USE OF OXYGEN-18 TO ASSESS COMPETITION FOR WATER BETWEEN GRASSLANDS AND SHRUBS IN THE SONORAN DESERT, MEXICO

J.P. Brunel 1 and D.G. Williams 2

1 ORSTOM-IMADES, Hermosillo, Mexico
2 University of Arizona, Tucson, USA

email: brunel@cideson.mx

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INTRODUCTION

As a consequence of the introduction of cattle more than one century ago in the semi-arid grasslands of North America, there has been a decreasing of grass density and invasion by shrubs (Mesquite trees, Prosopis sp.). This trend is apparent in the semi-arid grasslands of the Upper San Pedro Basin, in southeastern Arizona, USA and northeastern Sonora, Mexico (figure on right).

Using stable isotopes of water to infer the use of water by this plant community can provide an understanding of the competition between shrubs and grass.

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SITE

High plateau (1500m) in the Sonoran desert, North Mexico

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METHOD

• Sampling vegetation for extraction of sap water from tree stems by azeotropic distillation along the transect and from July to October 1997.
• Sampling rainfall and groundwater.
• Mass spectrometer Oxygen-18 analysis
  

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PRELIMINARY RESULTS FROM THE 1997 CAMPAIGN

(i)	results indicate that groundwater recharge occurred mainly from winter fall,
(ii)	mesquite trees on the top of the plateau can use deep groundwater but will preferably use surface water whenever possible.

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CONCLUSION

There is a real competition between grass and shrubs, the Mesquite taking certainly advantage when succession of drought years.

Next campaign would allow to determine the partition of water use by the two communities using isotopic signatures of the different sources of water vapour.