Project Number: 3098-11000-001-062-A
Project Type: Cooperative Agreement
Start Date: Aug 1, 2018
End Date: Jul 31, 2023
The research will identify the population dynamics of seeds, rhizomes, and established plants of an exotic invasive grass, Johnsongrass (Sorghum halapense), including the rate of spread in established switchgrass (Panicum virgatum) and mixed species native grassland stands as a function of soil water, soil nitrogen availability, and the species composition of the native community. Research will meet the following specific objectives: 1. Quantify how the spatial demography of established Johnsongass clones is influenced by water availability, nitrogen fertilization and species composition. 2. Investigate the effect of emergence timing (priority effect) on the outcome of Johnsongrass competition with native prairie grasses.
Objective 1: The persistence of Johnsongrass clones, once established, is well documented. From that point on, management options tend to focus on population containment, rather than eradication. To examine the demography of Johnsongrass to variation in soil water, soil nitrogen, and community composition, the student will conduct the first year sampling of rates of shoot initiation and survival, and their location relative to the clonal edge, in stands of restored native grassland and in plots of a factorial water x nitrogen experiment set up in established switchgrass stands for long-term research at the USDA Temple facility. Demographic parameters to be quantified will include seed production, viability and seedling survival, with the overall aim to assemble a “spatial integrodifference” model of Johnsongrass demography. As additional funds are added to the agreement in subsequent years, the sampling will be repeated. The approach will allow us to examine the sensitivity of model parameters, and thus the rate clonal spread, to management practices, including combinations of irrigation, fertilization and species richness of the plant community. From previous experiments we know that Johnsongrass’ competitive advantage in shoot-to-shoot competition is explained largely by its rapid shoot development relative to native species, which leads to overtopping and severe competitive suppression within a month of emergence. Therefore, our initial hypothesis is that clonal spread will be most contained by the inclusion of species that become active early in spring (at lower temperatures) and management practices that specifically alleviate resource limitations to these species. Objective 2: The student will conduct a greenhouse experiment examining the effect of size differences on competition between Johnsongrass and native C4 grasses. Plants will be grown alone and in pairs in long pots that minimize constraints on root development of plants. Our initial hypothesis is that competitive interactions between Johnsongrass and native C4 grasses are approximately equivalent, or possibly disadvantaging Johnsongrass, when competitive pairs are of equal size. Size relationships will be manipulated through staggering germination times. Results from this experiment will help the interpretation of field experiments by adding a mechanistic perspective to an essential component of the demographic model we propose to develop.