Transformation of cover-abundance values for appropriate numerical treatment - Alternatives to the proposals by Podani
Article first published online: 24 FEB 2009
2007 IAVS - the International Association of Vegetation Science
Journal of Vegetation Science
Volume 18, Issue 5, pages 767–770, October 2007
How to Cite
van der Maarel, E. (2007), Transformation of cover-abundance values for appropriate numerical treatment - Alternatives to the proposals by Podani. Journal of Vegetation Science, 18: 767–770. doi: 10.1111/j.1654-1103.2007.tb02592.x
- Issue published online: 24 FEB 2009
- Article first published online: 24 FEB 2009
- Received 15 January 2007; Accepted 13 May 2007
- Geometrical series;
- Logarithmic scale;
- Metric scale;
Abstract. Two alternatives are offered to Podani's proposals, based on the claim that Braun-Blanquet cover-abundance estimates cannot be properly analysed by conventional mul-tivariate methods.
1. The ordinal transform scale, based on an extended Braun-Blanquet cover-abundance scale, comes close to a metric cover percentage scale after (1) the abundance values r (very few individuals), + (few ind.), 1 (abundant) and 2m (very abundant, cover < 5%) are replaced by cover percentage estimates and (2) the higher Braun-Blanquet values, notably 4 and 5, with cover intervals 50-75% and 75-100%, respectively, are interpreted as estimates of considerably higher cover values than the usual visual projection on the ground (because of the position of stems and leaves in several layers). I propose the equation ln C= (OTV −2) /a, where C= Cover%, OTV is the 1 to 9 Ordinal Transfer Value and a is a factor weighting the cover values. With this equation cover values in a geometric series are achieved for the nine values in the extended Braun-Blanquet scale from 0.5 % (OTV 1) to 140% (OTV 9) for a= 1.415, and for a= 1.380 from 0.6 % to 160%.
2. This makes use of an earlier developed ‘optimum-transformation’ of cover-abundance values. For each species a frequency distribution of cover-abundance values is determined for a large data set, i.e. of dune slack vegetation. Tiny species have low values (OTVs 1–3) with high frequencies and hardly occur with higher OTV values; here all scores are considered ‘optimal’. In dominant species OTVs 7 to 9 have the highest frequencies and only these values are considered optimal. Species with intermediate OTV ranges have optimum ranges with low-bound OTV = 2, 3, 4 and 5, respectively. No species were found in the dune slack data set with a frequency distribution justifying an optimum range with low-bound OTV = 6.
For mathematically correct numerical treatments’ optimum scores’ can be converted to 1 and sub-optimal scores to 0 in order to approach a presence/absence situation.
Both alternatives are suggested to be acceptable approximations to a metric basis for numerical analyses.