Submitted to: Journal Of Plant Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 31, 2001
Publication Date: May 17, 2002
Citation: Polley, H.W., Johnson, H.B., Tischler, C.R. Woody invasion of grasslands: evidence that CO2 enrichment indirectly promotes establishment of Prosopis glandulosa. Plant Ecology. 2002. v. 164. p. 85-94. Interpretive Summary: Invasion of the world's grasslands by woody plants is among the dominant changes in Earth's vegetation during the last two centuries. Woody plants displace grasses and reduce the value of grasslands for cattle production. Woody invasion traditionally has been attributed to overgrazing, fire suppression, and elimination of small mammals that kill woody seedlings, but may also be related to the 30% increase in concentration of carbon dioxide gas in air that has occurred during the last 200 years. Grasses use water more slowly as carbon dioxide levels increase. Consequently, grassland soil may remain wetter during drought when carbon dioxide concentrations are high. Such an increase in soil water may indirectly have promoted woody invasion by increasing survival of shrub seedlings during drought. We evaluated the importance of this indirect effect of rising carbon dioxide on the invasive shrub honey mesquite. Mesquite seedlings were taller, heavier, and survived better in grassland when soils were wet than dry. Increasing carbon dioxide concentration on grassland increased soil moisture levels by reducing grass deletion of water. However, the rise in carbon dioxide required to increase soil water content enough to benefit mesquite seedlings was about twice that of the last 200 years. It appears, therefore, that the past increase in carbon dioxide concentration was too small to cause mesquite invasion by increasing soil moisture levels. Carbon dioxide concentration is continuing to rise, and is expected to double during the next century. An increase in carbon dioxide of this magnitude could reduce water limitation to seedlings enough to expand woody invasion of grassland.
Technical Abstract: Grasslands worldwide have been invaded by woody species during the last 200 years. Atmospheric CO2 enrichment may have indirectly facilitated invasion by reducing soil water depletion by grasses. We used a two-step correlative approach to test this hypothesis with the invasive 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 ppm 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. With these data, we tested the hypothesis that grasses reduce mesquite establishment by depleting soil 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. Soil water content over the 12 weeks that seedling survival was studied was higher following the first than second planting and in plots from which grass roots were excluded. Both emergence and survival of mesquite correlated positively with soil water content. Increasing CO2 by 200 umol/mol increased soil water content by an amount that was similar to the change caused by experimental treatments. Results indicate that the expected increase in CO2 concentration could indirectly promote woody invasion of grassland by lessening water limitation to woody seedlings.