Grazers and granivores have the potential to affect seed banks. Several studies have examined the impact of these herbivores on the aboveground vegetation, but few have looked at how they influence the seed bank. I asked whether both grazers and granivores alter the seed bank at the Sevilleta National Wildlife Refuge. Long-term experimental plots were installed in 1996 to exclude grazers and granivores from a grassland and shrubland. Soil samples were collected from these plots and seeds were germinated in a greenhouse. The grassland had significantly more species in its seed bank than the shrubland. Also, the seed bank composition differed significantly between the two sites. However, the number of species in the seed bank did not vary among herbivore treatments nor did total seed numbers vary among treatments at the grassland. At the shrubland, in contrast, plots that excluded both herbivores had fewer total seeds than control plots and plots where only grazers were excluded. Therefore, although herbivores play some role in the shrubland, herbivores do not reduce seed numbers at either site. Thus, seed bank size is not controlled by the consumption of seeds from these herbivores, but by some other factor (e.g. disturbance or abiotic events).
Collecting and Processing soil samples
Soil samples were collected at each site for each treatment in both mid March and October in 2004 for a total of 720 samples(15 samples per treatment x 3 treatments per block x 4 blocks per site x 2 sites x 2 seasons). There are 30 points between vegetation subplots in each of the Small Mammal Exclosure Study plots. Fifteen of these points were randomly selected from each plot. Soil samples were taken using a square electrical box with dimensions of 10cm(length) x 10cm (width) x 2cm(depth). These soil samples were then stored in paper bags and labeled with a unique sample number, indicating the site, block, treatment, and sample point, using a sharpie marker. Soil samples were then taken to the University of New Mexicos greenhouse. Due to space constraints in the greenhouse, only half of the samples from each season could be censused at one time. Therefore, I randomly selected half of the samples from each plot for each census. Soil samples were spread in a thin layer over a soil mixture containing half sand and half Metromix 360 in 8 x 11 flats. Flats were arranged into eight blocks each containing one sample from each plot to minimize variance due to greenhouse location. Control flats of sand and Metromix 360(with no field soil) were also distributed in the eight blocks to account for any seeds that might have contaminated the sand. The samples were then watered using a sprinkler system three times a day for five-minute durations where the thermostat temperature ranged from 16 to 25 Celsius for seven weeks. All seedlings were counted and identified to species. These procedures were repeated for the remaining half of the soil samples for each season.
Metadata entered in Microsoft Access. 26 May 2009. tk
Data were visually assessed for any errors.
Additional Information on the Data Collection Period
Soil samples were taken in mid March and October of 2004.
Additional Study Area Information
Study Area 1
Study Area Name: Five Points Grass Core Site
Study Area Location: Five Points is the general area which emcompasses the Black Grama Grassland (known as Five Points Grassland) and Creosote Core (Five Points Larrea) study sites and the transition between Chihuahuan Desert Scrub and Desert Grassland habitats. Both core sites are subject to intensive research activities, including measurements of NPP, phenology, pollinator diversity, and ground dwelling arthropod and rodent populations. There are drought rain-out shelters in both the Grassland and Creosote sites, as well as another set in the mixed ecotone with co-located ET Towers. The grassland Small Mammal Exclosure Study is located here, as well as many plots related to patch mapping and biotic transitions.Elevation: 1616 m
Vegetation: Desert Grassland habitat is ecotonal in nature and the Black Grama Core site is no exception, bordering Chihuahuan Desert Scrub at its southern boundary and Plains-Mesa Grassland at its northern, more mesic boundary. There is also a significant presence of shrubs, dominantly broom snakeweed (Gutierrezia sarothrae), along with less abundant fourwing saltbush (Atriplex canescens), Mormon tea (Ephedra torreyana), winterfat (Krascheninnikovia lanata), tree cholla (Opuntia imbricata), club cholla (O. clavata), desert pricklypear (O. phaeacantha), soapweed yucca (Yucca glauca), and what are presumed to be encroaching, yet sparsely distributed, creosotebush (Larrea tridentata). Characteristically, the dominant grass was black grama (Bouteloua eriopoda). Spike, sand, and mesa dropseed grasses (Sporobolus contractus, S. cryptandrus, S. flexuosus) and sand muhly (Muhlenbergia arenicola) could be considered co-dominant throughout, along with blue grama (B. gracilis) in a more mesic, shallow swale on the site. Notable forb species included trailing four o’clock (Allionia incarnata), horn loco milkvetch (Astragalus missouriensis), sawtooth spurge (Chamaesyce serrula), plains hiddenflower (Cryptantha crassisepala), blunt tansymustard (Descurania pinnata), wooly plaintain (Plantago patagonica), globemallow (Sphaeralcea wrightii), and mouse ear (Tidestromia lanuginosa).North Coordinate:34.3381South Coordinate:34.3381East Coordinate:106.717West Coordinate:106.717
Study Area 2
Study Area Name: Rio Salado
Study Area Location: Rio Salado is about 3 km West I-25 just south of the Rio Salado. Site is accessed by taking San Acacia exit, going west and then taking the frontage road back north to the Sevilleta gate. After entering the refuge turn left after about .2 mi and take this road about 1.4 mi to a T in the road at the power lines. An earthen berm stops road travel here and the station is located about 300 m west on the blocked road.Elevation: 1503 mNorth Coordinate:34.296South Coordinate:0East Coordinate:0West Coordinate:106.9267