Data set ID:
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 environ
ment 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 (< 2 mm diameter) or coarse > 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 > 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.
field data sheets
Protocol for measuring vegetation upslope of erosion pans: Three large (2 m x 2 m) quadrats will be placed upslope from the pan, centered on a tape anchored at the edge of the pan. The quadrats are not permanently marked but are delineated during the measurements by extending a meter stick or measuring tape one meter on either side of the centrally anchored tape. All large plants (height > 25 cm) rooted within these quadrats will be measured (maximum diameter, maximum perpendicular diameter, and height -- each to the nearest centimeter). Plants will be identified by species acronym (see Jornada LTER plant species list) and by functional group: shrub, perennial grass, succulent, or subshrub. Annuals and perennial forbs will not be measured. Six smaller quadrats (50 cm x 50 cm) will be placed, two in each of the large quadrats, centered on the tape and the downslope side of the quadrat placed at the 0 cm and the 150 cm marks within each 2 m interval. They are numbered 1 through 6 sequentially from the downslope end of the tape (at the top of the erosion pan). All small plants (greater or equal to 3 cm maximum diameter, but less than or equal to 10 cm in height) will be measured as recorded as above. If an entry is recorded by the observer that is less than 3 cm, this entry should be deleted from the data set. Plants must be rooted in the quadrat to be measured. A plant being measured which is in the understory of a plant rooted outside of any quadrat is not considered to be an understory plant. A plant being measured which is in the understory of a plant rooted in the current quadrat or the adjacent quadrat is considered to be an understory plant and percent undercover of that plant is measured. The understory portion is estimated to the nearest 5 percent. Two plants of the same species whose canopies touch will be considered a single individual and measured as such. The basal stem must be at least 50% within the quadrat for the individual to be measured.
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. Each of the 48 plots of the plant diversity experiment contains 5 pans or trays on the downslope side used to collect material moved by wind or water. The vegetation measurements are taken within 4 large quadrats (2 m x 2 m) and 6 small quadrats (50 cm x 50 cm) immediately upslope from each erosion pan collector. (Furthest distance for both large and small quadrats is 6 m from edge of pan). Erosion pans are located on downslope side of each plot, just outside the boundary of the central 15 m x 15 m zone within each plot. Pans are located at roughly equal intervals along the bottom line of the 15 m x 15 m square, with the two outermost pans (A and E) placed roughly a meter from the outside corner, pan C placed at the center of the 15 m line, and B and D midway between the corner pans and the center pan. Pans are identified A - E from left to right when standing below them facing upslope.