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United States Department of Agriculture

Agricultural Research Service

Research Project: Susceptibility to Encroachment by Invasive Plant Species of Monocultures and Mixtures

Location: Grassland, Soil and Water Research Laboratory

2013 Annual Report


1a.Objectives (from AD-416):
The purpose of this cooperative agreement is to support one MS student in summer research, supervised by Dr. Susan Schwinning. The student will conduct research in conjunction with a Masters degree program to conduct ecological research examining the susceptibility of common forage grasses, bioenergy feedstocks, and low-input mixed species grassland to invasion by Johnsongrass (Sorghum halapense) and other exotic invasive species. Research will meet the following specific research objectives: 1. Examine growth and nitrogen allocation in Johnsongrass and several native C4 grasses to identify periods of maximum nitrogen demand. The student will conduct greenhouse experiments where common forage grasses (Cynodon dactylon, Bermudagrass), a native grass and candidate bioenergy feedstock (Panicum virgatum, switchgrass), tallgrass prairie natives (Andropogon gerardii or Sorghastrum nutans),as well as Johnsongrass are grown either as monocultures or in pairs with Johnsongrass as a competitor. The competitive success of plants growing in pairs will be related to temporal patterns of nitrogen demand growing alone to test the hypothesis that competitive interactions between two species are stronger when they have broadly overlapping periods of nitrogen demand. Based on the results of these experiments, further hypotheses will be developed regarding differences in the susceptibility of grassland systems (native prairie, switchgrass monoculture, coastal Bermuda monoculture) to invasion by Johnsongrass. 2. Quantify establishment success of Johnsongrass in field plots of Bermudagrass, switchgrass, and low input mixed species grassland. The student will conduct a field experiment to determine establishment success of Johnsongrass in the Long-term Biomass experiment. The experiment will compare how the establishment success of Johnsongrass changes as the long-term biomass plots progress from just-established to the mature climax state.


1b.Approach (from AD-416):
Objective 1: Plants will be kept either well watered, or subjected to drought. Grasses growing alone will be harvested periodically to be analyzed for above- and belowground biomass and N concentrations in roots, stems, leaves and reproductive parts. Grasses growing in pairs will be harvested at peak biomass only, and analyzed for above- and below-ground biomass. Objective 2: Plots will be planted with seedlings of Johnsongrass in the spring of 2010, and subsequently censused periodically to determine mortality rates, growth and development indices, and physiological status through measuring rates of leaf gas exchange. This experiment will be repeated annually.


3.Progress Report:

This agreement funded research expenses and summer 2011 salary for a master's student, who conducted a greenhouse experiment at Texas State University-San Marcos examining growth and physiological responses of the non-native invasive grass species Johnsongrass (Sorghum halapense) and several co-occurring native grass species to understand the mechanisms by which invasive species may out-compete natives. The research discovered several aspects of Johnsongrass growth and physiology that may explain its competitiveness. These results will help pinpoint species that are better competitors with S. halepense and thus are good candidates for use in biofuel production. Results were presented at the Ecological Society of America 2012 Annual Meeting in Portland, OR, and the student completed the required final oral presentation of the results of thesis research in Fall 2012. Completion of the written portion of the thesis remains, and is anticipated by August 2013.


Last Modified: 12/18/2014
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