Data set ID:
Arid and semi‐arid ecosystems often exhibit diverse plant growth forms in water‐limited environments, but it is unclear whether resource competition (interference) is actually important in structuring communities. We chose a diverse Chihuahuan desert shrubland to examine the response of the plant community to experimental removals of selected perennial plant species or groups of species. Four treatments involved the removal of all individuals of all species of a single functional group (functional group removals: shrub removal, succulent removal, subshrub removal, perennial grass removal). Three other treatments involved removing species within functional groups. These seven treatments plus a control (no plants removed) were replicated six times each in 25×25 m experimental plots, in summer 1995. Permanent belt transects were surveyed for number and sizes of all vascular plants in spring and fall in 1997, 1999, 2000, and 2001. Those plots from which the dominant shrub, Larrea tridentata, was removed had not recovered in total plant cover or volume by 2001, but cover and volume in all other treatments were similar to those in control plots. Relatively few species demonstrated a positive response to the removal of other species or functional groups. The perennial grass group and forbs were the most responsive; perennial grass cover increased in the shrub removal treatment relative to the control but treatment differences diminished after dry growing seasons in 2000 and 2001. Results over the first five years suggest that either environmental conditions or intrinsic biological characteristics limit the ability of most plant species to respond to the removal of substantial fractions of community biomass and composition in the short term. Such slow response by both dominant and less abundant components of the community has implications for the recovery of semi‐arid systems after human disturbance or other events leading to the reduction of biological diversity. This study is complete.
For more information, see:
Buonopane, M., Huenneke L., and Remmenga, M. 2005. Community reponses to removals of plant functional groups and species from a Chihuahuan Desert shrubland. Oikos 110:67-80.
field data sheets; tape recorder (varied by day and observer)
A 250×150 m area was located on the slope and gridded into 25×25 m plots. Plots containing conspicuous drainage channels were eliminated from use in the experiment. Remaining plots were evaluated by the use of two diagonal line – intercept transects run through each, along which the cover of specific shrub and succulent species (and of bare ground or openings) was recorded to the nearest cm. These preliminary data did not reveal any gradient from west to east (along the slope) in total vegetative cover or in the relative abundance of particular woody species. However, a gradient from south to north in P. glandulosa abundance was detected. Due to this spatial pattern in vegetative composition, and probable environmental gradients from upslope to down, we chose to use a randomized complete block design with 48 plots (8 treatments×6 blocks).
Treatments were established by the selective removal of plant species or of all species of a functional group within a plot. There are eight treatments: a control (C, no removals); four functional group removal treatments (PG, perennial grass removed; S, shrubs removed; SSh, subshrubs removed; Succ, succulents removed), and three treatments where richness within a functional group was manipulated.The latter include the simplified treatment (Simp, where only the single most abundant species of each growth form remains, while all other species of those growth forms are removed), the reduced‐Larrea treatment (rL, where the dominant of each growth form is removed, and minority components remain), and a second form of the reduced treatment (rP, where Prosopis rather than Larrea is removed as the shrub dominant).
We established treatments in summer/fall 1995. Plants were removed by cutting at the soil surface (no soil disturbance or herbicide application). Within each treatment, all plants removed from at least one plot were both field weighed and oven dried to constant weight. These data were used to construct regressions of dry weight versus live biomass weight. In the remaining plots of each treatment, removed plants were weighed in the field by species. Cumulative live mass totals for each species in each plot were used in the regressions to calculate the amount of dry mass removed. Dead material from shrubs and subshrubs was removed and weighed separately; this material was regarded as dry matter and no regression was used.
SAS programs were used for quality assurance and statistical analysis.
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) was minimum during the duration of the study.
When resources permit and vegetation seems to deserve it, spring sampling was also carried out (2 x per year).
This study is complete.