Sedgwick Reserve

30 Jun, 05 PM - 29 Jun, 05 PM

One of the largest and most persistent conversions of a native plant community worldwide is the invasion of 9.2 million ha of native perennial grassland in California by exotic annual grasses and forbs from the European Mediterranean region. In addition to the loss of native plant biodiversity, this invasion has potentially altered basic ecosystem processes through increased nitrate leaching, altered fire regimes, and decreased carbon storage. The conversion of native plant communities may also have affected the value of this land for agriculture by decreasing the quality of summer grazing. In a previous study, the Principle Investigators employed a mutual invasibility approach to the California grassland system that yielded intriguing results. Whereas the geographic scale of the California invasion and its persistence for over a century suggests that there have been irreversible shifts in the plant community, results from the prior study reveal that native plant recovery is limited by the lack of sufficient. Furthermore, once seeds are available, native perennials out-compete the invasive annuals for important resources such as nitrogen and water. The proposed research is designed to refine our understanding of the nature of seed limitation as a factor limiting the reintroduction of native species. In addition to analyzing the effect of seed density on restoration success, this research will investigate resource competition as a mechanism mediating competition be exotic and native grasses after seed-limitation has been overcome by native species. Finally, the effect of species complementarity will be studied by analyzing the ability of communities of various mixes of native species to re-invade exotic annual grasslands. The proposed experiments will expand the previous results geographically (by using three University of California Natural Reserves that span a 700 km latitudinal gradient) and conceptually by employing modeling efforts to use data generated in the experiments to look into the future with regard to restored plant communities. These robust approaches ? refined field experiments, expanded geographic breadth, and modeling to extend the results in time, will yield results that both elucidate the underlying patterns and processes of invasions and restorations, and provide specific guidelines for the restoration of native California grasslands. Research Design Seed-limitation of the California native grassland flora The objective of this experiments is to determine if the restoration of native grasses depends on initial seeding density. The basic experimental manipulation in this study is the addition of increasing density of native seed of our three target native perennial grass species (B. carinatus, E. glaucus, and N. pulchra) to established annual grassland communities. Seed will be added at four densities: 100, 330, 500 and 1000 seeds m-2. The maximum seeding rate in this study close to the seed production rate of annual (968 seeds m 2) and perennial (1164 seeds m 2) communities at Sedgwick Reserve, the rates that led to successful establishment of perennial grasses in our previous experiments. Each of the twelve treatments (3 species x 4 densities) will be replicated in three separate grasslands at each of the three reserves for a total of 108 plots. Seed will be added to each of the 2x2 m plots in the fall prior to the first rain on three consecutive years. We will use seed collected from the reserve to ensure that genotypes are adapted to local conditions and to eliminate potential contamination of local populations with alien genotypes. It should be noted that this entails using seed from separate populations at each site so there is a potential confounding between site and the local genotype. However, we consider the use of local genotypes a more realistic test of the competitive ability of the native grasses. Furthermore, from a practical perspective, it is generally desirable for restorations to use local seed. In each of the plots we will count the number of individuals of the seeded species, and we will estimate individual fecundity by counting inflorescences and sub-sampling seeds. We will tag up to 10 individuals of the perennial species in the first year?s cohort to track individual survival rates over time. We will also estimate total cover of all species in the plot using a pinframe. Each pinframe sample consists of 100 vertical wires arrayed in a 10 cm grid in the center of each plot. We will record the identity of all species that touched a wire. Community Complementarity and Invasibility The objective of this experiment is to determine if a suite of native species is more effective at invading stands of exotic annuals than a single species at the same density. This experiment will conducted at the same location as the seed density experiment (see above). However, the basic manipulation will be the number of species in the seeding mix as opposed to the density of seed. For this experiment, we will hold total density of seed added constant at 1000 seeds m-2 (see seed density experiment). In this way we will be able to compare the success of a mixed seeding of natives relative to single species sowings while controlling for total seed density. Because the single species stands are a part of the seed density experiment, this experiment requires only four additional treatments to each of the blocks described above (three pairwise species combinations and a three species mixture). Sampling methodology will be identical to the methods described above. Effects of Disturbance, Nitrogen Availability and Competitive Dominance in California Grasslands The objective of this experiment is to investigate the role of resource availability and disturbance in controlling the ability of native grasses to recolonize stands of exotic annuals species. This experiment will be conducted at Sedgwick Reserve in an area that is currently open to cattle grazing. The core treatments will be a factorial combination of three levels of nitrogen addition (0, 4, and 10 g m-2 yr-1) and addition of native grass seed (seeded or control). Seed addition rates will be set at the level of the three species treatment in the Community Complementarity Study and nitrogen will be add quarterly as NH4NO3. These six treatments