Strategies of Acanthosicyos horridus (!nara) to exploit alternative atmospheric moisture sources in the hyper-arid Namib Desert

Abstract

The Namib Desert is considered hyper-arid due to its low annual precipitation and high potential evaporation rates. However, fog originating offshore sustains life and drives high levels of adaptation in this coastal desert. Organisms may employ different strategies to utilise vital atmospheric moisture. Acanthosicyos horridus (Welw. ex Hook. f.), locally known as !nara, is a member of the Cucurbitaceae family and is endemic to the Namib. This keystone species forms tangled thickets that accumulate wind-blown sand and plant litter, resulting in hummock formation. These hummocks harbour a host of biota and provide essential ecosystem services in this desert environment. There is, however, limited information on !nara plant-fog relations, and how hummock formation contributes to this interaction. The effect of the hummock on water balance in the soil was investigated by measuring moisture content and temperature. Various strategies to exploit fog and dew as alternative sources of water were tested. These include direct aerial absorption through the stems and the movement of coalesced water along the stems. A Wingscape time-lapse camera together with a Lascar humidity logger, and a Leaf-Wetness Sensor were used to determine presence and duration of moisture. Chlorophyll a fluorescence was employed to evaluate the overall vitality of the plants before and after fog events. Preliminary results indicate likely direct absorption of water through the stem and an increase in the chlorophyll a fluorescence intensity after a fog events. These results indicate that the !nara plant may greatly benefit from fog as a supplementary source of moisture