Evidence for scale-dependent root-augmentation feedback and its role in halting the spread of a pantropical shrub into an endemic sedge

Abstract

Vegetation pattern formation is a widespread phenomenon in resource-limited environments, but the driving mechanisms are largely unconfirmed empirically. Combining results of field studies and mathematical modeling, empirical evidence for a generic pattern-formation mechanism is demonstrated with the clonal shrub Guilandina bonduc L. (hereafter Guilandina) on the Brazilian Island of Trindade. The mechanism is associated with water-conduction by laterally spread roots and root augmentation as the shoot grows – a crucial element in the positive feedback loop that drives spatial patterning. Assuming precipitation-dependent root-shoot relations, the model accounts for the major vegetation landscapes on Trindade Island, substantiating lateral root augmentation as the driving mechanism of Guilandina patterning. Guilandina expands into surrounding communities dominated by the Trindade endemic, Cyperus atlanticus Hemsl. (hereafter Cyperus). It appears to do so by decreasing the water potential in soils below Cyperus through its dense lateral roots, leaving behind a patchy Guilandina-only landscape. We use this system to highlight a novel form of invasion, likely to apply to many other systems where the invasive species is pattern-forming. Depending on the level of water stress, the invasion can take two distinct forms: (i) complete invasion at low stress that culminates in a patchy Guilandina-only landscape through a spot-replication process, (ii) incomplete invasion at high stress that begins but does not spread, forming isolated Guilandina spots of fixed size, surrounded by bare-soil halos, in otherwise uniform Cyperus grassland. Thus, drier climates may act selectively on pattern-forming invasive species, imposing incomplete invasion and reducing the negative effects on native species.

Microbial Factors May Contribute to the Persistence of Australian Fairy Circles

“…The apparent spatial similarity between NFC patterns and AFC patterns (Figure 1a-c) has been attributed to the same general positive feedback that both systems share, a scale-dependent feedback between local vegetation growth and water transport to the growth location [15,30], albeit with diff erent water transport mechanisms [2,16,31]. NFCs patterns form in deep aeolian sands, characterized by high water infi ltration rates.…”

“…Another possible implication of our fi ndings is related to the multi-scale nature of patterns is related to the multi-scale nature of patterns observed in Namibian and Australian FCs-largescale FC patterns and small-scale patterns within the scale FC patterns and small-scale patterns within the vegetation matrix. In NFCs, the small-scale pattern often consists of spots [9,31], whereas in AFCs, it consists mostly of rings (Figure 1d) [2]. Vegetation rings have commonly been explained by negative plant-soil feedbacks [23,45] and by plant-water scale-dependent feedbacks [46][47][48][49][50][51].…”

“…NFCs patterns form in deep aeolian sands, characterized by high water infi ltration rates. The water-transport mechanism, in this case, is soil-water lateral diff usion towards denser and, thus, drier vegetation patches [5,31,32] and the soil water content is on average higher in the center then in the fairy circle periphery. By contrast, in Australia, the top soil layer is a barely permeable claypan that generates overland water fl ow towards denser vegetation patches at the gap edges (Figures 2a,3c) [2,3,14] and the soil water content in the center of the AFC is lower than in the periphery.…”

Habitat isolation diminishes potential of self-organised pattern formation to promote local diversity in metacommunities

“…There, the formation of a spatial pattern can promote coexistence between (autotrophic) consumer species that differ in their dispersal strategies (Nathan et al, 2013; Eigentler and Sherratt, 2020) or through relaxing competition and even enabling facilitation between species in the emergent heterogeneous environmental conditions (Cornacchia et al, 2018; L. Eigentler, 2021; Bennett et al, 2023). These studies however neglect the fragmentation of many natural landscapes into discrete habitat patches that may be only partially connected by dispersal links and therefore often have irregular spatial structures (Holland and Hastings, 2008).…”

Phenotypic plasticity: A missing element in the theory of vegetation pattern formation

“…Observations of vegetation patterns have primarily been reported in drylands ( 1 – 4 ), which occupy more than two-fifths of the terrestrial earth area and are expected to expand as climate change proceeds ( 5 ), but are not limited to drylands ( 6 – 9 ). Pattern formation theory ( 10 – 13 ) has been highly instrumental in understanding the emergence of vegetation patterns and their dynamics ( 14 – 23 ).…”