EDGE is located at six grassland sites that encompass a range of ecosystems in the Central US - from desert grasslands to short-, mixed-, and tallgrass prairie. We envision EDGE as a research platform that will not only advance our understanding of patterns and mechanisms of ecosystem sensitivity to climate change, but also will benefit the broader scientific community. Identical infrastructure for manipulating growing season precipitation will be deployed at all sites. Within the relatively large treatment plots (36 m2), we will measure with comparable methods, a broad spectrum of ecological responses particularly related to the interaction between carbon fluxes (NPP, soil respiration) and species response traits, as well as environmental parameters that are critical for the integrated experiment-modeling framework, as well as for site-based analyses. By designing EDGE as a research platform open to the broader scientific community, with subplots in all replicates (n = 180 plots) set-aside for additional studies, and by making data available to the broader ecological community EDGE will have value beyond what we envision here.
The six sites were selected to capture the key environmental and ecological gradients of Central US grasslands and represent the major grassland ecosystem types (desert, shortgrass, mixedgrass, and tallgrass) of the region. Site selection criteria included: site characteristics (mean annual precipitation and temperature, dominant vegetation), access and site security, permission to build experimental infrastructure, participation in an existing or future network (e.g., LTER, NEON), and available site support and supporting data (e.g., LTER, USFWS or ARS).
Experimental Treatments and Plots
Our approach will be to impose a significant reduction in growing season precipitation (-66 % of ambient) over a 4-yr period. This is the equivalent of a ca. 50% reduction in annual precipitation because at all sites about 60-75% of annual precipitation falls in the growing season. We will impose this long-term drought either by reducing the size of each rainfall event (event size reduction, E) or by reducing the number of events (delayed rainfall treatment, D).
The control (C) treatment is included for comparison. At each site, the ambient (C) rainfall pattern will be reduced in two ways to impose a severe drought over a 4-yr period.
For the event size reduction treatment (E), each rainfall event will be passively reduced by a fixed proportion. Note that rain event number and the average number of days between events does not differ from ambient treatment.
For the reduced event number (D) treatment, shelters roofs will be removable to permit periods of complete rain exclusion alternating with periods of ambient rainfall inputs. Here, a + 10 mm rule is used to determine when roofs are on or off. When the cumulative precipitation amount in this D treatment falls 10 mm below the E treatment, the roofs are removed until the cumulative precipitation total is 10 mm greater than the E treatment. In this way, total precipitation amounts will be similar at the end of the growing season, but event number will be reduced and the average number of days between events increased, with no change in event size compared to the C treatment.
At each site, we will establish replicate 6 x 6 m experimental plots (n = 10 per treatment, including the control treatment) in a relatively homogeneous area (similar soils, vegetation, etc.) that is representative of the overall site. Plots will be arrayed such that each treatment will be co-located in a single block (n=10 blocks per site), with each block located at least 5 m apart.
The blocking will help control for environmental gradients if present. For each site, all plots within a block (including the control) will be located at least 2 m apart and trenched to 1-1.5 m and surrounded by a 6 mil plastic barrier to hydrologically isolate them from the adjacent soil, and each plot will be covered by the rainfall manipulation infrastructure. The 6 x 6 m plot size includes a 0.5 m external buffer to allow access to the plots and minimize edge effects associated with the infrastructure. The resulting 5 x 5 m area will be divided into 4 2.5 x 2.5 m subplots. One subplot will be designated for plant species composition sampling, two will for destructive sampling (ANPP, belowground productivity, soil sampling, etc.), and the fourth set aside for opportunistic studies.
Rainfall Manipulation Infrastructure
We will passively alter rainfall reaching the plots by using a version of a rainfall reduction shelter (Fig. 6) designed by Yahdjian and Sala (2002). Versions of these shelters (ranging from ~2 to 100 m2 ) are being used by the co-PIs at the Sevilleta, Konza Prairie and Shortgrass Steppe LTERs, as well as by many other ecologists, and thus, they are proven technology. The most significant environmental artifacts of these shelters are a 5- 10% reduction in light due to the acrylic Vshaped shingles and a ~ 20 cm edge effect (Yahdjian and Sala 2002). Shelters will consist of a steel frame that supports a roof. To cover the 36 m2 plots, the shelters will be constructed as modular 3 x 3 m units, with four units per plot. The roof of each modular unit will be slanted at 15° toward the edge of the plot, creating a 6 m long peak along the mid-line of the plot, with two lower 6 m long edges with gutters to move rainwater away from the plots. The peaked roof will facilitate run-off of rainfall and access to the plot, and the lower edge will be oriented to the prevailing wind direction to minimize blow-in. Average leaf canopy height varies among the desert/short-, midand tallgrass prairie sites (~0.2 to 0.6 m), and to maintain a consistent roof-to-canopy distance, peak height of the shelters will be 1.3, 1.55 and 1.8 m, with lower edges of the shelters at 0.5, 0.75 and 1.0 m, respectively, for the four grassland types. Construction of the shelters will begin in Yr 1 (after pretreatment measurements are taken) and treatments will be operational by the early spring of YR 2. For the ESR treatment, the roof will consist of clear acrylic (high light transmission, low yellowness index, UV transparent) v-shaped shingles arrayed at a density to passively reducing each rainfall event by ~66% (Fig. 6). For the REN treatment, the roof will consist of clear, corrugated polycarbonate (high light transmission, low yellowness index, UV transparent) to completely exclude rainfall. For both treatments, the roofs will be constructed to facilitate easy removal via a clamping system. The REN treatment roofs will then be manually deployed and removed at intermittent intervals (see Fig. 6 for more detail). Ambient plots will have a deer netting roof to achieve an average reduction in light similar to the rainfall reduction roofs.
Plant species composition, species traits, stem density, and light availability
In the subplot designated for species composition, we will establish a permanent 2 x 2 m sampling plots, which will be divided into four 1 x 1m quadrats in which canopy cover of each species will be visually estimated to the nearest 1%. For each site, these measures will be repeated at least twice during the growing season of each year to sample early and late season species. Maximum cover values of each species will be used to determine richness, diversity and dominance and changes in composition, species turnover, and species associations over time.