will be nested within a grazing exclosure treatment for a total of 12 treatment combinations (3 levels of N, 2 levels of seeding, and 2 levels of grazing) that will be replicated in 5 widely dispersed blocks at the Sedgwick Reserve. Resource Use Patterns and Competitive Dominance in California Grasslands The objective of this work is to quantify how competition for nitrogen, water, and light determines the outcome of competitive interactions between annual and perennial species. We will use laboratory incubations and assays, and in situ studies to measure N cycling rates availability in four settings: naturally occurring stands, seeded plots, experimental plots, and monocultures. Naturally Occurring Stands: Three pairs of established, naturally occurring stands of nonnative annual-dominated and native perennial-dominated grasses will be chosen at all three reserves for a total of 18 plots. The pairs of exotic and native grasslands plots will be chosen to have similar slope, aspect, oak canopy cover, and soil type. We will collect soil cores to measure pools of available N in each plot. We also measure in situ nitrification, C mineralization, N mineralization, and potential nitrification three times per year at points critical points in plant phenology: immediately after the first autumn rains when annual seeds germinate and perennial roots begin to elongate and take up water, in the spring when annual species are in flower, and when perennials are in flower and the annuals have senesced. Seeded Plots At each of the three reserves, the high seed density plots and seeding control plots established in the Seed Density Experiment will be sampled for measurement of soil N availability. Three replicate plots of each treatment (either seeded or unseeded) will be set up for a total of 18 plots across the three Reserves. The analyses conducted on these soils will be the same as those done on Naturally Occurring Stands (see above). Experimental Grasslands In 1997, 3m2 experimental plots of seeded annual and perennial grasses were established at Sedgwick Reserve. Measurement of soil microbial processes and N cycling is currently underway in control and N fertilized plots that contain 3 species of either annual or perennial grasses. Sampling in these plots began in the spring of 2002, and will continue through the late spring of 2003. In addition to the sampling proposed for the Naturally Occurring Stands, we have been using an 15N pool dilution study to measure in situ N partitioning between soil, microbes, roots, and above ground plant parts to assess differences in N use between nonnative annual and native perennial grassland communities, We have also begun measurements of N losses from these experimental plots through the processes of leaching and denitrification. Deep lysimeters (90cm) were installed in January 2002 for the collection and analysis of NO3- and DON in groundwater. Denitrification potential assays and intact core acetylene inhibition measurements of gaseous N loss from the two grassland community types are in progress. Monocultures In the fall of 2000, we set up an experimental set of monocultures (1 m2) composed of 25 species selected from 5 distinct functional groups as well as bare ground plots. To the degree possible, we selected several native and exotic species within each group to represent the common flora in our experiments. We have maintained these plots as single-species stands for the past two years. We propose to begin sampling plant and soil parameters in these monocultures plots. We will sample these plots on the same schedule used for the Naturally Occurring Stands. In the proposed research, we will focus on the three native perennial grasses that we are using in our seeding experiments and five annual grasses that are the dominant species in the exotic annual matrix into which the native will be sown (Avena barbata, Bromus madritensis, Bromus hordeaceous, Bromus diandris, Hordeum murinum). We plan to use 15N isotope dilution for a study of plant/soil partitioning in each species growing in the monoculture plots. During each of the sample periods we will clip a 0.10.75 m strip at ground level in order to estimate total biomass and aboveground productivity. Within this strip, we will take 2 soil cores (2 cm diameter by 30 cm deep), and separate them into the following depth categories: 0-5, 5-10, 10-20, 20-30. In each depth subsample, we will measure root biomass and soil moisture. In the lab, we will conduct 10 day incubations to determine the ratio of C mineralization to N mineralization, and conduct assays of nitrification potential at each soil depth. In addition to the nitrogen analyses, we will measure photosynthetically active radiation at ground level at weekly intervals to track the phenology of the individual species. As each species matures, we will collect 10 individuals that we will use to measure plant mass, seed mass, and seed number. These measurements are critical parameters for the population modeling work that we are conducting in parallel with this field research. Potential Impacts The project does not require additional resources from the reserve. It is unlikely that this work will impact other projects at the reserve. Ongoing experimental manipulations in this project involve the addition of nitrogen and water at rates that are well within naturally occurring levels. In addition, this project involves planting native perennial grass seed collected on the reserve. It is unlikely that this will impact ongoing research as the seed is planted in sites that have been specifically selected to be remote from existing populations. A portion of this project is being conducted in experimental grasslands that have been planted with seeds from off the reserve. The disposition of these experimental grasslands is covered under a separate agreement between the PI's and the Reserve Director.

Visit #2174 @Sedgwick Reserve

Approved

Under Project # 1739 | Research

The role of seed limitation, resource competition, and community complementarity in invasions and restoration

faculty - University of Minnesota

Reservation Members(s)

Eric Seabloom Jun 30, 2003 - Jun 29, 2004 (366 days)
Group of 2 Faculty Jun 30, 2003 - Jun 29, 2004 (366 days)
Eric Seabloom Jun 30, 2003 - Jun 29, 2004 (366 days)

Reserve Resources(s) | Create Invoice