Hydraulic redistribution by deep roots of a Chihuahuan Desert phreatophyte.

Downward redistribution of soil water through plant roots has important consequences for water and nutrient balance of arid and semi-arid ecosystems. Nevertheless, information on the patterns and magnitudes is lacking for all but a few plant species. We measured sap flow in the taproot and three main lateral roots of a 10-yr old Juglans major tree on an ephemeral drainage in southeastern Arizona, to determine how patterns of redistribution respond to pulses of monsoonal precipitation. Groundwater was beyond rooting depth, and a hardpan prevented recharge of surface moisture to deep soil layers. Reverse flow (downward redistribution) commenced in early August after a series of moderate precipitation events, and abruptly ceased after all the shallow roots were experimentally severed in mid-August. On some days reverse flow continued in the deep lateral roots during periods of daytime transpiration, and the daily volume of reverse flow (deep lateral roots plus taproot) ranged from 10 to nearly 60% of daily transpiration. The persistent pattern of reverse flow demonstrates that in some plants, water potential gradients from soil to leaf during transpiration are often smaller than those between soil layers within the rooting zone. Downward redistribution may be an important component of the water balance of juvenile phreatophytes by facilitating root growth in deep soil layers and by transferring water away from shallow rooted competitors