Collecting the Data:
Net primary production data is collected twice each year, spring and fall, for both sites. Spring measurements are taken in April or May when shrubs and spring annuals have reached peak biomass. Fall measurements are taken in either September or October when summer annuals have reached peak biomass but prior to killing frosts. Winter measurements are taken in February before the onset of spring growth.
Vegetation data is collected on a palm top computer. A 1-m2 PVC-frame is placed over the fiberglass stakes that mark the diagonal corners of each quadrat. When measuring cover it is important to stay centered over the vegetation in the quadrat to prevent errors caused by angle of view (parallax). Each PVC-frame is divided into 100 squares with nylon string. The dimensions of each square are 10cm x 10cm and represent 1 percent of the total area.
The cover (area) and height of each individual live (green) vegetative unit that falls within the one square meter quadrat is measured. A vegetative unit consists of an individual size class (as defined by a unique cover and height) of a particular species within a quadrat. Cover is quantified by counting the number of 10cm x 10cm squares filled by each vegetative unit.
Niners and plexidecs are additional tools that help accurately determine the cover a vegetative unit. A niner is a small, hand-held PVC frame that can be used to measure canopies. Like the larger PVC frame it is divided into 10cm x 10cm squares, each square representing 1% of the total cover. However, there are only nine squares within the frame, hence the name “niner.” A plexidec can help determine the cover of vegetative units with covers less than 1%. Plexidecs are clear plastic squares that are held above vegetation. Each plexidec represents a cover of 0.5% and has smaller dimensions etched onto the surface that correspond to 0.01%, 0.05%, 0.1%, and 0.25% cover.
It is extremely important that cover and height measurements remain consistent over time to ensure that regressions based on this data remain valid. Field crew members should calibrate with each other to ensure that observer bias does not influence data collection.
Grasses-To determine the cover of a grass clump, envision a perimeter around the central mass or densest portion of the plant, excluding individual long leaves, wispy ends, or more open upper regions of the plant. Live foliage is frequently mixed with dead foliage in grass clumps and this must be kept in mind during measurement as our goal is to measure only plant biomass for the current season. In general, recently dead foliage is yellow and dead foliage is gray. Within reason, try to include only yellow or green portions of the plant in cover measurement while excluding portions of the plant that are gray. This is particularly important for measurements made in the winter when there is little or no green foliage present. In winter, sometimes measurements will be based mainly on yellow foliage. Stoloniferous stems of grasses that are not rooted should be ignored. If a stem is rooted it should be recorded as a separate observation from the parent plant.
Forbs, shrubs and sub-shrubs (non-creosote)-The cover of forbs, shrubs and sub-shrubs is measured as the horizontal area of the plant. If the species is an annual it is acceptable to include the inflorescence in this measurement if it increases cover. If the species is a perennial, do not include the inflorescence as part of the cover measurement. Measure all foliage that was produced during the current season, including any recently dead (yellow) foliage. Avoid measuring gray foliage that died in a previous season.
Cacti-For cacti that consist of a series of pads or jointed stems (Opuntia phaecantha, Opuntia imbricata) measure the length and width of each pad to the nearest cm instead of cover and height. Cacti that occur as a dense ball/clump of stems (Opuntia leptocaulis) are measured using the same protocol as shrubs. Pincushion or hedgehog cacti (Escobaria vivipara, Schlerocactus intertextus, Echinocereus fendleri) that occur as single (or clustered) cylindrical stems are measured as a single cover.
Yuccas-Make separate observations for the leaves and caudex (thick basal stem). Break the observations into sections of leaves that are approximately the same height and record the cover as the perimeter around this group of leaf blades. The caudex is measured as a single cover. The thick leaves of yuccas make it difficult to make a cover measurement by centering yourself over the caudex of the plant. The cover of the caudex may be estimated by holding a niner next to it or using a tape measure to measure to approximate the area.
Height is recorded as a whole number in centimeters. All heights are vertical heights but they are not necessarily perpendicular to the ground if the ground is sloping.
Annual grasses and all forbs-Measure the height from the base of the plant to the top of the inflorescence (if present). Otherwise, measure to the top of the green foliage.
Perennial grasses-Measure the height from the base of the plant to the top of the live green foliage. Do not include the inflorescence in the height measurement. The presence of live green foliage may be difficult to see in the winter. Check carefully at the base of the plant for the presence of green foliage. If none is found it may be necessary to pull the leaf sheaths off of several plants outside the quadrat. From this you may be able to make some observations about where green foliage is likely to occur.
Perennial shrubs and sub-shrubs (non-creosote)-Measure the height from the base of the green foliage to the top of the green foliage, ignoring all bare stems. Do not measure to the ground unless the foliage reaches the ground.
Plants rooted outside but hanging into a quadrat-Do not measure the height from the ground. Measure only the height of the portion of the plant that is within the quadrat.
Additional Information on the personnel associated with the Data Collection / Data Processing
Nathan Gehres 2014-present; Michell Thomey 2012-2014
Larrea tridentata (Creosote Shrub) is a generalist plant that provides floral resources in the form of nectar and pollen to a wide variety of bee species. The aim of this study is to evaluate the extent of this interaction at the Sevilleta Creosote Shrubland. Specifically, bee individuals directly interacting with L. tridentata were captured in order to give an accurate description of the number of species dependent on Creosote for resources.
Begun in spring 2013, this project is part of a long-term study at the Sevilleta LTER measuring net primary production (NPP) across three distinct ecosystems: creosote-dominant shrubland (Site C), black grama-dominant grassland (Site G), and blue grama-dominant grassland (Site B). Net primary production is a fundamental ecological variable that quantifies rates of carbon consumption and fixation. Estimates of NPP are important in understanding energy flow at a community level as well as spatial and temporal responses to a range of ecological processes.
