Abstract
AbstractQuestionsThe cover and abundance of individual plant species have been recorded on ordinal scales for millions of plots world‐wide. Ordinal cover data often need to be transformed to a quantitative form (0%–100%), especially when scrutinising summed cover of multiple species. Traditional approaches to transforming ordinal data often assume that data are symmetrically distributed. However, skewed abundance patterns are ubiquitous in plant community ecology. The questions this paper addresses are (a) how can we estimate transformation values for ordinal data that account for the underlying right‐skewed distribution of plant cover; (b) do different plant groups require different transformations; and (c) how do our transformations compare to other commonly used transformations within the context of exploring the aggregate properties of vegetation?LocationGlobal.MethodsWe assigned Braun‐Blanquet cover‐abundance ordinal values to continuous cover observations. We fitted a Bayesian hierarchical beta regression to estimate the predicted mean (PM) cover of each of six plant growth forms within six ordinal classes. We illustrate our method using a case study (2,809 plots containing 95,812 observations), compare the model‐derived estimates to other commonly used transformations and validate our model using an independent dataset (2,227 plots containing 51,497 observations) accessed through the VegBank database.ResultsOur model found that PM estimates differed by growth form and that previous methods overestimated cover, especially of smaller growth forms such as forbs and grasses. Our approach reduced the cumulative compounding of errors and was robust when validated against an independent dataset.ConclusionsBy accounting for the right‐skewed distribution of cover data, our alternate approach for estimating transformation values can be extended to other ordinal scales. A more robust approach to transforming floristic data and aggregating cover estimates can strengthen ecological analyses to support biodiversity conservation and management.
Highlights
Field-based assessment of the cover and abundance of individual plant species is complex
The Braun-Blanquet cover-abundance (BBCA) scale is perhaps the most common ordinal scale used in plant ecology
The method we describe is applicable to data in any ordinal scale, can be extended to allow for differences in vegetation type or among biomes and can accommodate alternative aggregate properties of plant data such as growth forms, life forms, functional or taxonomic groups
Summary
Field-based assessment of the cover and abundance of individual plant species is complex. Too, counting cryptic, clonal, or copious numbers of plants can be complicated. Owing to this complexity, vast numbers of floristic plots across many continents have been surveyed using ordinal scales (Schaminée et al 2009; Dengler et al 2011; Chytrý et al 2016). In Braun-Blanquet (1932) originally described an abundance-dominance scale, the practical, on-ground application of this scale is to assess plant cover, and where cover is less than 5%, abundance is assessed. The Braun-Blanquet cover-abundance (BBCA) scale is perhaps the most common ordinal scale used in plant ecology. Within the vegetation plot database sPlot v2.1
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