Plant rings | Namibia | Australia | Grass | Desert

Plant Rings

Plant rings may look a little similar to fairy circles, but rings and circles differ from each other in various ways. First of all, fairy circles are primarily gaps in vegetation like holes in a Swiss cheese. Plant rings look more like a doughnut and form primarily on bare ground with no or sparse vegetation cover. With their usual diameters of 4-8 m, fairy circles are also much larger than typical plant rings, which are usually between 40-80 cm. However, some exceptional plant rings around the globe can also reach diameters of several meters. Finally, fairy circles are highly ordered and connected at the landscape scale, while plant rings are typically disordered and not interacting with each other. Before the year 2000, fairy circles have been sometimes called fairy rings because after years of drought, the remaining grass stubbles at the periphery of the circle may indeed look like a ring. But the name “fairy ring” has been used in scientific journals already since the mid-20th century for arctic and temperate regions, hence long before fairy circles of the Namib Desert were known. These fairy rings, also known as “elf rings”, are caused by a fungal mycelium and are not the subject of this website, which deals with dryland ecosystems.

Types of plant rings

Ring formation in plants of the global drylands has attracted great interest in the research community, particularly regarding the mechanisms that generate these symmetric shapes. For example, various plant rings are known from countries such as Bolivia, Israel, Jordan or the deserts of the United States. In most cases, these plant rings are so-called “tussock rings”, which result from slow vegetative growth processes of perennial tussock plants. The inner ring is usually caused by central dieback, which is often due to a lack of resources such as water and nutrients in the center of the aging plant. But the causes for the doughnut-like shape of the perennial plant can vary, especially among the more than eighty clonal species that exert ring formation worldwide. In some cases like in spinifex rings of Australia, microbes may also play a role in causing the ring.

However, there is also a new class of plant rings, which we named “collective plant rings” because annual plants collectively deplete the inner soil moisture of the ring. Collective plant rings are composed of several individuals of annual grasses or annual forbs or a mixture of both, and they pop up spontaneously after rainfall events along the Namib Desert. These rings appear to have the same local function as the fairy circles, as the exclusive access to internal soil moisture allows a disproportionately higher biomass to grow along the periphery of the circle or ring. This gives the plants on the periphery of the ring a competitive advantage over the surrounding weaker plants. The logical reason for the grasses forming a ring is that a circle has the smallest circumference-to-area ratio of all geometric shapes. As a result, only a minimum of individual plants growing on the edge have to share the valuable soil water within the ring. This optimizes their water usage inside the ring and keeps the competing grasses from the matrix effectively outside. However, plant rings are different from fairy circles because the latter are much larger and much more ordered than plant rings, and they are primarily vegetation gaps rather than rings. Nevertheless, it is intriguing that completely different grass and forb species like the annual grass Schmidtia kalahariensis or the forb Limeum argute-carinatum form circular rings, just as the Stipagrostis grasses form fairy circles. This is a strong hint that similar self-organizing processes form fairy circles and plant rings along the Namib Desert.

During the years 2020 to 2022, we discovered these collective plant rings in the Namib Desert. Moreover, we found more than a dozen of grass and forb species that all form rings in the Namib, making this desert region a genuine “diversity hotspot” of plant-ring formation. But these rings with typical diameters between 15 to 150 cm have never been studied before and information is not available in the scientific literature. We are currently investigating these plant rings in detail in Namibia. This research involves drone mapping of plant rings at various farms, continuous soil-moisture measurements with solar-driven data loggers, soil-moisture recordings with a mobile sensor, infiltration measurements, microbiological soil analyses, root and shoot measurements of the grasses and forbs and photo documentation using camera traps.

First research results from 2023

In the rainy season 2023, we focused on two exemplary perennial grass species, Eragrostis nindensis and Stipagrostis namaquensis, that both form tussock rings due to a central dieback. The Eragrostis species forms large palatable tussocks in the moister interior of Namibia but towards the drier western part of the country, doughnut-like rings form due to increased aridity stress. The ring diameters vary from a few centimeters to several dozen centimeters. Across 900 km, we mapped the outer diameter of this grass and the interior ring diameter at more than 22 study sites, spanning a rainfall gradient from 90 to 250 mm mean annual precipitation (MAP). The most southern study sites were in the Karas Mountains and the most northern sites were at Brandberg. Somewhat unexpectedly, there was no correlation between the size of the grasses or their ring diameters and rainfall variation, indicating that variation in local habitat conditions must have been a more dominating determinant of ring formation than MAP per se.

The results from the analysis of Eragrostis nindensis show that, for example, large inner ring diameters can form under quite dry site conditions with only 120 mm MAP, but the same extent of central dieback can also be found when precipitation is almost twice as high at 220 mm. Our observations suggest that local habitat constraints such as micro-topography and associated limitations in water availability must be key factors for the formation of tussock rings. For this reason, we focused in a very detailed study on the formation of rings in the grass species Stipagrostis namaquensis. This grass species is also known as river bushman grass which grows in and along riverbeds. At Landsberg farm in southern Namibia, this grass species forms rings with diameters ranging between 16 to 160 cm and rings can be very old. We mapped this grass species at an image resolution of 2 cm/pixel with a drone in a 200 m × 200 m plot, which had drainage lines running through its center from south to north. But before we digitized the grasses, we also tested the detectability of the rings using a higher image resolution of 1 cm/pixel obtained for a 100 m × 100 m plot. Johanna Ottenbreit, who is doing her B.Sc. thesis on these rings could thereby verify that an image resolution of 2 cm/pixel is sufficient to locate all the rings in a 200 m × 200 m plot. The digitization of the plants then resulted in 1164 grass tussocks that had not (yet) formed rings, 634 small rings with a diameter of 16 to < 50 cm, 327 medium-sized rings with a diameter of 50 to < 80 cm, and 91 large rings with a diameter of 80 to 160 cm. We then created a digital terrain model (DTM) which revealed the micro-topography of the plot and a height variation of less than 70 cm across the 200 m × 200 m. We used a so-called Berman test to analyze in how far the rings and grass tussocks were significantly associated with the drainage lines. The results show that all three classes of ring diameters were significantly associated with low elevation values. In other words, the grass tussocks transformed over time into rings, but predominantly only within the drainage lines. In contrast, the grass tussocks were not significantly associated with elevation, which can be attributed to their initial random dispersal within the entire study plot. We did many more related spatial analyses, revealing, for example, that rings were significantly associated with low soil moisture in the plot and that ring patterns were disordered and did not indicate competitive interactions among the plants. All these results support the view that these investigated tussock rings reflect an individual-level response to aridity stress. Ring formation of these grasses demonstrates also a high degree of phenotypic plasticity of such desert-adapted plants.

More examples of plant rings along the Namib