Because water is the primary limiting resource for many organisms in arid systems, changes in the biota which translate into changes in water distribution or availability will be strong drivers of a change in state. One critical stage is the infiltration of water into the soil (versus its evaporation from the surface or its horizontal transfer or loss to run-off). Vegetative cover modulates the impact energy of raindrops, reducing the amount of sediment dislodged and transported during heavy storms (e.g. Rogers and Schumm 1991). The presence of rooted plants provides root channels which in turn enhance deep percolation of water into the soil profile; the nature of the plant canopy influences the proportion of rainfall that is intercepted and that falls either as throughfall or as stemfiow (West and Giftbrd 1976; Tromble 1987; Navar and Bryan 1990). Stemfiow apparently redistributes precipitation to the deep roots of shrubs and trees, favoring established vegetation at the expense of small plants growing under the canopy (Nulsen et al. 1986). Termites and their tunneling exert significant control over the rate of infiltration versus runoff of precipitation (Elkins et al. 1986; Whitford 1991); ant colonies may have similar effects (Elmes 1991; Blom et al. 1994). Jones et al. (1993) list several examples of burrowing desert animals influencing soil structure and hydrology, ranging from ants to naked mole rats to crested porcupines.
Conversion of vegetation from one structure to another (e.g. change from homogeneous grass cover to patchy shrub cover) will alter the spatial pattern of infiltration and runoff, which will in turn alter the spatial location and the depth of water storage in the soil. Elimination of a group of plants actively using soil water at a particular season, or from a particular depth in the soil, could lead to a decrease in productivity if that water is lost from the system (by evaporation or by percolation beyond the roots of remaining plants). These effects arc most likely where all members of such a "group" respond to disturbance (drought or grazing) in a similar fashion.
Field data are equivocal on the question of whether the substitution of one plant group for another, or the presence of multiple groups versus a single "type", will affect total ecosystem water balance. Dugas and Mayeux (1991) and Carlson et al. (1990) found that increased herbaceous cover following shrub removal resulted in little net change in water distribution or total évapotranspiration from dry rangeland. There is growing evidence that desert vegetation in most areas, even where cover is sparse, is capable of absorbing and transpiring virtually all water received as precipitation, thus preventing the movement of water down below the rooting zone (Link et al.
1994; Phillips 1994). These references suggest that changes in the relative abundance of growth forms, or of species within growth forms, do not appear to affect the net outcome of total water use.
On the other hand, one published exception came from a lysimeter experiment where most weedy species were able to "dry up" the moisture content of the lysimeter, but the shallow roots and short lifespan of an annual non-native, Bromus tectorum. failed to prevent deep drainage (Gee et al. 1994). Jn a field experiment in semi-arid steppe, shrubs and grasses used largely different sources of soil water, and neither group could compensate fully for the removal of the other in terms of total water use (Sala et al. 1989). Both root system morphology and phenology or timing of water use are critical to determining the effectiveness of water uptake (Pelaez et al. 1994).
Water inputs from dew and fog are significant additions to the available moisture in some coastal deserts; however, it is unclear whether organisms play a role in adding substantial water beyond that for their own consumption.
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