Submitted to: Tree Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/1996
Publication Date: N/A
Interpretive Summary: Seedlings of trees and shrubs often die on grasslands because water availability is too low. Low soil water levels or drought have, for example, been shown to limit abundance of the aggressive woody invader mesquite on grasslands in the southwestern U.S. The doubling in concentration of carbon dioxide in the atmosphere that is expected during the next century may, however, reduce death of shrub seedlings by increasing woody growth and tolerance of drought. Mesquite was grown from seed at the current carbon dioxide concentration and at two elevated levels possible during the next century. Watering was discontinued when seedlings emerged from soil, and soil water levels declined as seedlings grew. Seedlings maintained a more favorable internal water status and initially grew faster at high carbon dioxide concentrations. As soil water content declined, seedlings at high carbon dioxide levels shed leaves faster than did those grown at today's carbon dioxide concentration. More than twice as many seedlings survived 65 days without irrigation at high than low carbon dioxide concentrations. Results suggest that increases in carbon dioxide concentration during the next century may increase mesquite invasion of grasslands by improving seedling tolerance of drought.
Technical Abstract: Low water availability reduces establishment of the invasive shrub Prosopis on some grasslands. Survival of water deficit and traits that may contribute to the postponement or tolerance of plant dehydration were measured on seedlings of P. glandulosa Torr. var. glandulosa (honey mesquite) grown at CO2 concentrations of 370, 710, and 1050 ppm. Soil water content was depleted at similar rates across CO2 treatments by increasing the number of seedlings per container at elevated concentrations. Elevating CO2 increased root biomass and the ratio of lateral root to total root biomass of seedlings, but plants grown at elevated CO2 shed more leaves and retained smaller leaves than did those grown at the current CO2 concentration. These changes, together with apparently reduced transpiration/leaf area at elevated CO2, may have contributed to a slight increase in xylem pressure potentials at 1050 ppm during the first 37 days of growth (0.26 to 0.40 MPa). Osmotic potential was not affected by CO2 treatment. Elevating CO2 concentration to 710 and 1050 ppm more than doubled percentage survival of seedlings from which water was withheld for 65 days. Significantly, CO2 enrichment increased survival from 0% to about 40% among seedlings that experienced lowest soil water levels. By increasing seedling survival of drought, rising CO2 concentration may increase abundance of P. glandulosa on grasslands where low water availability limits its establishment.