[R-sig-eco] Question on block effect significance in 'adonis' PERMANOVA

[Alexandre Fadigas de Souza]

Dear friends,

   In performing a PERMANOVA in VEGAN's 'adonis' function, we can use strata
to indicate a blocking variable. Contrary to standard statistical packages,
however, adonis' output does not include the block variable explicitly among
its Sources of Variation.

   Sometimes, as is my present situation, the blocking variable is
interpretable and we want to know whether it varied significantly.

   I could not find this discussion in the log files of this list.

   Our response variable is the abundances of tree species in 18 square plots
in southern Brazil, our explanatory variables are distance to a power dam
(near x far, binary) and altitude. Our strata/blocking variable is hill, since
treatments near or far from the power dam were replicated on the slopes of
three different hills.

   It would be important to know if there were compositional differences
between hills but this result does not appear in the output. We tried to force
its appearance using the following formula, but I do not know if this distorts
the analysis:

> adonis (species ~ hill + dam + altitude + declivity, method=”Chao”, strata 
= environment$hill, perm=4999)

  Thanks in advance for your attention,

  Alexandre