COA and absolute frequencies

From: Agustin Lobo (
Date: Mon Nov 12 2001 - 23:18:05 MET

Bonjour there!

I've run a COA using the R function
CAIV (as the algorithm is the same than the one in ADE,
I think that this list is better suited for this
question than R-help).

My dataset consisits on forest inventory data
(area covered x sps), including trees and shrubs.

A simple COA using this data matrix has the problem that some
sites with a low tree cover appear cery close to sites with high tree
cover in the biplot,
because in both cases they are dominated by the same species (P.
I understand this comes from the fact that
COA standardises the data, and relative frequencies are more important
than absolute frequencies.

Therefore, I have selected the site with the highest vegetation cover and
an artificial species "bare ground", which for this site has a cover of 0
and for the rest of sites has a cover equal to the difference between the
vegetation cover and the observed maximum. Using this new "species", sites
low cover but dominated by P.halepensis no longer appear close to sites
that are also dominated by P.h. but with a high tree cover.

I'd appreciate your critical opinions and alternative suggestions.


Dr. Agustin Lobo
Instituto de Ciencias de la Tierra (CSIC)
Lluis Sole Sabaris s/n
08028 Barcelona SPAIN
tel 34 93409 5410
fax 34 93411 0012

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