Soil water use by co-existing 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 Desert (Mexico) was investigated after an experimental irrigation equivalent to a 75 mm rainfall. Three shrubs (Flourensia cernua,Larrea tridentataandProsopis glandulosa) and one grass (Hilaria mutica) were studied. Irrigation water did not percolate deeper than 40 cm. This soil layer contained more than 75% of the roots in all species, exceptP. glandulosawhere a less developed, deeper root system was detected (but not quantified). Root distribution indicates that the water stored in the 0–40-cm soil layer after the experimental irrigation was available for the four species. However, predawn xylem water potential (XWP) ofF. cernuaandH. muticawere strongly influenced by soil water present in the 0–40-cm layer, whereas those ofL. tridentataandP. glandulosawere not. Differences in predawn XWP between watered and unwatered individuals were greatest inF. cernuaandH. mutica, and smallest inP. glandulosa. Changes in tissue osmotic potential (TOP) values as a consequence of watering were sharply marked in all species exceptP. glandulosa. H. muticaXWP approached zero for a few days in response to small rain events. The results indicate that adult individuals of grasses and shrubs are potential competitors for soil resources (to a variable degree according to the shrub species). Their co-existence in the arcs is probably favoured by a process of slow competitive displacement as long as the recruitment of new shrubs takes place mainly by colonization 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 suppressed by water competition diminishes in the following order:F. cernua,L. tridentata,P. glandulosa. A drastic reduction in grass biomass because of grazing would depress the competitive ability of the grasses and may preferentially facilitate the establishment of livestock dispersed species likeP. glandulosa. A consequent shift to a more shrubby community seems unavoidable since the recovery of the grass strata will probably not suppress the newly-established shrubs tapping water from deep water sources.