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 and and semi-arid ecosystems. Nevertheless, information on the seasonal patterns and magnitudes of redistribution is lacking for all but a few plant species. We measured sap flow in the taproot and three main lateral roots of a 10-year-old Juglans major Torr. tree, on an ephemeral catchment in southeastern Arizona, to determine how patterns of redistribution respond to pulses of summer precipitation. Groundwater was beyond rooting depth and a hardpan prevented recharge of surface water to deep soil layers. Reverse flow (hydraulic descent) commenced in the taproot and deep lateral roots in early August after a series of moderate precipitation events, and abruptly ceased after all shallow roots were experimentally severed in mid-August. On some days, hydraulic descent continued in the deep lateral roots during periods of daytime transpiration, and the daily volume of hydraulic descent (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. Hydraulic descent may be an important component of the water balance of phreatophytic trees by facilitating root growth in deep soil layers and by transferring water away from shallow-rooted competitors.