were factorially applied, yielding a control plot, single
factor plots, and plots with varying degrees of combinations of
factors. Original treatments applied to these 16 plots in 1939 were
clearing (hand removal of shrubs severed at ground surface),
furrowing (shallow, hand raked furrows designed to trap surface
water), seeding (broadcast application of seeds of native
perennials), and lagomorph exclusion. Only the lagomorph exclusion
and shrub removal treatments were continued in later years. Poultry
netting with a 2.5 cm mesh buried about 15 cm in the ground and
extending above-ground about 75 cm was used to exclude lagomorphs.
Shrubs (not including suffrutescents) have been removed from the
cleared plots immediately following all resampling periods listed
above. Seeded species were black grama, spike dropseed, mesa
dropseed (Sporobolus flexuosus), and fourwing saltbush (Atriplex
canescens). All treatments were applied after the initial vegetation
measurements were recorded in 1938, except that the lagomorph
exclusion fences were constructed in 1939. Livestock were excluded
from the study area in 1938, and the site has not been grazed by
livestock at any time during the study period.
The 4 treatments are indicated by columns in the included data file
following the description in Havstad et al. 1999. Plot 15 is an
untreated control.
References
Havstad, K.M., R.P. Gibbens, C.A. Knorr, and L.W. Murray, (1999).
"Long-term influences of shrub removal and lagomorph exclusion
on Chihuahuan Desert vegetation dynamics." Journal of Arid
Environments 42: 155-166. https://doi.org/10.1006/jare.1999.0516
Tansley, A. G., & Chipp, T. F. (1926). Aims and methods in the
study of vegetation.
