Biodiversity plots vegetation transects

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)