How Desert Plants Find a Drink

AFew images of adaptation are as durable as the desert shrub that sinks a root to the water table and drinks while everything around it dies. The image is not baseless: taproots reaching thirty metres have been excavated in the Kalahari, and hydraulic lift, by which deep roots release water into dry surface soil at night, was described in the 1980s and has since been confirmed in dozens of species. Yet the isotopic surveys that began to accumulate after 1998 have been persistently awkward for it. When the hydrogen and oxygen signatures of water drawn from a stem are matched against the signatures of rain, dew, fog and groundwater, the water moving up most desert shrubs in most years turns out to have fallen recently and to have travelled no more than a metre. Whether that discrepancy means the deep root has been overrated, or merely that the surveys have been sampling the wrong months, is now the central quarrel of dryland plant physiology.

BYusra Nabti, a plant physiologist at the Arid Lands Institute in Tucson, holds that the deep root has been miscast rather than disproved. Between 2011 and 2020 her group tracked 84 individuals of four shrub species across a rainfall gradient in the Sonoran Desert, running 2,140 isotope analyses in all. In years of average rainfall, water drawn from below three metres supplied 12 per cent of what the shrubs transpired. In the two drought years within the series, the same individuals took 61 per cent of their water from that depth. The mean across the decade, 19 per cent, is the figure that reached the review articles, and it is, she argues, the least informative number her group produced. A root system, on her account, is not a straw but a portfolio: the shallow layers are worked whenever they are wet because they are cheap, and the expensive deep connection is held back. Averaging the two states describes neither of them.

CThe account makes a prediction that can be dug up, and Nabti has dug it up. If deep water is insurance rather than income, then severing the deep connection should cost a shrub nothing until a drought arrives. At 31 plots her group trenched a barrier to a depth of three metres around mature creosote bushes and left them for eight years. Through five ordinary years the trenched plants were indistinguishable from the controls in leaf area and stem growth; in the sixth year, the driest of the series, mortality among them reached 44 per cent against 6 per cent in the controls. The effect was confined to individuals that had been established for more than five years before trenching; seedlings, whose roots had never reached that depth, died at the same rate on both sides of the barrier. Nabti is careful to describe deep rooting as a buffer against rare years rather than as the ordinary supply, and she accepts that where rain fails for a decade the buffer empties too.

DLars Petersen, who studies soil and plant hydrology at the Namib Research Centre in Gobabeb, answers that the quarrel about depth has distracted the field from water that never enters the soil at all. His measurements on 148 individuals of a coastal succulent recorded sap moving down the stem, from leaf towards root, on 71 of the 104 nights on which fog settled, and estimated that fog absorbed through the leaf supplied roughly a third of the plant's annual water. Water taken in through a leaf, he notes, arrives without the carbon cost of building and maintaining root tissue, which in some dryland shrubs consumes more than half of what the plant fixes. He concedes two things. His site lies within forty kilometres of a cold current and receives fog on some 120 nights a year, a frequency few deserts match; and leaf wetting was registered by surface sensors rather than by any direct measurement of water crossing the cuticle.

EThe dispute left the laboratory some time ago. Since 2021 several national restoration programmes have required that planting stock be raised in deep containers and watered through buried pipes, on the reasoning that a young plant which fails to reach moisture below two metres will not survive its first drought. Petersen points out, awkwardly for his own position, that fog harvesting structures are cheapest to justify exactly where fog is already frequent, so the mechanism he trusts recommends itself least in the interiors where restoration is most needed. Nabti replies that this taste for whatever can be trenched or excavated is precisely what keeps night-time leaf absorption out of the models, since a film of water on a leaf leaves no evidence a spade can find. Both regret one habit above all: that restoration projects go on reporting survival at twelve months, an interval short enough to exclude the droughts that decide which planting actually persists.

FA truce of sorts is taking shape in which the two accounts are assigned to different conditions rather than ranked against each other. Where rain arrives often enough to keep the upper soil damp, the shallow layers do the ordinary work and depth is an expense held in reserve; where the air delivers more water than the sky does, as it does along fog coasts, the leaf can act as a second root and the reach of the root system matters less than its capacity to survive. The unresolved question is what becomes of the fog: the warming that is drying the interiors also alters the temperature contrasts on which coastal fog depends, and no record of fog frequency yet runs long enough to say which way it will move. What both researchers reject is the older assumption that a plant can be explained by naming the single place its water comes from.