Erosion zone vegetation

Overview:
In semiarid ecosystems the diversity of plant functional types (
grasses, shrubs, succulents, and so on) and of species may interact
with the severe stresses imposed by the desert environment to influence
ecosystem processes. Erosion and transport of surface sediment by wind
and water is one process that may be affected by the physical structure
of the plant community. The Jornada plant diversity experiment, in
which the diversity and structure of the plant community have been
manipulated in large (25 m x 25 m) plots, offers the opportunity to
examine the relative importance of vegetation characteristics and
landscape position in determining rates of sediment movement within
the plots.
Each of the 48 plots of the plant diversity experiment contains
5 pans or trays on the downslope side; these accumulate sediments and
plant litter that are moving within the plots (carried by wind or by
water). Data have been collected on the amount of sediment accumulated
in the pans during rainy and during dry seasons, with material sorted
and weighed as fine ( less than 2 mm diameter) or coarse more than 2 mm) mineral sediment,
plant litter, or rabbit/jackrabbit pellets. Previous statistical analyses
found that the mass of material collected per plot is explained only poorly
by the treatment (plant community manipulation) of the plot and by block
(a rough indication of location on the slope).

Objectives:
We will test the relative significance of the following variables
in explaining plot-level accumulations of sediment and litter: treatment,
block, position on slope (the row, from 1 (top of slope) to 10 (bottom),
in which the plot is located), the treatment of the plot immediately upslope
from the plot, and indices of plant cover and volume (total and by
functional group) from the plot-level sampling of vegetation (using
fall 1997 data).
In addition, we will test the significance of the following variables
in explaining the accumulation of sediment and litter in individual pans
within a plot: all variables listed above for the plot, plus indices of
the vegetative cover and volume located immediately upslope of the pan
(weighted for distance from the pan itself).

Response variables:
Vegetative cover measurements are made immediately upslope of
erosion pans to estimate plant cover and volume. This is done at
two scales. The three large quadrats (2 m x 2 m) are used to look
at all large plants (height more than 25 cm) rooted within them. The six
small plots (50 cm x 50 cm) are used to look at all small plants
(greater or equal to 3 cm maximum diameter, but less than or equal
to 10 cm) rooted within them. Maximum diameter, maximum perpendicular
diameter, and height are measured to the nearest centimeter.