Effects of Elevated Atmospheric CO² on Invasive Weed Species in Managed Terrestrial Ecosystems of the Southeastern US. S.A. Prior1, G.B. Runion1, A.J. Price1, E. van Santen², H.H. Rogers1, D.H. Gjerstad³, and H.A. Torbert1; 1USDA-ARS National Soil Dynamics Laboratory, Auburn, AL; ²Department of Agronomy & Soils, Auburn University, AL; 3School of Forestry & Wildlife Sciences, Auburn University, AL. (sprior@acesag.auburn.edu)

ABSTRACT

One neglected aspect of global change is the consideration of how invasive plants might react to the increasing CO2 concentration in the atmosphere. This recently funded National Institute for Global Environmental Change (Southeast Regional Center) research project will place primary emphasis on assessing cogongrass [Imperata cylindrical (L.) Beauv.] responses to CO2 enrichment. The work will be conducted in three phases: (1) phase 1, individual invasive plants representative of various guilds will be assessed for their responses to elevated CO2; (2) phase 2, will consist of herbicide trials as affected by elevated CO2; and (3) phase 3, based on phase 1 & 2 findings, will be a series of competition studies conducted under elevated CO2 conditions. This presentation will cover the different invasive species being investigated along with the specific experimental hypotheses that will be addressed.

INTRODUCTION -

Invasive weeds are estimated to cost U.S. agricultural and forest producers 34 billion dollars each year from decreased productivity and increased weed control costs (4). One neglected aspect of global change is how invasive plants might react to the increasing atmospheric CO2 concentration. Since elevated CO2 stimulates photosynthesis (1), resource use efficiency, and carbon allocation to belowground plant structures (6), it may impact the competitiveness of invasive plants. Bright (1998) summarizes, "Fast-growing, highly invasive plants may also be able to profit directly from the atmosphere's increased carbon content...any slower-growing natives would tend to lose out to the invaders." We propose to investigate the effects of elevated CO2 on growth, physiology, water relations, competitive ability, and control of invasive weed species detrimental to the Southeast economy.

GOALS and METHODS -

This project will emphasize assessment of cogongrass [Imperata cylindrical (L.) Beauv.] response to elevated CO2 and will be conducted in three phases. In the first phase, individual invasive plants (different functional guilds) will be assessed for their responses to increased CO2. The second phase will consist of herbicide trials as affected by increased CO2. Based on findings from the first and second phases, the third phase will be a series of competition studies conducted under elevated CO2 conditions.

Plants will be exposed to CO2 using an open top chamber system (5) located at the soil bin facilities at the USDA-ARS National Soil Dynamics Laboratory, Auburn, Alabama. Greenhouse established seedlings will be transplanted into large plastic containers using standard potting medium prior to CO2 exposure. Soil fertility will be reflective of southeastern U.S. conditions. Containers will be subjected to ambient rainfall and watered to prevent drought-induced plant mortality. Two levels of CO2 (ambient and twice ambient) will be maintained in a randomized complete block design (6 replications).

During the course of each phase, plant photosynthesis (net C assimilation) will be measured weekly for each species using a Li-Cor 6400 portable gas exchange system; conductance and transpiration are concomitantly determined while water use efficiency can be calculated from these data. At the end of each phase, plants will be separated into organ parts (i.e., leaves, stems, roots). Using standard practices, other aboveground parameters will be assessed (e.g., leaf area, stem number, height, diameter, and numbers of nodes, branches, and leaves). Roots will be separated from soil using the sieve method. Plant organs will be dried (55 oC) and dry weights recorded. Organ parts will be ground (0.2 mm sieve) and analyzed for total C and N with a LECO CN-2000 analyzer; C and N partitioning will be calculated. Carbon and nitrogen content of soils will also be determined. Statistical analysis of all data will be done using the Mixed procedure from SAS (3).

