In keeping with the Intergovernmental Panel on Climate Change (2007) global climate projections, this study aims to determine how a shift towards more extreme precipitation events punctuated with longer dry intervals will affect the ecophysiological response of two arid-semiarid grassland species, Bouteloua eriopoda and Bouteloua gracilis. Both species are dominants in their respective ecosystems (Chihuahuan Desert Grassland and Great Plains Short Grassland Prairie, respectively) and co-occur at an ecotone located at the Sevilleta National Wildlife Refuge (SNWR) in central New Mexico. Knowing how these dominant grassland species differentially respond to altered precipitation regimes is critical to understanding potential shifts in ecotone boundaries due to increased climate variability. I hypothesize that large, infrequent rainfall events will increase photosynthesis and stomatal conductance and decrease leaf water potential in both species. However, B. eriopoda will maintain higher rates of photosynthesis, stomatal conductance and lower leaf water potentials during dry intervals, which will increase aboveground net primary productivity (ANPP) when compared with B. gracilis. Thus, giving B. eriopoda an advantage and potentially shifting the ecotone boundary northward under proposed climate scenarios.
Each rainout shelter is constructed using two angle-iron frames that measure 1.52 m at the highest point and 1.0 m at the lowest point with 14o angled roof. Steel cables are used to connect each frame end and increase stability. All shelters are securely anchored in 2 ways: 1) each frame end is buried to a depth of 30.5 cm and 2) each corner post is tied down using metal cables extending from the top of each post down to metal stakes that were hammered into the ground. In addition, shelters were positioned against the prevailing wind to avoid potential lift. The roof of each rainout shelter consists of four 2.45 m x 0.61 m polycarbonate panels that are secured to each frame. Metal flashing was placed along the perimeter of each plot to prevent surface run-on and runoff. Within each study area, two 300 gallon water storage tanks were installed for water storage. Watering treatments include: 1) 10 mm event applied three times / month and 2) a 30 mm event applied once / month. All rainfall treatments and intervals lengths are based on 20 year regional precipitation patterns collected from the SNWR. Additional hardware installed at each study site includes solar panels and battery enclosures used to power the data loggers that store ambient data: air temperature, PAR, relative humidity and soil temperature.
Field measurements are non-destructive. All plots are sampled twice a year for estimates of plant cover and height (Aboveground Net Primary Productivity). Throughout the monsoon season (June-September), photosynthesis and soil CO2 flux are monitored bi-weekly using a portable photosynthesis system. Leaves removed (2-4) for photosynthesis measurements will be returned to the laboratory for further analysis. Leaf water status of selected plants is measured using a portable Scholander-type pressure chamber. Initially, soil cores (~ 30 cm in depth) were extracted once from each plot to determine soil texture; this physical property has been shown to influence the availability of soil water in aridland ecosystems. Soil moisture is monitored in each sheltered to a depth of 15 cm using soil moisture probes. Shelter effects will be assessed using (non-sheltered) data from nearby met stations.
Rain-out shelters were constructed at three separate locations on the SNWR: 1) ten sheltered plots were installed at the B. gracilis core site, 2) ten sheltered plots were installed at a B. eriopoda core site and 3) ten shelters were installed near Deep Well where both B. eriopoda and B. gracilis co-dominate. The total shelter area is ~ 250 m2 at each site.
LI-6400 portable photosynthesis system and Scholander Pressure Bomb
Additional Study Area Information
Study Area 1
Study Area Name: Blue Grama Core Site
Study Area Location: The Blue Grama Core Site is one of 5 current core SEVLTER study sites. Core studies include meteorology, rodent abundance, pollinator diversity, monthly phenology, and NPP. Additional studies have examined the Bootleg Canyon fire of 1998 and grass patch dynamics.
Elevation: 1670 m
Vegetation: Vegetation is characterized as Plains-Mesa Grassland, dominated by blue and black gramma (Bouteloua gracilis & B. eriopoda) and galleta grass (Hilaria jamesii)North Coordinate:34.3348South Coordinate:34.3348East Coordinate:106.631West Coordinate:106.631
Study Area 2
Study Area Name: Deep Well
Study Area Location: Deep Well is centrally located on McKenzie Flats and is home to the longest running SEVLTER Met Station 40, which was active in 1988. As such, many studies are located here. In addition to meterology, core line-intercept vegetation transects, as well as line-intercept transects from the 1995 & 2001 Deep Well Fires, the Mini-rhizotron root study, blue and black grama compositional comparison, blue and black grama patch dynamics, and blue and black grama and kangaroo rat dynamics are ongoing here.Elevation: 1600 m
Vegetation: The vegetation is Chihuahuan Desert Grassland, dominated by black grama (Bouteloua eriopoda & B. gracilis).
North Coordinate:34.3592South Coordinate:34.3592East Coordinate:106.691West Coordinate:106.691
Study Area 3
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 (Descarania obtusa), 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