Above-ground net primary production is the change in plant biomass, represented by stems, flowers, fruit and and foliage, over time and incoporates growth as well as loss to death and decomposition. To measure this change the vegetation variables in this dataset, including species composition and the cover and height of individuals, are sampled twice yearly (spring and fall) at permanent 1m x 1m plots within each site. A third sampling at Site C is performed in the winter. Volumetric measurements are made using vegetation data from permanent plots (SEV289, "Core Site Grid Quadrat Data for the Net Primary Production Study") and regressions correlating species biomass and volume constructed using seasonal harvest weights from SEV157, "Net Primary Productivity (NPP) Weight Data."
Data Processing Techniques to Derive Biomass and NPP:
Data from SEV289 and SEV157 are used used to calculate seasonal and annual production of each species in each quadrat for a given year. Allometric equations derived from harvested samples of each species for each season are applied to the measured cover, height, and count of each species in each quadrat. This provides seasonal biomass for winter, spring, and fall.
Seasonal NPP is derived by subtracting the previous season's biomass from the biomass for the current season. For example, spring NPP is calculated by subtracting the winter weight from the spring weight for each species in a given quadrat. Negative differences are considered to be 0. Likewise, fall production is computed by subtracting spring biomass from fall biomass. Annual biomass is taken as the sum of spring and fall NPP.
Other researchers involved with collecting samples/data: Chandra Tucker (CAT; 04/2014-present), Megan McClung (MAM; 04/2013-present), Stephanie Baker (SRB; 2013-present), John Mulhouse (JMM; 2013).
This dataset contains pinon-juniper woodland biomass data and is part of a long-term study at the Sevilleta LTER measuring net primary production (NPP) across four distinct ecosystems: creosote-dominant shrubland (Site C, est. winter 1999), black grama-dominant grassland (Site G, est. winter 1999), blue grama-dominant grassland (Site B, est. winter 2002), and pinon-juniper woodland (Site P, est. winter 2003). Net primary production is a fundamental ecological variable that quantifies rates of carbon consumption and fixation. Estimates of NPP are important in understanding energy flow at a community level as well as spatial and temporal responses to a range of ecological processes.
Above-ground net primary production is the change in plant biomass, represented by stems, flowers, fruit and and foliage, over time and incoporates growth as well as loss to death and decomposition. To measure this change the vegetation variables in this dataset, including species composition and the cover and height of individuals, are sampled twice yearly (spring and fall) at permanent 1m x 1m plots within each site. A third sampling at Site C is performed in the winter. Volumetric measurements are made using vegetation data from permanent plots (SEV278, "Pinon-Juniper (Core Site) Quadrat Data for the Net Primary Production Study") and regressions correlating species biomass and volume constructed using seasonal harvest weights from SEV157, "Net Primary Productivity (NPP) Weight Data."
Data from SEV278 and SEV157 are used used to calculate seasonal and annual production of each species in each quadrat for a given year. Allometric equations derived from harvested samples of each species for each season are applied to the measured cover, height, and count of each species in each quadrat. This provides seasonal biomass for winter, spring, and fall.
Other researchers involved with collecting samples/data: Chandra Tucker (CAT; 04/2014-present), Megan McClung (MAM; 04/2013-present), Stephanie Baker (SRB; 10/2010-present), John Mulhouse (JMM; 08/2009-06/2013), Amaris Swann (ALS; 08/2008-present), Maya Kapoor (MLK; 08/2003 - 01/2005, 05/2010 - 03/2011), Terri Koontz (TLK; 02/2000 - 08/2003, 08/2006 - 08/2010), Yang Xia (YX; 01/2005 - 03/2010), Karen Wetherill (KRW; 02/2000 - 08/2009); Michell Thomey (MLT; 09/2005 - 08/2008), Heather Simpson (HLS; 08/2000 - 08/2002), Chris Roberts (CR; 09/2001- 08/2002), Shana Penington (SBP; 01/2000 - 08/2000), Seth Munson (SMM; 09/2002 - 06/2004), Jay McLeod (JRM; 01/2006 - 08/2006); Caleb Hickman (CRH; 09/2002 - 11/2004), Charity Hall (CLH; 01/2005 - 01/2006), Tessa Edelen (MTE, 08/2004 - 08/2005).
Shrub encroachment is a global phenomenon. Both the causes and consequences of shrub encroachment vary regionally and globally. In the southwestern US a common native C3 shrub species, creosotebush, has invaded millions of hectares of arid and semi-arid C4-dominated grassland. At the Sevilleta LTER site, it appears that the grassland-shrubland ecotone is relatively stable, but infill by creosotebush continues to occur. The consequences of shrub encroachment have been and continue to be carefully documented, but the ecological drivers of shrub encroachment in the southwestern US are not well known.
One key factor that may promote shrub encroachment is grazing by domestic livestock. However, multiple environmental drivers have changed over the 150 years during which shrub expansion has occurred through the southwestern US. Temperatures are warmer, atmospheric CO2 has increased, drought and rainy cycles have occurred, and grazing pressure has decreased. From our prior research we know that prolonged drought greatly reduces the abundance of native grasses while having limited impact on the abundance of creosotebush in the grass-shrub ecotone. So once established, creosotebush populations are persistent and resistant to climate cycles. We also know that creosotebush seedlings tend to appear primarily when rainfall during the summer monsoon is well above average. However, high rainfall years also stimulate the growth of the dominant grasses creating a competitive environment that may not favor seedling establishment and survival. The purpose of the Mega-Monsoon Experiment (MegaME) is twofold. First, this experiment will determine if high rainfall years coupled with (simulated) grazing promote the establishment and growth of creosotebush seedlings in the grassland-shrubland ecotone at Sevilleta, thus promoting infill and expansion of creosotebush into native grassland. Second, MegaME will determine if a sequence of wet summer monsoons will promote the establishment and growth of native C4 grasses in areas where creosotebush is now dominant, thus demonstrating that high rainfall and dispersal limitation prevent grassland expansion into creosotebush shrubland.
Vegetation and soil measurements are taken in the spring and fall each year. Spring measurements are taken in May when spring annuals have reached peak biomass for the growing season. Fall measurements are taken in either September or October when summer annuals and all perennial species have reached peak biomass for the growing season, but prior to killing frosts. Vegetation cover is measured to assess growth and survival of grasses and shrubs. Bare soil and litter covers are also measured to monitor substrate changes that occur within the plots.
