RESEARCH ARTICLE Roots point to water sources of Welwitschia mirabilis in a hyperarid desert Joh R. Henschel 1,2,3 | Theo D. Wassenaar 1 | Angie Kanandjembo 4 | Michele Kilbourn Louw 4 | Götz Neef 1 | Titus Shuuya 1 | Keir Soderberg 5,6 1 Namib Ecological Restoration and Monitoring Unit, Gobabeb Research and Training Centre, Walvis Bay, Namibia 2 Arid Lands Node, South African Environmental Observation Network, Kimberley, South Africa 3 Centre for Environmental Management, University of the Free State, Bloemfontein, South Africa 4 Safety Health and Environmental Risks, Swakop Uranium, Swakopmund, Namibia 5 Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 6 Geochemistry, S.S. Papadopulos & Associates, Inc., Bethesda, Maryland Correspondence Joh R. Henschel, South African Environmental Observation Network, PO Box 11040 Hadison Park, Kimberley 8306, South Africa. Email: joh.henschel@saeon.ac.za Abstract 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. KEYWORDS fog, gypsum, long‐lived broadleaved evergreen, perched water, petrocalcic horizon, resilience to drought stress, root map 1 | INTRODUCTION The fundamental question of how plants source water is acute for long‐lived woody species in extremely water‐scarce conditions. An understanding of their adaptations to hyperaridity can help elucidate ecohydrological processes and strategies evolved by plants to avoid or survive dehydration. This should be especially true for the continuously growing broadleaved evergreen welwitschia, Welwitschia mirabilis Hook. Fil. (Figure 1), a long‐lived gymnosperm endemic to the Namib Desert. Land use changes, mining, agriculture, and potentially also climate change are increasing risks to biota, including welwitschia (Wassenaar et al., 2013). A large population of this species on the Welwitschia Plains, adjacent to one of the largest uranium mines in the world, is potentially threatened by interference with its water supply. It has therefore become critical to resolve uncertainties about its ecohydrology, including the roles of different water sources and its physiological and anatomical strategies to cope with continuous drought. The mean annual precipitation (MAP) across welwitschia's distribution range is ~20–100 mm. Yet this evergreen plant, which rarely employs the water‐saving crassulacean acid metabolism photosynthetic pathway (Schulze, Ziegler, & Stichler, 1976; Von Willert, Armbrüster, Drees, & Zabarowski, 2005) but primarily uses the C3 pathway (von Willert et al., 2005), transpires daily 0.2–1.4 L of water per square metre of leaf (von Willert & Wagner‐Douglas, 1994). The question of how it manages to survive in hyperarid condi- tions has intrigued generations of biologists (Henschel, Eller, Seely, & Received: 21 March 2018 Revised: 20 June 2018 Accepted: 21 August 2018 DOI: 10.1002/eco.2039 Ecohydrology. 2018;e2039. https://doi.org/10.1002/eco.2039 © 2018 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/eco 1 of 12