HYPOTHESES AND RESEARCH GOALS -

The three major hypotheses to be tested are: Hypothesis 1: Elevated atmospheric CO2 will result in positive growth responses of all invasive plant species. However, C3 plants will respond to a greater extent than C4 plants, N2-fixers will respond to a greater extent than non-fixers, and evergreens will respond to a greater extent than deciduous species; carbon inputs (in the form of aboveground biomass residues and roots) to the soil will follow these same patterns; Hypothesis 2: Herbicide tolerance of cogongrass will increase with CO2 enrichment, requiring increased application rates; tolerance will vary among ecotypes collected from across Alabama; and Hypothesis 3: Competition from invasive species will reduce growth and yield of crops. Effects of elevated CO2 on this competitive interaction will depend on physiological characteristics of both the crop and the invasive weed (as suggested in Hypothesis 1); however, growth in monocultures will not be a reliable predictor of response under competitive conditions. Further, while competition will not affect total biomass production, it will impact carbon dynamics by altering allocation among species.

The first phase will address Hypothesis 1. Individual invasive plant (different functional guilds) responses to CO2 level will be assessed: cogongrass [Imperata cylindrical (L.) Beauv.] and Johnson grass (Sorghum halepense), C4 grasses; purple nutsedge (Cyperus rotundus L.) a C4 sedge; sicklepod (Cassia obtusifolia L.) a C3 N2-fixing legume; musk thistle (Carduus nutans L.) a herbaceous C4 biennial; Chinese privet (Ligustrum sinense Lour.) a C3 broadleaf evergreen shrub; and possibly tropical soda apple (Solanum viarum Dunal) a C3 perennial.

The second phase will address Hypothesis 2 using herbicide trials. Plants will be treated (glyphosate and imazapyr) at the highest labeled rate as well as one-half and one-quarter of the highest labeled rate. Efficacy will be visually rated on a zero (no effect) to 100 (complete effect) and re-growth will be measured. Efficacy will be compared to a non-treated check for each CO2 level. Methods will be similar for the other species.

Based on the previous two phases findings, the third phase (Hypothesis 3) will be a series of competition studies using the following pairs: (1) cogongrass (C4) with evergreen conifers (C3) - e.g., loblolly pine (Pinus taeda) and/or longleaf pine (P. palustris); (2) sicklepod (C3 N2-fixer) and/or purple nutsedge (C4) with agricultural crops - e.g., soybean [Glycine max (L.) Merr.; C3 N2-fixer] and grain sorghum [Sorghum bicolor (L.) Moench; C4 grass]; (3) musk thistle and/or tropical soda apple and the pasture species bahiagrass (Paspalum notatum Fluegge; C4 grass). These combinations represent problems commonly encountered by Southeastern producers and represent species which differ in lifeform, growth habit, and physiology.

Invasive plant pests have the capacity to disrupt terrestrial ecosystems; nowhere is this threat greater than in the southeastern U.S., with its numerous ports of entry and mild climate. This research project will generate information that helps combat invasive plants in a future CO2-enriched world.

LITERATURE CITED

1. Amthor, J.S. 1995. Terrestrial higher-plant response to increasing atmospheric [CO2] in relation to the global carbon cycle. Global Change Biol. 1:243-274.
2. Bright, C. 1998. Life Out of Bounds: Bioinvasion in a Borderless World. W.W. Norton & Company, NY.
3. Littell, R.C., G.A. Milliken, W.W. Stroup, and R.D. Wolfinger. 1996. SAS System for Mixed Models. SAS Institute, Inc., Cary, NC.
4. Pimentel, D. 2002. Biological Invasions: Economic and Environmental Costs of Alien Plant, Animal, and Microbe Species. CRC Press, Boca Raton, FL.
5. Rogers, H.H., W.W. Heck, and A.S. Heagle. 1983. A field technique for the study of plant responses to elevated carbon dioxide concentrations. Air Poll. Control Assn. J. 33:42-44.
6. Rogers, H.H., G.B. Runion, and S.V. Krupa. 1994. Plant responses to atmospheric CO2 enrichment with emphasis on roots and the rhizosphere, Environ. Pollut. 83:155-189.