One meter2 vegetation quadrats are used to measure the cover of all plants present in each m2. There are 10 quads in each plot, checkered along on side of the plot. There is a tag on one rebar of each quad with the representative quad number.
General vegetation measurements
The cover is recorded for each species of live plant material inside the quadrat. Vegetation measurements are taken in two layers: a ground level layer that includes all grasses, forbs, sub-shrubs, and a litter and bare soil, and a “shrub” layer that includes the canopy of Larrea tridentata. The purpose of this approach is to include Larrea canopies, while allowing the cover values of the ground level layer to sum to approximately 100%. The dead plant covers are not included in the measurement, thus the total amount may not equal 100%. It is assumed that the remaining cover missing from the 100% is a combination of dead plant material.
The quadrat boundaries are delineated by the 1 m2 PVC-frame placed above the quadrat. Each PVC-frame is divided into 100 squares with nylon string. The dimensions of each square are 10cm x 10cm and represent 1 % of the total quadrat area or cover. The cover and height of all individual plants of a species that fall within the 1m2 quadrat are measured. Cover is quantified by counting the number of 10cm x 10cm squares intercepted by all individual plants of a particular species, and/or partial cover for individual plants < 1%.
Vegetation cover measurements
Cover measurements are made by summing the live cover values for all individual plants of a given species that fall within an infinite vertical column that is defined by the inside edge of the PVC-frame. This includes vegetation that is rooted outside of the frame but has foliage that extends into the vertical column defined by the PVC-frame. Again, cover is quantified by counting the number of 10cm x 10cm squares intercepted by each species. Do not duplicate overlapping canopies, just record the total canopy cover on a horizontal plane when looking down on the quadrat through the grid.
Larger cover values will vary but the smallest cover value recorded should never be below 0.1%. When dealing with individual plants that are < 1.00%, round the measurements to an increment of 0.1. Cover values between 1.00% and 10.00% should be rounded to increments of 1.0, and values > 10.00% are rounded to increments of 5.
Larrea tridentata canopy is estimated using the portion of the canopy that falls within the quadrat. The canopy edge is defined by a straight gravity line from the canopy to the ground (i.e. imagine a piece of string with a weight on the end being moved around the canopy edge). ForLarrea seedlings the code LSEED is used and is a separate measurement from the Larrea canopy measurements. The cover measurement for LSEED is simply a count of individuals, not actual cover, as it is assumed that they would have a cover of < 1.00%.
To determine the cover of a grass clump, envision a perimeter around the central mass or densest portion of the plant excluding individual long leaves, wispy ends or more open upper regions of the plant. Live tissue is frequently mixed with dead tissue in grass clumps.
The cover of forbs is the perimeter around the densest portion of the plant. Measure all foliage that was produced during the current season.
Cacti and Yucca
The cover of cacti and yucca is made by estimating a perimeter around the densest portion of the plant and recorded as a single cover. For cacti that consist of a cluster of pads or jointed stems (i.e., Opuntia phaecantha, Opuntia imbricata), estimate an average perimeter around the series of plant parts and record a single coverage measurement.
Vine cover (and some forbs) is often convoluted. Rather than attempt to estimate cover directly, take a frequency count of 10X10X10cm cubes that the vine is present in.
As with other vegetation measurements, the smallest cover value for seedlings should never be <0.1%. If the value of a seedling’s cover is less, round up to 0.1%.
Non-Vegetation cover measurements
Materials other than vegetation that are measured in the drought plots include soil and litter.
Measure the cover of the area occupied by abiotic substrates. Cover is quantified by summing the number of 10cm x 10cm squares intercepted by abiotic substrates. Cover values < 10.00% should be rounded to increments of and cover values > 10.00% should be recorded in increments of 5. If there is no soil in the quadrat, record “SOIL” in the species column for that quadrat and record a “0” for cover.
Measure the cover of the area occupied by litter, which is unattached dead plant material. Cover is quantified by summing the number of 10cm x 10cm squares intercepted by abiotic substrates. Cover values < 10.00% should be rounded to increments of 1 and cover values > 10.00% should be recorded in increments of 5. If there is no litter in the quadrat, record “LITT” in the species column for that quadrat and record a “0” for cover.
Clipping grass at Ecotone Site
After measurements are taken at the Ecotone Site, grass is clipped down to the soil and removed from half of the quads in each plot. The goal is to assess the impact of competition on successful creosote seedling germination. The following quads, # 2, 4, 6, 7, and 10, get clipped in every plot at the ecotone site.
The watering schedule varies based on seasonal rainfall. Our goal is to increase average monsoon precipitation (150mm) by 50%, so we shoot for a total of 225mm on the plots during the summer monsoon.
Additional Information on the personnel associated with the Data Collection:
Stephanie Baker 2014-present
Megan McClung 2014-present
Chandra Tucker 2014-present
The purpose of this project is to test the hypothesis that the smallest 50% of precipitation events during the monsoon season affect microbial functioning and grassland productivity in mixed grasslands of B.eriopoda and B. gracilis at the SNWR. At the SNWR, the summer monsoon season accounts for 60% of total annual precipitation and drives the majority of vegetation productivity during the year; the largest 25% of precipitation events account for the majority of this precipitation. I predict that important ecological variables such as nutrient and soil moisture availability are disproportionately influenced by smaller events. The proposed project will help tease apart the importance of precipitation event classes on nutrient availability and grassland aboveground net primary production (ANPP). This research will also provide a basis for understanding how increased aridity in the U.S. southwest due to increasing global surface temperature and altered precipitation could affect grassland communities at the SNWR.
