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Elevated CO2 Pumps Up Water Use Efficiency of Mesquite Seedlings
Peterson, A.G. and Neofotis, P.G. 2004. A hierarchial analysis of the interactive effects of elevated CO2 and water availability on the nitrogen and transpiration productivities of velvet mesquite seedlings. Oecologia 141: 629-640.

What was done
Velvet mesquite (Prosopis velutina Woot.) seedlings were grown for six weeks from their time of planting (as seeds) in small pots within environmentally controlled growth chambers that were maintained at atmospheric CO2 concentrations of 380 and 760 ppm and two levels of water availability (high and low).

What was learned
Although they did not see a significant CO2-induced increase in plant growth, Peterson and Neofotis say that by the end of their six-week study, they observed a highly significant reduction of approximately 41% in the volume of water transpired by P. velutina in response to the experimental doubling of the air's CO2 content. "This large reduction in whole-plant water use," as they describe it, "occurred because the reduction in transpiration per unit leaf area at elevated CO2 was not offset by a proportional increase in total leaf area."

What it means
The pair of scientists from the Biosphere 2 Center near Oracle, Arizona, USA, say their findings suggest that "under a future [high-CO2] scenario, seedlings may deplete soil moisture at a slower rate than they do currently," and that "this could facilitate seedling survival between intermittent rain events," noting that their work "corroborates the conclusions of Polley et al. (1994, 1999, 2003) that increasing levels of atmospheric CO2 may facilitate the establishment of mesquite seedlings through a reduction in soil water depletion." That such has indeed occurred is suggested by the fact, again quoting Peterson and Neofotis, that "mesquites and other woody species in the semiarid southwestern United States have shown substantial increases in population density and geographic range since Anglo-American settlement of the region approximately 120 years ago," in support of which statement they cite the studies of Van Auken and Bush (1990), Gibbens et al. (1992), Bahre and Shelton (1993), Archer (1995), Boutton et al. (1999), Van Auken (2000), Ansley et al. (2001), Wilson et al. (2001) and Biggs et al. (2002).

Ansley, R.J., Ben Wu, X. and Kramp, B.A. 2001. Observation: long-term increases in mesquite canopy cover in a north Texas savanna. Journal of Range Management 54: 171-176.

Archer, S. 1995. Tree-grass dynamics in a Prosopis-thornscrub savanna parkland: reconstructing the past and predicting the future. Ecoscience 2: 83-99.

Bahre, C.J. and Shelton, M.L. 1993. Historic vegetation change, mesquite increases, and climate in southeastern Arizona. Journal of Biogeography 20: 489-504.

Biggs, T.H., Quade, J. and Webb, R.H. 2002. 13C values of soil organic matter in semiarid grassland with mesquite (Prosopis) encroachment in southeastern Arizona. Geoderma 110: 109-130.

Boutton, T.W., Archer, S.R. and Midwood, A.J. 1999. Stable isotopes in ecosystem science: structure, function and dynamics of a subtropical savanna. Rapid Communications in Mass Spectrometry 13: 1263-1277.

Gibbens, R.P., Beck, R.F., Mcneely, R.P. and Herbel, C.H. 1992. Recent rates of mesquite establishment in the northern Chihuahuan desert. Journal of Range Management 45: 585-588.

Polley, H.W., Johnson, H.B. and Mayeux, H.S. 1994. Increasing CO2: comparative responses of the C4 grass Schizachyrium and grassland invader Prosopis. Ecology 75: 976-988.

Polley, H.W., Tischler, C.R., Johnson, H.B. and Pennington, R.E. 1999. Growth, water relations, and survival of drought-exposed seedlings from six maternal families of honey mesquite (Prosopis glandulosa): responses to CO2 enrichment. Tree Physiology 19: 359-366.

Polley, H.W., Johnson, H.B. and Tischler, C.R. 2003. Woody invasion of grasslands: evidence that CO2 enrichment indirectly promotes establishment of Prosopis glandulosa. Plant Ecology 164: 85-94.

Van Auken, O.W. 2000. Shrub invasions of North American semiarid grasslands. Annual Review of Ecological Systems 31: 197-215.

Van Auken, O.W. and Bush, J.K. 1990. Importance of grass density and time of planting on Prosopis glandulosa seedling growth. Southwest Naturalist 35: 411-415.

Wilson, T.B., Webb, R.H. and Thompson, T.L. 2001. Mechanisms of Range Expansion and Removal of Mesquite in Desert Grasslands of the Southwestern United States. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station.

Reviewed 15 December 2004