plot(fm1) also works with the lme function. If you calculate R^2 as
keep in mind that it is in fact only a pseudo R^2 as it does not account for the random structure. Alternatively you can calculate marginal and conditional R^2 as described here:
This is the publication that describes it in detail
Nakagawa, S., and H. Schielzeth. 2013. A general and simple method for obtaining R2 from generalized linear mixed-effects models. Methods in Ecology and Evolution 4(2): 133-142.
There is also a package called lmmfit although I haven't used it, yet