We will implement 10 open plots (control) and 10 precipitation exclosure plots(treatment; 20 total plots) at a mixed blue and black grama grassland site at the SNWR. In this experiment, treatment plots will only receive the largest 50% of precipitation events. This will maintain statistically similar total precipitation between control and treatment plots because the smallest 50% of events have an insignificant effect on total seasonal precipitation. How these small events are linked to microbial activity and vegetation productivity is still very much unknown. I predict that soil microbial activity and nutrient availability will differ between control and treatment plots and will result in differing vegetation ANPP between them. These effects may become more distinct as time progresses, which is the reason for conducting this research for a series of monsoon seasons.
Existing precipitation exclosures (2.45 m x 2.45 m) will be employed at the mixed grassland site. We will implement 20 total plots (10 control, 10 treatment; approx. 500 m2 total area). Temporary site infrastructure will include 10 precipitation exclosures, a water tank (1100 gal.) and soil moisture probes. This infrastructure currently exists at the mixed grassland site and will be adopted from Michell Thomey's project entitled, "Soil moisture extremes and soil water dynamics across a semiarid grassland ecotone."
Precipitation is the only independent variable in this experiment. Using precipitation exclosures, I will remove all ambient precipitation from treatment plots from DOY 182-273. Ambient daily precipitation thatexceeds the estimated 50% threshold will be delivered to the plots within 24 hours of an event. Delivered precipitation will be adjusted for atmospheric demand differences.
Dependent variables in this experiment are vegetation ANPP, soil nitrogen content, soil enzymatic activityand soil moisture content. Vegetation biomass will be collected from the sites on DOY 181 and 274. Soil enzymatic activity will be determined approximately 4 times per monsoon season using plot soil samples. Soil nitrogen content will be measured under vegetation using nitrogen probes. Volumetric soil moisture content [m3 m-3] will be measured continuously using soil moisture probes (30 cm depth).
Biological soil crusts (BSCs) are complex assemblages of fungi, lichens, bacteria, mosses and green algae that stabilize surface soils and manage and traffic photosynthate, nutrients and water to diverse microbial and producer communities in arid environments worldwide. In Sevilleta grasslands, BSCs occupy much of the open space between clumps of vegetation and vary substantially in terms of structure.
BSCs have important biological and physical roles. They have been termed ‘mantles of fertility’ because of their general importance in biogeochemical cycling and net primary production in arid ecosystems. It has been proposed that BSCs play a role in the rapid movement of N, C and water from open areas to plants (see below). BSCs stabilize soils, and physical and chemical disturbances of BSCs lead to topsoil loss and dust storms. BSCs are therefore critical components in efforts to understand implications of both climate change and physical disturbance. Related to this, it has been suggested that BSC diversity can be used to inform conservation policies.
BSCs have been the subject of several previous Sevilleta LTER studies. Green et al. showed that stable-isotope carbon and nitrogen could be transferred bi-directionally between BSCs and adjacent plants. This led Collins et al. to propose that fungal hyphae provide connections between plant roots and BSCs that allow for transport between the two, a proposal known as the “fungal loop hypothesis.” Porras-Alfaro et al. have surveyed the diversity of fungi in BSCs from Sevilleta grasslands using molecular methods. We have also shown that thermophilic fungi are common in BSCs (unpublished results), a result that is not unexpected given the high summer temperatures attained in Sevilleta surface soils. Yet, many questions remain regarding the organisms present in BSCs, their biological roles and how long it takes for BSCs to re-establish after disturbance. Long-term, we are interested in the types of fungi present in BSCs and in how fungi function in transporting nutrients between BSCs and adjacent plants. We are also interested in the extent to which specific fungi provide structure to BSCs and in how they help protect from stress agents such as desiccation. We are interested in the extent to which fungi might help BSCs tolerate high summer soil temperatures, which often reach ≥ 60C. We therefore have a special interest in thermophilic fungi present in the BSCs. To date, little has been done to actually culture fungi from Sevilleta BSCs, hence the need for the current study.
In summary, BSCs are one of the most important features of aridland ecosystems and form a critical interface between physical and biological domains. Understanding the roles of BSCs in protecting soil structure, and in the cycling of carbon, water and nitrogen, is fundamental to aridland ecology. The work proposed here continues efforts to characterize the specific fungi associated with Sevilleta BSCs. It is a modest but important step toward addressing the long-term goals mentioned above.
For each sampling site and sampling period a small amount of surface crust (approx. one teaspoon per sample) was taken from each of 10 locations at approximately 1 meter intervals across a transect. Samples were transported back to the laboratory in plastic bags.
On rare occasions, a larger sample of 0.5 liter volume or less may have been removed at one or two sampling stations.
Data was collected at: LTER PJ site (N 34 23’ 08.7” W 106 31’ 27.0”), a sand dune above the railroad tracks near the Sevilleta wetlands (N34 18' 06.5" W106 51' 14.1"), gypsum outcroppings (N34 12' 40.5" W106 45' 35.5"), grasslands near the Sev LTER warming and monsoon sites (N34 21' 34.3" W106 41' 29.4" and N34 20' 38.1" W106 43' 34.5"), and the Rio Grande Bosque (N34 19'45" W106 51'40").
The distribution, structure and function of mesic savanna grasslands are strongly driven by fire regimes, grazing by large herbivores, and their interactions. This research addresses a general question about our understanding of savanna grasslands globally: Is our knowledge of fire and grazing sufficiently general to enable us to make accurate predictions of how these ecosystems will respond to changes in these drivers over time? Some evidence suggests that fire and grazing influence savanna grassland structure and function differently in South Africa (SA) compared to North America (NA). These differences have been attributed to the contingent factors of greater biome age, longer evolutionary history with fire and grazing, reduced soil fertility, and greater diversity of plants and large herbivores in SA. An alternative hypothesis is that differences in methods and approaches used to study these systems have led to differing perspectives on the role of these drivers. If the impacts of shared ecosystem drivers truly differ between NA and SA, this calls into question the generality of our understanding of these ecosystems and our ability to forecast how changes in key drivers will affect savanna grasslands globally. Since 2006, an explicitly comparative research program has been conducted to determine the degree of convergence in ecosystem (productivity, N and C cycling) and plant community (composition, diversity, dynamics) responses to fire and grazing in SA and NA.
