Conny-- Note that Jari's surface fitting is using ordination scores on the right-hand predictor size of the formula, with some z as the response.
If you need something about species composition as your _response_ variable in a linear model (e.g., with time, disturbance type, and treatment as predictors, and perhaps site as a random effect), why not use each stand's dissimilarity/distance from your reference forest sites? The trend line would be compositional distance or dissim v. time, with color/symbols/whatever for different treatments. That would have the advantage of being easily & directly interpretable. [The use-case where that would fail is >>100% turnover so lots of 0 similarities to the reference forests, so step-across or nmds might help put those large distances in order.] You might be able to set up the equivalent to your GLM in adonis to get permutation significance tests. I hope that this helps, or at least gives you a different way to think about your problem, or else is so stupid that Jari gets annoyed and blasts it with a valid solution. Tom 2 ------ Tom Philippi Quantitative Ecologist & Data Therapist National Park Service On Sun, Jan 17, 2016 at 8:04 PM, Conny <constanze_k...@hotmail.com> wrote: > Thanks a lot for all the helpful responses and info. > > But I’m actually still not sure how to use both NMDS axes as a response > (y) in a regression model - is this even possible?? > > My overall goal is to model species compositional change over time in a > restoration project (is the system getting more similar to the reference > forest). I would like to create a trend line here in a graph, rather than > just using an ordination plot. > > I thought about using the fitted values returned by ordisurf(), but as I > understood it (please correct me if I’m wrong) it will use my restoration > time again as a response and my axes scores as predictors. > > So the z values will represent fitted age values rather than my sample > scores (?) – so it would make no sense to plot it against my restoration > time… > > I’m sorry if this is getting a bit confusing. > > Cheers, > Conny > > _______________________________________________ > R-sig-ecology mailing list > R-sig-ecology@r-project.org > https://stat.ethz.ch/mailman/listinfo/r-sig-ecology > [[alternative HTML version deleted]] _______________________________________________ R-sig-ecology mailing list R-sig-ecology@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology