Data set ID:
The purpose of this study is to determine whether or not the activities of small mammals regulate plant community structure, plant species diversity, and spatial vegetation patterns in Chihuahuan D
esert shrublands and grasslands. What role if any do indigenous small mammal consumers have in maintaining desertified landscapes in the Chihuahuan Desert? Additionally, how do the effects of small mammals interact with changing climate to affect vegetation patterns over time? This study will provide long-term experimental tests of the roles of consumers on ecosystem pattern and process across a latitudinal climate gradient. The following questions or hypotheses will be addressed. 1) Do small mammals influence patterns of plant species composition and diversity, vegetation structure, and spatial patterns of vegetation canopy cover and biomass in Chihuahuan Desert shrublands and grasslands? Are small mammals keystone species that determine plant species composition and physiognomy of Chihuahuan Desert communities as Brown and Heske (1990a) and Gibbens et al. (1993) suggest? Do small mammals have a significant role in maintaining the existence of shrub islands and spatial heterogeneity of creosotebush shrub communities? 2) Do small mammals affect the taxonomic composition and spatial pattern of vegetation similarly or differently in grassland communities as compared to shrub communities? How do patterns compare between grassland and shrubland sites, and how do these relatively small scale patterns relate to overall landscape vegetation patterns? 3) Do small mammals interact with short-term (annual) and long-term (decades) climate change to affect temporal changes in vegetation spatial patterns and species composition?
field data sheets
Experimental Design ------------------- A creosotebush shrub study site and a black grama grassland study site have been established at both the Jornada and Sevilleta, for a total of four study sites. The study sites were subjectively chosen to represent typical creosotebush shrub and black grama grassland communities on lower bajada or peidmont slopes at both the Jornada and Sevilleta. The study sites are located in areas that are not grazed by domestic livestock (except the Jornada grassland site; see experimental design below), to eliminate the confounding effects of livestock on vegetation and soil. Each of the four study sites are approximately 1 km by 0.5 km in area. Three rodent trapping webs and four replicate experimental blocks of plots were randomly located at each of the four study sites to measure vegetation responses to the exclusion of small mammals (Figure 2). Treatments within each block include one unfenced control plot, one fenced plot to exclude rodents and rabbits, and one fenced plot to exclude rabbits only. The three treatments were randomly assigned to each of the four possible plots in each block independently. The Jornada grassland site is grazed by cattle, so an additional treatment plot of cattle fencing was randomly assigned to one plot in each of the four blocks. Each of the three or four plots in a replicate block are separated by 20 meters. Each block of plots is situated near a rodent trapping web. Distances between the four replicate blocks of plots at each study site varies among sites from 30 meters to 800 meters, depending upon the random coordinates. Each block of experimental measurement plots consists of one unfenced control plot, and two (or three if cattle are present) fenced animal exclosure plots (Figure 2). Each experimental measurement plot measures 36 meters by 36 meters. A grid of 36 sampling points are positioned at 5.8-meter intervals on a systematically located 6 by 6 point grid within each plot. A 3- meter wide buffer area is situated between the grid of 36 points and the perimeter of each plot. A permanent one-meter by one- meter vegetation measurement quadrat is located at each of the 36 points (Figure 3). The control plots are not fenced. One year of pre-treatment or pre-fencing measurement data will be collected in 1995 from all of the plots. Analysis of the pre-treatment data will reveal any differences between plots that are independent of the treatments. Fences will be constructed in the winter of 1995, and the long-term small mammal exclosure experiment will commence in spring of 1996. One set of exclosure plots will be fenced with three-foot high wire hardware cloth of 1/4-inch mesh size to exclude rodents, and four-foot high poultry-wire fencing of two-inch mesh size to exclude rabbits. The hardware cloth screen will be buried inches to prevent animals from burrowing underneath the fence. The four-foot high poultry-wire fence will be positioned inside of and flush with the screen fence to exclude rabbits from the plot. A six-inch wide strip of metal flashing will be placed along the top of the hardware cloth, and attached outside of the poultry-wire, to prevent rodents from climbing over the fences. Steel fence posts and reinforcing bar (rebar) will be used to support the fences. The second set of exclosure plots will be fenced with poultry-wire to exclude rabbits, but not rodents. Two-inch diameter poultry-wire mesh will allow access for all local rodent species at each site. A four-foot high poultry-wire fence will be positioned around the perimeter of each plot to exclude rabbits. Steel fence posts and rebar will be used to support the fences. Fencing with hardware cloth and poultry wire will impede the ground surface movement of organic litter. Rainfall runoff transports plant litter in ground-surface sheet-flow, and in small ephemeral surface drainages or rills. To alleviate the problem of fences interfering with transport, litter accumulations on the up-slope sides of fences will be manually removed and lifted over the fences once every two months. This procedure will allow for the natural movement of soil surface organic materials on to and off of the fenced plots. Litter catchment screens made of hardware cloth and poultry wire will be placed in all small drainages crossing the up-slope perimeters of each plot, including the unfenced control plots. Litter caught on the screens will be manually lifted over the fences of treatment plots, and placed just across the perimeters of control plots. Use of catchment screens should allow normal litter transport in rills onto and off of the plots. Use of catchment screens on the control plots will account for differences in water flow that the screens might create on the fenced plots. Cattle are present (one-month low intensity winter grazing) at the Jornada black grama grassland site. Barbed-wire strands will be added to the rodent and rabbit, and rabbit only exclosure fences to exclude cattle from those plots. Additionally, a fourth measurement plot was added to each replicate block that will be fenced with barbed-wire to exclude cattle only to measure cattle effects on vegetation and soil. The cattle exclosure treatment is not intended to be part of the overall experimental design, but rather a way a accommodate for the presence of cattle at the site. There are no ungrazed black grama grassland sites at the Jornada that are large enough in area to support the small mammal exclosure study. The same experimental design is proposed for the Mapimi research site, except that a creosotebush study site and a tabosa grassland study site will be installed instead of creosotebush and black grama grassland sites as at Sevilleta and Jornada. Tabosa is the dominant grassland at Mapimi, and no black grama grasslands occur at Mapimi. Additionally, the Mapimi site is grazed by cattle, so a fourth cattle exclosure plot will be included in each block of plots at the creosotebush and tabosa grassland study sites. Rodent trapping webs are being used to determine the composition of rodent species at each study site, and to estimate densities of each species over time. The use of webs and distance measures to estimate rodent densities is statistically more robust than grid plot sampling and mark-release indices (Anderson et al. 1983, Buckland et al. 1993). Each rodent trapping web consists of a series of 12 equally spaced lines radiating from a central point. Each line consists of 12 trap stations. The first trap station is located 5 meters from the center, the next three at 5 meter intervals, and the remaining 8 at ten meter intervals. Each trap line is 100 meters long, and each web is 200 meters in diameter. The above rodent trapping web design has been used for six years at the Sevilleta LTER, and has recently been adopted by the US Centers for Disease Control and Prevention, as a standard technique for monitoring rodent populations. Vegetation will be measured two ways on each of the measurement plots. Plant species composition, cover, and above ground foliage height will be measured from each of the 36 quadrats on each plot. The maximum height (cm) and total canopy cover (cm2) of each plant species will be measured from a one- meter by one-meter measurement frame that will be placed over each permanent vegetation measurement quadrat. The plant measurement frames are constructed of 1/2-inch diameter PVC pipe. The frames are one-meter on a each side with an internal grid of 100 ten-centimeter by ten-centimeter squares made from string. Four height-adjustable legs are attached to each corner of the frame to measure vegetation from ground level up to two meters in height. Vegetation ---------- This vegetation measurement frame design has been used successfully at the Jornada LTER site to measure plant canopy cover and heights for the past six years. Biomass estimates will be made for each plant species by use of species specific regression equations that have been developed at the Jornada LTER based on clipped foliage harvests. Vegetation measurements will be taken twice each year in the late spring (April-May) to include spring annual plants, and again in the late summer (September-October) at the end of the growing season to include summer annuals and peak perennial biomass. Low-level aerial photographs will be taken to record total vegetation canopy cover and plant species composition throughout controlled camera positioned above each 36-meter by 36-meter study plot on a small weather balloon that will be held in position by tether lines. The weather balloon has been used successfully for low-level aerial photographs at the Sevilleta LTER site. Four separate aerial photographs will be taken of each 13 meter by 13 meter quarter of each plot to increase photographic resolution. Resolution of such photographs should be less than 1 centimeter, based on similar photographs taken at the Sevilleta LTER site. Complete plot vegetation map overlays will be prepared from the four photographs for each plot. Photographs will be ground-checked to verify plant species. Balloon photographs will be taken once each year at the end of the growing season in the autumn.
SAS program will be used to analyze data
The Jornada LTER site is located at the US Department of Agriculture, Agricultural Research Service, Jornada Experimental Range in southern New Mexico (Figure 1). The Jornada is located at 32 north latitude, and the grasslands and shrublands are at about 1,300 meters elevation. The long-term mean annual precipitation is 230 mm, and the mean annual temperature is 15 C. The Jornada is primarily creosotebush and mesquite (Prosopis glandulosa) shrubland with less extensive black grama grass communities. The grassland site is one kilometer west of the IBPE grassland site, and the creosotebush site is three kilo- meters southeast of the Lower Trailer. The Sevilleta LTER site is located at the US Fish and Wildlife Service, Sevilleta National Wildlife Refuge in central New Mexico (Figure 1). The Sevilleta is located at 34 north latitude, and the grasslands and shrublands are at about 1,550 meters in elevation. The grassland and shrubland areas have long- term mean annual precipitation of about 280 mm and mean annual temperature of about 13 C. The Sevilleta is predominantly black grama (Bouteloua eriopoda) and blue grama (Bouteloua gracilis) grassland, with less extensive creosotebush shrub communities. The Mapimi Biosphere Reserve is located in the south-central Chihuahuan Desert in the northeastern portion of the state of Durango, Mexico (Figure 1). The Mapimi site is located at 27 north latitude, and the creosotebush shrublands are at 1,100 meters elevation. Mapimi has a long-term mean annual precipitation of 271 mm, and the average annual temperature is 21C. The Mapimi research site is primarily creosotebush shrubland with mesquite. Tabosa (Hilaria mutica) grasslands occur in the bottoms of basins.
Twice each year through 1995: April and October Once every 5 years after 1995: October