Saturday, January 21, 2017

Accord of the Rings - another stunning paper in Nature by Corina

Nature arrived on Thursday with this front cover. When I saw it I thought at once that it would be a paper by Corina Tarnita and her colleagues and sure enough it is. "tanquam ex ungue leonem." (or rather leanem I think)

The paper, A theoretical foundation for multi-scale regular vegetation patterns, develops an integrated model which explains the emergence of fascinating self-organised vegetation patterns such as North American Mima mounds, Brazilian murundus, South African heuweltjies, and Namibian fairy circles.

There has been a lot of controversy, some fairly heated, as to whether the best explanation is scale-dependent feedback (SDF) or the activities of subterranean ecosystem engineers such as termites, ants, and rodents.  However as the paper shows conclusively, both mechanisms are important and the correct explanation comes from integrating both factors in a single model.

When they did this they explained all the observed features of the system, but now comes the completely beautiful part....
Fig 4 from Tarnita et al. a, Panorama
showing matrix-vegetation clumps.
b, Low-altitude (10-m) image of matrix
vegetation  Scale bar as in c. c, Model
output used for comparison with b.
d, Normalized radial spectra of field
images (n = 27 samples) and model
simulations (n = 52 samples), as
functions of wavenumber. 
Our coupled model also predicts a previously unrecognized feature of these Namib Desert landscapes. Prior studies have focused exclusively on the Fairy Circles and have largely ignored the matrix in between. In our model, SDF induces dynamic self-organization of the matrix vegetation, but at smaller spatial scales that are more compatible with ecohydrologically realistic grass–water feedback distances (Supplementary Video 3). Following wet seasons, small, regular clumps of matrix vegetation emerge, interspersed with larger, randomly distributed clumps (Fig. 2c). These larger clumps are rare in the SDF-only model without termites, but arise in the coupled model from small-scale soil-moisture variability in the matrix (Fig. 2d; consistent with published data)—itself a ripple effect created by the Fairy Circles (Extended Data Fig. 6). To evaluate these theoretical predictions, we photographed NamibRand Nature Reserve matrix-vegetation distributions from 10-m height in February 2015 and characterized both observed and model-predicted patterns using Fourier-transform analyses (see Methods). We found strong agreement between model outputs and field data (Fig. 4).

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