Population dynamics of the tropical annual grass Sorghum intrans in relation to local patchiness in its abundance

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Abstract
Communities of Sorghum intrans, a tropical annual grass of north‐west Australia, contain sparse patches several metres in diameter within otherwise dense swards. These patches are persistent for at least ca. 4 years, despite the fact that these species are annuals which lack a persistent seed‐bank.Three possible causes of the sparse patches were investigated at a site near Darwin: loss of viable seeds through granivory or death; high plant mortality; or poor fecundity. This was done by augmenting the seed‐banks of S. intrans in the sparse patches, and thinning stands of seedlings in the surrounding, naturally dense areas, and comparing the results of these perturbations with controls.Granivory did not appear to be a cause of the sparse patches. Furthermore, the seeds in the sparse patches did not become inviable over the dry season. Compared with the plants of S. intrans in the naturally high‐density areas, survival of the plants in the augmented patches was not reduced, but vigour and seed production were. Conversely, plants in the thinned stands had a higher survival rate and very much greater vigour and seed production than plants in either the undisturbed patches or the undisturbed surrounding areas. Biotic constraints to plant growth such as grazing and disease appeared to be unimportant.These sparse patches result from poor site potential for S. intrans which limits its growth and reproductive output.