Hi every body.
I have some questions about Mantel Test. I expect that with experience in
spatial analysis of people in this list I can resolve them. Your
explanations and suggestions will help me so much, so, thanks in advance.
For contextualizing. I want to explore the association between some
environmental variables: precipitation, temperature, bio-climatic
indicators (construction of "bio variables" proposed in WorlClim), EVI
(Enhanced Vegetation Index from MODIS-NDVI), NPP (Net Primary Product from
MODIS), and VCF (Vegetation Continues Fields for trees from MODIS).
*1. *At first instance, I thought to calculate Pearson´s correlation
coefficient from Mantel Test approach, however, I have some doubts about
what I am actually doing with this.
I think that I can obtain a more accurate correlation coefficient with out
the problem of spatial auto-correlation, due to permutation implicit in
Mantel Test, using as matrices for calculate it two grids of the set of
environmental variables.
My real questions comes from the definition of matrices used in Mantel
Test, because they are proposed from literature to represent a relationship
between points included in them (often distances), but the grids do not
represent this, they are the raw values of environmental variables. In
fact, Legendre recommends that Mantel tests should not be used to test
hypotheses about the relationships of the original data tables.
So am I correct in try to perform a correlation coefficient with out the
problem of spatial auto-correlation between any two grids using the Mantel
Test?
*2. *An other option that I have thought for involving distances is to
perform the Mantel Test using a distance matrix between the two grids (this
distance measure could be the correlation coefficient) and the geographic
distance between grid points (Euclidean distance), since this is a way to
explore if a variable is spatially autocorrelated, even this is less used
than Moran Index or Correlogram.
However, I am not sure if with this option I would test the spatial
correlation of the distance measure used, or I would obtain a correlation
index of the association of the two grids (distance measure used) with out
the problem of spatial auto-correlation due to permutation. Or both?
*3.* How else can you recommend me to measure the relationship between two
environmental grids trying to minimize the effect of spatial
auto-correlation?
Regards to all and thanks in advance for your help.
--
Jaime Burbano Girón
Maestría en Ciencias Biológicas - Ecología
Universidad Nacional de Colombia
Bogotá - Colombia
(57) 3168573713
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