SOIL-WATER USE BY COEXISTING SHRUBS AND GRASSES IN THE SOUTHERN CHIHUAHUAN DESERT, MEXICO

Soil water use by shrubs and grasses of vegetation patches (vegetation arcs) occurring in two-phase mosaics of the Southern Chihuahuan Deser t (Mexico) was investigated after an experimental irrigation equivalen t to a 75 mm rainfall. Three shrubs (Flourensia cernua, Larrea trident ata and Prosopis glandulosa) and one grass (Hilaria mutica) were studi ed. Irrigation water did not percolate deeper than 40 cm. This soil la yer contained more than 75% of the roots in all species, except P. gla ndulosa where a less developed, deeper root system was detected (but n or quantified). Root distribution indicates that the water stored in t he 0-40-cm soil layer after the experimental irrigation was available for the four species. However, predawn xylem water potential (XWP) of F. cernua and H. mutica were strongly influenced by soil water present in the 0-40-cm layer, whereas those of L. tridentata and P. glandulos a were not. Differences in predawn XWP between watered and unwatered i ndividuals were greatest in F. cernua and H. mutica, and smallest in P . glandulosa. Changes in tissue osmotic potential (TOP) values as a co nsequence of watering were sharply marked in all species except P. gla ndulosa. H. mutica XWP approached zero for a few days in response to s mall rain events. The results indicate that adult individuals of grass es and shrubs are potential competitors for soil resources (to a varia ble degree according to the shrub species). Their co-existence in the arcs is probably favoured by a process of slow competitive displacemen t as long as the recruitment of new shrubs takes place mainly by colon ization of the upslope fringe of the arcs where grass biomass is low. As development of the vegetation progresses in the colonization front and the grass canopy is almost closed, the chances of a shrub being su ppressed by water competition diminishes in the following order: F. ce rnua, L. tridentata, P. glandulosa. A drastic reduction in grass bioma ss because of grazing would depress the competitive ability of the gra sses and may preferentially facilitate the establishment of livestock dispersed species like P. glandulosa. A consequent shift to a more shr ubby community seems unavoidable since the recovery of the grass strat a will probably not suppress the newly-established shrubs tapping wate r from deep water sources. (C) 1995 Academic Press Limited.