I have a general question about using thin plate splines in the surf and ordisurf routines. My rudimentary knowledge of a gam is that with each predictive variable there is a different smooth for each one and then they are added together with no real interaction term (because they don't handle this well?). Now, If I have two variables that have a high D^2 score and a low GCV score (I am thinking of this as a goodness of fit which may be wrong) I would like to add these together to produce one surface fitted to the ordination. Is this possible? Am I going down the right track? Any help is greatly appreciated.
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