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)