The contribution of pathogenic soil microbes to ring formation in an iconic Australian arid grass,

Abstract

Ring-forming species of spinifex grasses (Triodia spp.) are a dominant feature across much of Australia's arid and semi-arid zone. Researchers have long been curious about the mechanisms underpinning their striking growth form. However, none of the factors investigated to date provide a convincing explanation for ring formation. Here, we asked whether an accumulation of pathogenic soil microbes might impede seedling emergence and subsequent growth in the centre of Triodia basedowii rings. We collected soil from inside and outside naturally occurring spinifex rings and compared plants grown in soil with live microbes to plants grown in sterilised soil. Consistent with our hypothesis, we found that emergence of T. basedowii seedlings was lower in live soil from inside the rings than in live soil from outside the rings. Further, seedling emergence in soil from inside the rings increased significantly in response to soil sterilisation. We found no significant difference in growth between sterile and live soils. However, this might be due to a lack of power caused by high rates of seedling mortality in all treatments. Overall, our study provides evidence for the role of soil pathogens in shaping this iconic Australian grass.

Definition of “fairy circles” and how they differ from other common vegetation gaps and plant rings

“…In the small area of Western Australia where FCs exist, the tussock rings of T. basedowii commonly reach diameters of 2 m, whereas FCs at the same area have mean diameters of 4 m, and they may partly exceed 7 m (Getzin et al, 2016a). In the study of Ross and Moles (2021) the sampled T . basedowii rings had internal diameters from 0.3 m to 1 m and they state that large Triodia rings can have diameters of more than 2 m. These Triodia rings have nothing in common with genuine Australian FCs that have been described separately from rings (Getzin et al, 2016a).…”

“…Plant rings such as the large Triodia rings (or spinifex‐grass rings) in Australia originate mostly from a single older plant that expands laterally over long periods, whereby mechanisms such as microbial pathogen effects in the center of the plant may induce the ring (Ross & Moles, 2021). Other reasons for plant‐ring formation in arid environments are, for example, negative feedbacks between sediment deposition and vegetation growth inside the grass, which leads to central dieback in grasses of the Chihuahuan desert (Ravi et al, 2008) and in Asphodelus ramosus geophytes of the Negev Desert in Israel (Herooty et al, 2020; Yizhaq et al, 2019).…”

“…FCs have been named "fairy" because unlike all other vegetation-gap patterns, the origin of the FC phenomenon was so mysterious for decades. In recent years, too Plant rings such as the large Triodia rings (or spinifex-grass rings) in Australia originate mostly from a single older plant that expands laterally over long periods, whereby mechanisms such as microbial pathogen effects in the center of the plant may induce the ring (Ross & Moles, 2021). Other reasons for plant-ring formation in arid environments are, for example, negative feedbacks between sediment deposition and vegetation growth inside the grass, which leads to central dieback in grasses of the Chihuahuan desert (Ravi et al, 2008)…”

“…Graminoid plants that eventually form a ring over time can be classified as “tussock rings” (Table 1). Notably, such tussock rings are not restricted to clonal growth only because obligate seeders, such as the hummock grass T. basedowii , also can form large rings (Getzin et al, 2016a; Ross & Moles, 2021).…”

“…For example, common grassland gaps that are obviously created by harvester termites and that can be found in many regions of the Australian continent have been synonymously called FCs (Walsh et al, 2016), despite the fact that these nest‐associated gaps differ significantly in many structural and spatial characteristics from the genuine FCs (Getzin et al, 2016b, 2019a, 2021a). Recently, plant rings have been mixed up with FCs (Ross & Moles, 2021; Zhao et al, 2021), despite the fact that both are different phenomena. Also, underwater structures in Mediterranean seagrass (Ruiz‐Reynés et al, 2017) and even the microscopic nanostructures of carbon‐based graphene and graphite surfaces (Phan et al, 2019) have also been called FCs.…”