The Detection of Small-Scale Patterns in Vegetation

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Abstract
Pattern at the scale of a few centimeters may be important for understanding the determinants of species-distributions, but may be difficult to demonstrate using quadrats. A transect sampling system is described, together with 2 methods of analysis, for detecting small-scale positive and negative associations between species. The 1st method considers the frequency of juxtaposition of species, and tests whether this frequency is significantly greater or less than a random expectation. The 2nd method divides the transect into regions, and tests whether the number of occurrences of each species within the regions of occurrence of each other species is significantly greater or less than outside those regions. Indices of association may be generated by either method. The 2nd method may also be used to examine the effect or association patterns of changing the size of regions, and to determine whether some species may show stronger patterns when both of 2 other species are present than when either one alone is present. Both methods were used to analyse transect data collected in an old-field in Illinois [USA], and the results were compared. [It contained, in order of decreasing abundance, Festuca elatior L., Poa compressa L., Coronilla varia L., Dactylis glomerata L., Agropyron repens (L.) Beauv., Phleum Pratense L. and Bromus inermis Leyss. Infrequent were Pastinaca sativa L., Prunella vulgaris L., Solidago nemoralis Ait., S. rugosa Ait., Melilotus alba Desr. and Trifolium pratense L.] These methods made it relatively simple ti detect patterns of association for the stems and tillers of the 7 most abundant species at a scale of 4 cm.