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Roots point to water sources of Welwitschia mirabilis in a hyperarid desert

Welwitschia mirabilis is a long‐lived evergreen in the hyperarid Namib Desert; at our study site, rainfall is rare (mean annual precipitation = 31 mm), groundwater deep (57–75 m), and fog frequent (50–90 events per year). By examining root architecture in relation to soil moisture and analysing the isotopic composition of hydrogen and oxygen of plant and soil water, we established whether welwitschia sources water from a stable supply of deep groundwater, or from shallow moisture originating from fog and dew, or from rainwater at infiltration depth. Isotopes suggested rainwater as principal water source. Most (55%) major roots and fine roots occurred in 10‐ to 66‐cm‐deep layers of gypsum containing 10% moisture. A further 25% of both root types grew in moist sand in petrocalcic horizons at 93‐ to 125‐cm depths. A high density of fine roots (14% of total) grew upwards towards the ground surface in a 1.5‐m radius around plants, an area occasionally wetted by run‐off of fog and dew. We conclude that welwitschia mainly relies on rainwater obtained in perched horizons. Supplemental water is obtained from fog and dew from the surface and potentially from gypsum blocks. Multiple strategies enable this extremely long‐lived evergreen to be resilient against dehydration in hyperarid conditions.