Getting to the root of the water question in water-scarce environments

By Joh Henschel, Manager, SAEON Arid Lands Node Tweet

Water is of primary importance for xeric plants (in arid conditions). It therefore comes as a surprise that in the extremely dry Namib Desert, Welwitschia mirabilis, an exceptionally long-lived plant, appears to flaunt conditions of water scarcity by being evergreen, broadleaved, continuously growing and maintaining high evapotranspiration (about a litre of water per day), but somehow avoiding and surviving dehydration for centuries to millennia.

From where and how does it get sufficient water?

Its remarkable water story may not be a Just So Story, but points to some fundamental principles of xeric ecohydrology, as described in our recent publication.

Our study of Welwitschia mirabilis was conducted on the Welwitschia Plains near Swakopmund, Namibia (Photo: Joh Henschel)

Examining welwitschia root architecture 

In 2012–2014, the need for the Husab uranium mine to remove seven welwitschias to make way for the construction of a pipeline and road presented an opportunity to study the roots of these plants. 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 obtains water from 57 to 75-metre deep groundwater, or from shallow moisture during 50 to 90 fog events per year, or from rainfalls, annually averaging 31 millimetres.

The project entailed collaboration between seven institutions, with 30 people involved in fieldwork, four in labwork, and a draughtsman, as well as generous measures of effort and goodwill by many. Many tons of gravelly soil were moved to painstakingly map roots ranging from 22 to 62 metres in length per plant, growing sideways from the stem by 2.4-9.0 metres, and down to depths of 1.2-1.8 metres.

We were startled when our results indicated that rainwater was the principal water source. Rainfall is usually very low (median 25 mm per annum) and extremely erratic (range 1-98 mm per annum) in this part of the Namib Desert.

A dense root network of this female welwitschia penetrates gypsum layers at 10-66 cm depth (Photo: Joh Henschel)                                                                                                                                                                                                                                                                Many dedicated hands made the task of excavating and mapping welwitschia roots possible, including (l-r) Dr Ndafuda Shiponeni from the University of Namibia, Theresa Henschel from EnviroMend and Frank Löhnert from the Namib Botanical Gardens, Swakopmund (Photo: Joh Henschel)

Fog water contributed a smidgen more, perhaps not to be scoffed at because of its much greater frequency of occurrence than rain. With roots penetrating less than two metres deep, welwitschia did not get to groundwater, confirmed by isotopes – the commonly-assumed idea of welwitschia being a phreatophyte was not applicable.

The complex root architecture (see Figure S1 in https://onlinelibrary.wiley.com/action/downloadSupplement?doi=10.1002%2Feco.2039&file=eco2039-sup-0001-Data_S1.pdf) revealed more details and some further surprises. A concrete-like flat layer of calcrete – termed petrocalcic horizon – at depths of 93-125 cm may have prevented rainwater from infiltrating any deeper, resulting in high levels of moisture at this depth.

About a quarter of welwitschia’s major roots and fine roots grew horizontally in this moist layer of sand on the solid calcrete floor. Some 14% of the fine roots grew upwards towards the ground surface in a 1.5 m radius around plants, an area occasionally wetted by fog, perhaps even runoff from the corrugated leaves.

So, it is true that welwitschia is a bit of a fog-collecting plant, though only to a small extent; the bulk of its water comes, as we have seen, from rain, scarce as that may be.

Surprisingly, most (55%) welwitschia roots were concentrated in a dense network in a higher layer between 10-66 cm deep, a layer where gypsum infused the gravel. Gypsum is a hydrophilic salt, which would presumably draw water from roots, so why did over half of the roots grow in this layer?

In the field, soil at 10-66 cm warms up a bit by one or more degrees during the day, and cools down again at night. In the laboratory we found that warming released some 600 mm of water per 1 ^oC of warming in the gypsum layer around welwitschia roots and gained this water again from atmospheric moisture when cooling. If welwitschia roots managed to collect water released during warm times and move it away from the gypsum layer before it cooled, it could effectively harvest water from gypsum.

It took a total of 37 days in the field to excavate seven welwitschia plants and map their roots, often entailing die-hard efforts to get to the bottom. The yellow stem next to Theresa marks the original position of the removed welwitschia stem (Photo: Joh Henschel)          Excavated welwitschia plants were moved, roots and all, to a new site where they were replanted. Only one transplant survived, this being the smallest plant shown here, for which the volunteer field assistant, otherwise tour operator, Werner de Hilster, carefully embedded the roots among blocks of gypsum (Photo: Joh Henschel)

The gypsum may, in turn, replenish its hydrophilic demand from evaporated moisture coming from deeper layers or vapour penetrating from the soil surface at night. The mechanisms employed by welwitschia roots to obtain and retain water remain to be confirmed and understood, but it is clear that roots enable this extremely long-lived evergreen to reduce risks of running out of water even in the driest phases of a hyperarid desert.

Resilience against dehydration

The demonstration of welwitschia’s resilience against dehydration – though coming from a case study of an extremophile – is instructive for SAEON’s future studies of ecohydrology of soil at root level. It also demonstrates so clearly that long-held assumptions and inferences cannot replace empirical studies.

Given that drought and its effects on plants and animals is currently a topic of such looming importance across southern Africa, there is comfort in the idea that there is one species that has seemingly overcome the risks of drought – welwitschia, an enigmatic plant which never ceases to amaze anew.