Thus far, initial support has been found for convergence at the ecosystem level and divergence at the community level in response to alterations in both fire regimes and grazing. However, there have also been two unexpected findings (1) the ways in which fire and grazing interact differed between NA and SA, and (2) the rate of change in communities when grazers were removed was much greater in NA than in SA. These unexpected findings raise a number of important new questions: (Q1) Will exclusion of grazing eventually affect community structure and composition across all fire regimes in SA? (Q2) Will these effects differ from those observed in NA? (Q3) What are the determinants of the different rates of community change? (Q4) How will these determinants influence future trajectories of change? (Q5) Will the different rates and trajectories of community change be mirrored by responses in ecosystem function over time? This project is based on a large herbivore exclusion study established within the context of long-term (25-50+ yr) experimental manipulations of fire frequency at the Konza Prairie Biological Station (KPBS) in NA and the Kruger National Park (KNP) in SA. The suite of core studies and measurements include plant community composition, ANPP, and herbivore abundance and distribution at both study sites to answer these research questions.
We used comparable experimental designs and sampling procedures at both URF and KPBS. At URF we used three replicate plots (not hayed or mowed) that have been burned every 1 and 3 years in the spring, and those left unburned (N=9 plots). At KPBS, we established replicate plots in experimental watersheds burned every 1 and 4 years in the spring, and those left unburned (N=9 plots). Thus, the only difference in design between NA and SA was the intermediate burn frequency. In 2005 at both sites we established four 2x2m areas in each replicate of the 1-yr, 3-4 yr burned, and unburned plots (N=36 subplots). We then randomly selected two of the subplots for the fertilization treatment and the other two subplots served as controls (Fig. 1). Starting in 2006 at KPBS and 2007 at URF, we began adding 10 gN/m2/yr as NH4+NO3- to assess the interactive effects of fire frequency and nitrogen limitation on plant community composition, structure and dynamics.
Fig. 1. Experimental design and sampling for the proposed studies: A) the role of long-term fire regimes (without megaherbivores), B) the importance of grazing and grazing/fire interactions, and C) the role of megaherbivore diversity. Moveable exclosures (3/plot) will be used to estimate ANPP in the grazed plots. N addition subplots (2 x 2 m) will be divided into 4 1 x 1 plots, with two designated for plant species composition sampling and the other two for destructive sampling. Soil samples will be collected from areas not designated for ANPP or plant composition sampling. Note that the same annually and infrequently burned plots at Kruger and Konza will be used in (B) and (C). In addition, similar plots will be established minus the N addition subplots in the 1-yr and 4-yr burned blocks of the Buffalo enclosure for (C).
Each of the 2x2m subplots was divided into four 1x1m quadrats. Annually since 2005 (prior to nitrogen addition) canopy cover of each species rooted in each quadrat was visually estimated twice during the growing season to sample early and late season species. As a surrogate for aboveground production, we measured light availability at the end of the growing season above the canopy at the ground surface in each quadrat (N=4 per subplot) using a Decagon ceptometer.
Net primary production measurements: Prior to the 2005 growing season we established plots (13.7 m by 18.3 m) in ungrazed areas burned annually, at 3–4-year intervals, and unburned (n = 3 per fire treatment) at both KBPS and URF. Areas with trees or large shrubs were avoided as our main goal was to evaluate responses in the herbaceous plant community. ANPP was estimated from end-of-season harvests starting in 2005 (September for KBPS, April for URF). In 10, 0.1-m2 (20 cm by 50 cm) quadrats randomly located in each plot (n = 30/treatment/site), we harvested the vegetation at ground level and separated it into grass, forb, and previous year’s dead biomass. Samples were dried at 60C to a constant weight. For annually burned plots, total biomass harvested represents ANPP. For the intermediate and unburned sites, we calculated ANPP by summing all but the previous year’s dead component.
To assess the impacts of fire on ANPP in grazed areas, we established herbivore exclusion treatments in KBPS in North America and KNP in South Africa. Herbivore exclosures in grazed areas in KPBS and KNP were erected prior to the 2006 growing season. The exclosures were 7 m in diameter, 2 m tall, and constructed of diamond mesh (5-cm diameter). Seven exclosures were established in each of three blocks of the three fire treatments— annually burned, intermediate burn (3- years for KNP or 4-years for KPBS), and unburned (n = 21 exclosures/treatment/site). As our focus was on ANPP responses of the herbaceous layer, exclosures were not located beneath trees or where dense shrub patches were present. Additionally, in the Satara region of KNP is a 900-ha permanent enclosure containing 80–90 adult African buffalo (S. caffer). This enclosure was erected in 2000 and was divided into six areas (100–200 ha each), with these burned on a rotational basis including plots burned annually and plots that were unburned. We used the unburned and annually burned areas in the buffalo enclosure to provide a direct comparison for determining the effects of a single-species large grazer in KNP and KPBS, and to assess the effects of large herbivore diversity at adjacent sites in KNP. Similar exclosures were built in the African buffalo enclosure at KNP. We placed 7 exclosures in the three blocks of each fire treatment (annually burned and unburned) resulting in 21 exclosures/treatment. We sampled ANPP by harvesting plant biomass from three 0.1 m2 quadrats per herbivore exclosure at the end of the growing season starting in 2006.
Data are collected twice each year at each site. Sample periods are equivalent to spring and late summer at each study site (December/January and March/April in South Africa, May and September in North America.
Where the Data were Collected:
Ukulinga Research Farm, Pietermaritzburg, South Africa; Satara Region of Kruger National Park, South Africa; Konza Prairie Biological Station, North America
Additional Geographic Metadata:
Ukulinga Research Farm (URF), South Africa. The URF of the University of KwaZulu-Natal is located in Pietermaritzburg, in southeastern South Africa (30o 24’ S, 29o 24’ E). The site is dominated by native perennial C4 grasses, such as Themeda triandra and Heteropogon contortus, that account for much of the herbaceous aboveground net primary production (ANPP). Mean annual precipitation is 790 mm, coming mostly as convective storms during summer (Oct-Apr). Summers are warm with a mean monthly maximum of 26.4oC in February, and winters are mild with occasional frost. Soils are fine-textured and derived from shales. There has been no grazing at this site for >60 years. Long-term experimental plots were established at URF in 1950 with the objective of determining the optimal fire and/or summer cutting regime to maximize hay production. The experiment is a randomized block (three replicates) split-plot design with four whole-plot haying treatments and 11 subplot fire or mowing treatments. Subplot sizes are 13.7 x 18.3 m.
