FOG AS A SUPPLEMENTAL WATER SOURCE FOR PLANTS IN THE CENTRAL NAMIB DESERT

The Namib Desert on the southwestern coast of Africa is hyper-arid in terms of rainfall, but experiences up to 100 days of fog each year. This fog is a more regular and potentially more plentiful source of water than the highly seasonal rainfall, which averages 20-80mm/yr. Several studies have demonstrated fog use strategies of Namib biota, and the mechanism for fog uptake has been shown for three endemic plants (Stipagrostis sabulicola, Trianthema hereroensis, Arthraerua leubnitziae). Groundwater availability is currently threatened by proposed increases in abstraction for copper and uranium mining in the area. The idea that fog is a major water source for the ecosystem is used to justify these increases in groundwater utilization. This study uses stable isotopes of water from five sources -- fog, rain, groundwater, soil water, and plant water -- to estimate the amount of fog water utilized by nine common plant species in the Central Namib, notably including the enigmatic and very long-lived Welwitschia mirabilis. Initial results confirm a large isotope discrimination between fog (-0.6‰ δ¹⁸O, 2.1‰ δ²H) and groundwater/rainwater (-6.1‰ δ¹⁸O, -38.9‰ δ²H). Soil water (10 to 100cm) exhibits typical evaporative enrichment, and plant water shows an integration of these three water sources. Leaf water exhibits extreme isotopic enrichment of up to 100‰ in δ²H. The W. mirabilis population in the study area apparently does not make significant use of fog water, with stem water isotope values indicating groundwater or local rainfall as the predominant water source. The W. mirabilis values correspond fairly well with known phreatophytes, and are distinctly depleted relative to known fog-utilizing plants.