Data set ID:
Design and hypotheses. The treatments are designed to distinguish the effects of plant biomass per se from those of plant functional groups and plant species richness within functional groups.
We recorded the mass of plants removed from the plots when the treatments were imposed in 1995; the total biomass removed from a plot is an index of the disturbance associated with the start of the experiment. One hypothesis is that plant biomass, rather than the species or functional group composition of the plants, is the critical regulator of ecosystem function. If so, most response variables would be highly correlated with initial biomass removed in the early seasons of the experiment. If remaining species increase in biomass over time, to the point where all treatments support approximately equal biomass, that correlation (and the differences among treatments) should disappear. Another hypothesis is that the architecture and physiology of different groups of plants differ sufficiently that each functional group contributes in a unique way to ecosystem function. We therefore are testing the impact of removing various groups or growth forms from the plant community. Four treatments involve the removal of all individuals of all species of a given functional group of perennial plants: shrubs, subshrubs, perennial grasses, and succulents (both leaf and stem succulents). That is, one treatment involves the removal of all shrubs from a plot; another the removal of all perennial grasses, and so on. All other treatments (Control plus three other treatments) contain at least some species representative of all functional groups, so the contrast between the functional group removals and these other treatments should reflect the effect of functional group diversity. There are multiple species in each of the functional groups, so an additional hypothesis is that higher species richness within a functional group alters ecosystem function significantly. We have imposed a Simplified treatment, where only a single species (the most abundant species at the time of initiation) of each of the four groups remains, and all other species of those functional groups have been removed. That is, the Simplified plots contain a single shrub (Larrea tridentata), a single subshrub (Zinnia acerosa), a single succulent (Yucca baccata), and a single perennial grass (Muhlenbergia porteri). In contrast, we have also imposed two versions of a Reduced diversity treatment, where the dominant species have been removed and the subordinate species remain. (In one version of this treatment, Larrea is assumed to be the dominant shrub and was removed; in another version, Prosopis is assumed to be dominant and is removed. In both of these, the dominants of other growth forms are as identified above in the Simplified treatment.) Finally, in the Control, all species (dominant and subordinate) of all four functional groups remain. Hence the contrast among the Control, the Reduced, and the Simplified treatments should reflect the importance of species richness within functional groups. Response variables: The vegetation transects are measured once or twice a year to assess the response of the plant community to the manipulations of species and functional diversity. (Measurements are every fall at a minimum. When resources permit and vegetation seems to deserve it, spring sampling will also be carried out (2 x per year)
field data sheets; tape recorder (varied by day and observer)
Three 1 m x 15 m transects (total area 45 m2) cross each 15 m x 15 m central area in the plots: one at the western- most edge of each area, one 5 m to the east, and the third 10 m to the east of the western boundary. These transects are sampled by the use of meter sticks to frame individual quadrats (numbered 1-15 within each transect; the transects are numbered 1 to 3, from west to east in each plot). All plants rooted within the quadrat (or with more than half of the stem or basal area rooted within) are measured for maximum diameter, maximum perpendicular diameter, and height (all to nearest cm). The entire plant canopy is measured, not just that portion falling within the quadrat. Plants rooted outside the quadrat, whose canopies may overlap the quadrat, are not measured. Multiple individuals of the same dimensions in a single quadrat are given a "count" (if no count recorded, count = 1). Dead plants are identified as to species (where possible) and recorded as "condition = dead." High densities of fluffgrass in fall 1997 forced us to alter sampling, such that DAPU was measured only in odd-numbered quadrats for most of the transects. The following applies to readings of DAPU (fluffgrass): In Spring 97 all quads read for DAPU). Beginning with Fall 97 only odd numbered quads measured for DAPU. In Spring and Fall 99 DAPU and all forbs were read in only odd numbered quads. In Spring and Fall 2000 DAPU was read in odd numbered quads (forbs were read in all quads). In Spring 2001 DAPU and all forbs were read in odd numbered quads. In Fall 2001, all vegetation in every quad was read. In Spring 2002 all vegetation in all quads was read.
SAS Programs: BVT_FGANAL.SAS BVT_SPIDANAL.SAS BVT_RICHANAL.SAS Macros: BLOCK.SAS, FUNGRP.SAS, TRT.SAS, GENALL.SAS
The Plant Diversity Experiment: The experimental area is a 250 m x 250 m area located on the NMSU Chihuahuan Desert Rangeland Research Center, immediately northeast of the intersection of the Summerford powerline road and the road running along the southern boundary of the Jornada Experimental Range and the CDRRC. The area was gridded into 25 m x 25 m plots; because of some existing environmental gradients (the area slopes to the east, and mesquite abundance varies from north to south) blocks were established and treatments were randomly assigned to plots within blocks. A 15 m x 15 m study plot with a 5 m buffer area around it was established within each 25 m x 25 m plot.
Plots are oriented north-south with the west side paralleling the powerline road. There are 3 parallel, 1 m x 15 m transects per plot. Each transect is divided into 15 1-m2 quadrats. The southwest corner of Transect 1 begins at the southwest corner of the 15 m x 15 m study plot and extends north. The southwest corner of Transect 2 begins 5 meters from the southwest corner of Transect 1 along the south boundary of the 15 m x 15 m plot and extends north. The southwest corner of Transect 3 begins 10 meters from the southwest corner of Transect 1 along the south boundary of the 15 m x 15 m plot and also extends north. The transects run perpendicular to the bajada slope.
Every fall (1 x per year) is minimum.
When resources permit and vegetation seems to deserve it, spring sampling will also be carried out (2 x per year)