Shape matters in sampling plant diversity

The identification of shape and size of sampling units maximising species richness has major implications in conservation planning and monitoring actions. In this study we tested the effect of three sampling unit shapes (rectangles, squared, and randomly shaped sampling units) on the number of recorded plant species. We used a large dataset gathered in the network of protected areas of the Siena Province, Italy, as part of a joint project with the Italian National Forest Inventory. This dataset is composed of plant species occurrence data recorded from 604 plots (10m x10m), each divided in a grid of 16 contiguous subplot units (2.5 m x 2.5 m). Furthermore, we evaluated the effect of plot orientation along the main environmental gradient, related to slope, to understand how the selection of plot orientation (when elongated plots are used) influences the number of species collected. In total, 1041 plant species were recorded from the study plots. Species richness was significantly higher using randomly shaped sampling units compared to rectangular and squared units. Although rectangular units captured a significantly larger number of species than squared ones, plot orientation along the main environmental gradient (terrain slope) did not show a systematic effect on the number of recorded species. The decision of whether or not using rectangular versus squared plots should therefore depend upon the objectives of the survey and the characteristics of the area to be surveyed, with squared plots being more suitable for assessing species composition of more homogeneous vegetation units while rectangular plots being more suited for recording more species in the pooled sample of a large area. If the aim of a survey is to evaluate the biodiversity and conservation value of a ecosystem/community, sampling protocols that maximise the number of recorded species given the same sampling effort should be used. Furthermore, considerations on the orientation of rectangular plots should be carefully made to enhance reproducibility of collected data and correctly interpret the results of a survey. This information is crucial to develop cost-effective surveys and monitoring programs aimed at maximising the number of collected species.