Kruger National Park (KNP), South Africa. The KNP is a 2 million ha protected area of savanna grassland that includes many of the large herbivores common to southern Africa (22º 25' to 25º 2 32' S, 30º 50' to 32º 2' E). The extant abundance and grazing intensity of herbivores in KNP is considered moderate for regional savanna grasslands. In the south-central region of KNP where our research takes place, average rainfall is 537 mm with most falling during the growing season (Oct-Apr). The dormant season is mild, dry and frost free, and summers are warm with mean monthly maximum air temperature of 28.9oC in January. Because of the importance of fire in savanna grassland ecosystems, the Experimental Burn Plot (EBP) experiment was initiated in 1954 to examine the effects of fire frequency (control-no fire, 1-, 2-, 3-, 4- and 6-yr return interval) and season [early spring (Aug), spring (Oct), mid-summer (Dec), late summer (Feb), and fall (Apr)] on vegetation communities in the park. Four blocks of 12 plots (two were later split for the 4- and 6-yr trts), each ~7 ha (370 x 180 m) in size, were established in four primary vegetation types covering the two major soil types (granites and basalts) and spanning the precipitation gradient in the park. Each plot has 50+ years of known fire history, and native herbivores have had unrestricted access, thus fire and grazing effects are combined. This research focuses on the EBPs located near Satara where precipitation, soil type, and the mix of herbaceous and woody plants are similar to KPBS. Vegetation on the blocks is co-dominated by C4 grasses, such as Bothriochloa radicans, Panicum coloratum and Digiteria eriantha, and woody plants, such as Acacia nigrescens and Sclerocarya birrea. Soils are fine-textured and derived from basalts. Adjacent to one of the Satara blocks is the Cape buffalo enclosure, erected in 2000 for veterinary purposes. The 200 ha permanent enclosure contains 65-80 animals and is divided into 4 blocks burned on a rotational basis. The grazing intensity inside is comparable to the moderate levels imposed in the park and at KPBS. Two blocks are burned annually while others are burned infrequently (approximately once every 4-yr).
Konza Prairie Biological Station (KPBS), North America. The KPBS is a 3,487 ha savanna grassland in northeastern Kansas, USA (39o 05’ N, 96o 35’ W) dominated by native perennial C4 grasses such as Andropogon gerardii and Sorghastrum nutans that account for the majority of ANPP. Scattered shrub and tree species include Cornus drummondii, Gleditsia triacanthos, and Prunus spp. Numerous sub-dominant grasses and forbs contribute to the floristic diversity of the site. The climate is continental, with mean July air temperature of 27°C. Annual precipitation is ca. 820 mm/year, with 75% falling as rain during the Apr-Oct growing season. Soils are fine textured, silty clay loams derived from limestone and shales. KPBS includes fully replicated watershed-level fire and fire/grazing treatments, in place since 1977 and 1987, respectively. Replicate watersheds (mean size ~60ha) are burned at 1-, 2-, 4-, 10- and 20-yr intervals, mainly in April, to encompass a range of likely natural fire frequencies and management practices. A subset of watersheds has not been grazed for more than 30 years. To address the role of native grazers and fire/grazing interactions, bison (~260 individuals) were reintroduced to KPBS in a 1000-ha fenced area that includes replicate watersheds burned in the spring at 1-, 2-, 4- and 20-year intervals. The overall grazing intensity is considered moderate.
Study Area 1:
Study Area Name: Ukulinga Research Farm
Study Area Location: Near Pietermaritzburg, South Africa
Elevation: 840 m above sea level
Landform: Colluvium fan
Geology: Marine shales and dolerite colluvium
Soils: Dystric leptosols, Chromic luvisols, Haplic plinthisols
Vegetation: Native grassland
Climate: Mean annual precipitation is 844 mm, Mean annual temperature 17.6C
Site history: Ungrazed since 1950
Single Point: 29o 40’ S / 30o 20’ E
Study Area 2: Kruger National Park, South Africa
Study Area Name: Satara Experimental Burn Plots and Cape Buffalo Exclosure
Study Area Location: Near Satara rest camp
Elevation: 240-320 meters above sea level
Landform: Level Upland
Soils: Rhodic nitisols, Haplic luvisols, Leptic phaeozems
Climate: Mean annual precipitation 544 mm; mean annual temperature 21.2–23.3C
Site history: Grazed by native herbivores
Single Point: 23–25o S /30-31o E
Study Area 3: Konza Prairie Biological Station
Study Area Name: Konza Prairie
Study Area Location: Watersheds N20B, N4D, N1B, N4B; 1D, 4F, 20B
Elevation: 320-444 meters above sea level
Landform: Alluvial terrace
Geology: Cherty limestone and shale
Soils: Udic argiustolls
Climate: Mean annual precipitation 835 mm; mean annual temperature 12.7C
Site history: Ungrazed watersheds (since 1971), watersheds grazed by native herbivores (since 1987)
Single Point: 39o 05.48’ N / 96o 34.12’ W
Konza-Ukulinga fire by nitrogen project: We used comparable experimental designs and sampling procedures at both URF and KPBS (Figure 1). At URF we used three replicate plots (not hayed or mowed) that have been burned every 1 and 3 years in the spring, and those left unburned (N=9 plots). At KPBS, we established replicate plots in experimental watersheds burned every 1 and 4 years in the spring, and those left unburned (N=9 plots). Thus, the only difference in design between NA and SA was the intermediate burn frequency. In 2005 at both sites we established four 2x2m areas in each replicate of the 1-yr, 3-4 yr burned, and unburned plots (N=36 subplots). We then randomly selected two of the subplots for the fertilization treatment and the other two subplots served as controls (Fig. 1). Starting in 2006 at KPBS and 2007 at URF, we began adding 10 gN/m2/yr as NH4+NO3- to assess the interactive effects of fire frequency and nitrogen limitation on plant community composition, structure and dynamics.
