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Transpiration (Woody Plants: Dryland Shrubs) - Summary
The subject of this summary concerns two major questions.  First: how does atmospheric CO2 enrichment impact the transpiration rates of dryland shrubs?  And second: what are the implications of that impact?

In a FACE experiment designed to examine these questions that was conducted in a chaparral region of southern California, Roberts et al. (1998) exposed Adenostoma fassciculatum shrubs to atmospheric CO2 concentrations of 360 and 550 ppm.  After six months of treatment, it was clear that the elevated CO2 had reduced the stomatal conductances of the shrubs' leaves and decreased their rates of evaporative water loss, because the CO2-enriched shrubs exhibited leaf water potentials that were significantly more positive (less stressful) than those of the plants growing in ambient air, which is the answer to question number one.  The answer to question number two is that this enhancement of internal water status would be expected to help this woody perennial better withstand the periods of drought that commonly occur throughout its southern California range.

In a similar study that produced similar findings in Texas, Dugas et al. (2001) studied the response of whole-plant transpiration to atmospheric CO2 enrichment in the woody legume Acacia farnesiana, which occurs throughout south-central United States and is one of the most aggressive woody-plant invaders of grasslands worldwide.  Plants of this species were grown for a full year in greenhouse bays continuously maintained at atmospheric CO2 concentrations of either 385 or 980 ppm, after which whole-plant transpiration was assessed via sap flow measurements.  This protocol revealed that the mean transpiration rate of the plants that had been grown at an atmospheric CO2 concentration of 980 ppm was only about a fourth of that exhibited by the plants that had been grown at a concentration of 385 ppm; and the tremendous increase in water use efficiency implied by this result could well explain A. farnesiana's increasing ascendancy over grasses as the air's CO2 content has risen over the past century or more.

In yet another such study of dryland shrubs, Peterson and Neofotis (2004) sprouted and grew velvet mesquite plants in small pots within controlled environment chambers maintained at atmospheric CO2 concentrations of either 380 or 760 ppm.  Although they did not see a large CO2-induced increase in plant growth, they say that by the end of their six-week study there was a highly significant 41% reduction in the volume of water transpired by the mesquite seedlings in the CO2-enriched treatment.  "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."

The pair of scientists from the Biosphere 2 Center near Oracle, Arizona, say their findings suggest that in a future high-CO2 world, mesquite 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 in the real world of nature as the air's CO2 content has risen ever higher over the past century or more is said by Peterson and Neofotis to be demonstrated by the fact 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).

In conclusion, just as non-agricultural herbaceous plants are encroaching upon earth's deserts as the air's CO2 content continues to rise, so too are woody dryland shrubs following in their "rootsteps," as the greening of the earth continues.

References
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.  δ13C 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.

Dugas, W.A., Polley, H.W., Mayeux, H.S. and Johnson, H.B.  2001.  Acclimation of whole-plant Acacia farnesiana transpiration to carbon dioxide concentration.  Tree Physiology 21: 771-773.

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.

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.

Polley, H.W., Johnson, H.B. and Mayeux, H.S.  1994.  Increasing CO2: comparative responses of the C4 grass Schizachyrium and grassland invader ProsopisEcology 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 glandulosaPlant Ecology 164: 85-94.

Roberts, S.W., Oechel, W.C., Bryant, P.J., Hastings, S.J., Major, J. and Nosov, V.  1988.  A field fumigation system for elevated carbon dioxide exposure in chaparral shrubs.  Functional Ecology 12: 708-719.

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.

Last updated 22 June 2005