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Title: LEAF PHYSIOLOGY, PRODUCTION, WATER USE, AND NITROGEN DYNAMICS OF THE GRASSLAND INVADER ACACIA SMALLII AT ELEVATED CO2 CONCENTRATIONS

Author
item Polley, Wayne
item Johnson, Hyrum
item Mayeux Jr, Herman

Submitted to: Tree Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/21/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Shrubs recently have invaded many grasslands, influencing their usefulness for food and fiber production. Shrubs may, for example, reduce the amount of grass present for livestock by using water or soil nutrients like nitrogen that otherwise would have supported grass growth. Because the concentration of carbon dioxide gas in air affects plant growth and use of water and nitrogen, the expected doubling of carbon dioxide concentration during the next century may alter effects of invading shrubs on rangelands. Huisache, a leguminous woody invader on southern rangelands, is capable of converting nitrogen gas in the air to a plant-available form. It was grown at the current carbon dioxide concentration and at two levels possible in the next century. Growth of huisache was approximately tripled by tripling the carbon dioxide concentration. Despite this increase in growth, huisache used the same amount of water at high carbon dioxide concentrations as it did at the current level. Huisache grown at elevated carbon dioxide converted 3 to 4 times more gaseous nitrogen to plant nitrogen than did shrubs at the current concentration. On rangelands, much of this nitrogen will become available to grasses when huisache leaves fall and decompose on the soil. By increasing the amount of nitrogen that eventually can support growth of grasses, rising carbon dioxide concentration may increase forage on rangelands that contain woody legumes like huisache.

Technical Abstract: Invasion by woody legumes can alter hydrology and nutrient cycling on grasslands. The rate and magnitude of these changes likely will be sensitive to effects of atmospheric CO2 concentration on the growth of leguminous shrubs and their acquisition and utilization of water and nitrogen (N). Seedlings of Acacia smallii (huisache), a woody invader of grasslands in the southern U.S., were grown for 13 months at CO2 concentrations near 385, 700, and 1000 ppm to assess effects of rising CO2 on plant growth and water and nitrogen dynamics. Despite declines in leaf N concentration and photosynthetic capacity, elevating atmospheric CO2 to 1000 ppm increased biomass of huisache more than 2.5-fold by increasing leaf photosynthetic rates, leaf area, and N2-fixation. Water use efficiency of well-watered huisache increased by about the same relative amount as did CO2 concentration. Effects of an accompanying decline in leaf conductance on transpiration were offset by an increase in leaf area, however, and total evapotranspiration was similar across the N fixed by huisache at the current CO2 level. We suggest that in addition to contributing to a large growth response to CO2, stimulation of symbiotic N2 fixation in woody invaders will increase N availability and potential productivity on many grasslands.