Konza-Kruger fire by grazing project: For this study, we are utilizing the long-term experiments at KPBS and KNP in which native megaherbivore grazers are present and fire frequency is directly manipulated. To assess the effects of grazing and fire-grazing interactions, we constructed seven sets of permanent exclosures and adjacent control plots in three blocks at both sites. The exclosures and matching paired open plots were established in 2005 in the Satara EBPs that are burned every 1 and 3 years in the spring or left unburned and at KPBS in watersheds that are burned every 1 and 4 years or left unburned. (N=63 exclosures/site; Fig. 1). Within each exclosure and paired open plot, we sample plant community composition and light availability in permanent 2x2 m subplots. We collect ANPP at the end of each growing season from each exclosure, and throughout the growing season in grazed areas adjacent to the unexclosed plots using 1x1 m moveable exclosures (Fig. 1).
The use of stored resources to fuel reproduction, growth and maintenance to balance variation in nutrient availability is common to many organisms. The degree to which organisms rely upon stored resources in response to varied nutrients, however, is not well quantified. Through stable isotope methods we quantified the use of stored versus incoming nutrients to fuel growth, egg and fat body development in lizards under differing nutrient regimes. We found that the degree of capital breeding is a function of an individual’s body condition. Furthermore, given sufficient income lizards in poor condition can allocate simultaneously to storage, growth, and reproduction, which allowed them to catch up to better conditioned animals. In a parallel, inter-specific survey of wild lizards we found that the degree of capital breeding varied widely across a diverse community. These findings demonstrate that capital breeding in lizards is not simply a one-way flow of endogenous stores to eggs, but is a function of the condition state of individuals and the availability of nutrients during both breeding and non-breeding seasons. Here we explore the implications of these findings for our understanding of capital breeding in lizards and the utility and value of the capital-income concept in general.
For measures of capital breeding in wild lizards, females of seven species were caught April through July of 2008 under the approval of the University of New Mexico institutional animal care and use committee (UNM-IACUC #05MCC004). The species captured were: Cophosaurus texanus, Crotaphytus collaris, Eumeces multivirgatus, Phrynosoma modestum, Sceloporus undulates consubrinus, Urosaurus ornatus, and Uta stansburiana. Lizards deemed by palpation to be egg-bearing were returned to the lab, euthanized and reproductive tissues prepared for stable isotope analysis (see below).
Stable isotope treatments:
After the lizards were euthanized liver, fat body, and thigh muscle samples were harvested, freeze dried and a 0.5 mg sample was placed into a pre-cleaned tin capsule (Costech, #041074, Valencia, CA) for stable isotope analysis. Eggs and follicles were also harvested, their length and width measured and freeze dried. All lipids were extracted from freeze dried and ground muscle and eggs/follicles by a 2:1 chloroform and methanol bath; lizard muscle had undetectable amounts of lipids. The suspended lipids from eggs were pipetted into separate storage vials and air dried. Lipids and lipid-free egg tissues were then loaded into tin capsules. We measured the δ13C of each egg and follicle greater than 6mm in length (½ the length of shelled eggs and assumed to reflect reproductive allocation). Our stable isotope methodology follows standard methods and our protocol is described in detail in Warne et al. (2010a, 2010b). We report all isotope values in the standard delta notation (δX = (Rsample /Rstandard – 1) x 1000) in parts per thousand (‰) relative to the international carbon standard VPDB (Vienna Pee Dee Belemnite). Measurements were conducted on a continuous flow isotope ratio mass spectrometer in the UNM Earth and Planetary Sciences Mass Spectrometry lab. The precision of these analyses was ± 0.1‰ SD for δ13C based on long-term variation of the working laboratory standard (valine δ13C = -26.3‰ VPDB), samples of which were included on each run in order to make corrections to raw values obtained from the mass spectrometer.
Essential to this study is the observation that differences in photosynthetic biochemistry inherent to C3- and C4-plants produces distinct differences in the d13C of their tissues, which can be used to trace the movement of nutrients through consumers (Hobson et al. 1997, O'Brien et al. 2000). Because winter and summer monsoonal rains drive seasonally separated C3 and C4 plant production and resource flux in Chihuahuan Desert food webs (Warne et al. 2010b), we hypothesized that we could use natural variation in the δ13C of C3 and C4 resources to examine capital breeding in wild lizards. We predicted that during the late summer and early fall lizards would develop endogenous lipid stores (capital) from C4 derived sources because C4 plants (primarily grasses) comprise the bulk of primary production during this period. We also hypothesized that reproduction in the spring (the income source) would be fueled by C3 plants associated with winter rains. We subsequently sampled female lizards of a variety of species during April through June 2008 to gauge the relative use of capital (C4) versus income (C3) resources for their first clutch of the season. The lizards were collected from a mixed Creosote and gramma grassland.
We used tissue d13C values and a standard two-end-point mixing model to estimate the proportion of endogenous fat or muscle (capital) and incoming insect-dietary sources used to provision eggs. The mean δ13C value of insects feeding on C3 plants (-27.3‰) served as an income source (see Warne et al. 2010b). The discrimination (Δ13C) values used in this model for muscle (-1.9‰) and fat bodies (0‰) were experimentally determined for S. undulatus (Warne et al. 2010a).
Study Area 1:
*Study Area Name: Socorro NM
*Study Area Location: BLM land 11 miles south of Socorro, NM
*Study Area Description: Mixed creosote and gramma grass shrubland
North Coordinate: 33°56'54.88"N
West Coordinate: 106°57'6.26"W
Study Area 2:
*Study Area Name: Tres pistoles
*Study Area Location: BLM land 13 miles east of Albuquerque, NM
*Study Area Description: Mixed shrub and gramma grassland
North Coordinate: 35° 4'44.38"N
West Coordinate: 106°26'48.29"W
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