Woody invasion of grasslands: evidence that CO2 enrichment indirectly promotes establishment of Prosopis glandulosa

Grasslands worldwide have been invaded by woody species during the last 200 years. Atmospheric CO2 enrichment may indirectly have facilitated invasion by reducing soil water depletion by grasses. We used a two-step correlative approach to test this hypothesis with the invasive and native shrub honey mesquite (Prosopis glandulosa Torr. var. glandulosa). 1) Water content to 0.15 m depth was measured in grassland exposed to a CO2 gradient from 200 to 550 micromol/mol to evaluate the prediction that CO2 enrichment lessens soil water depletion by grasses. 2) Soil water content and emergence and survival of mesquite seedlings were measured in adjacent grassland plots from which grass roots were excluded to 0.15 m depth to reduce water depletion or that were irrigated to increase soil water levels. With these measurements, we tested the hypothesis that mesquite establishment is limited by water. Excluding grass roots doubled emergence of mesquite and almost tripled the fraction of emergent seedlings that survived for 12 weeks following the first of two plantings. Seedlings were taller, heavier, and had greater leaf area when grown without grass roots. Root exclusion did not measurably affect soil water during the 3-week period of seedling emergence, but soil water content over the 12 weeks that seedling survival was studied was higher in plots from which grass roots were excluded and following an April than May planting. Survivorship of mesquite seedlings correlated positively with soil water content. Percentage survival of seedlings increased from 1.5% to 15% and 28% at the soil water content measured in grassland exposed to CO2 concentrations of 270 (preindustrial), 360 (current), and 550 micromol/mol (future), respectively. We infer that recent and projected increases in atmospheric CO2 concentration may be large enough to increase establishment of invading mesquite seedlings in grasslands that are severely water-limited.