[R] lme and varFunc()

[Christoph Scherber]

Dear R users,

I am currently analyzing a dataset using lme(). The model I use has the 
following structure:

model<-lme(response~Covariate+TreatmentA+TreatmentB,random=~1|Block/Plot,method="ML")

When I plot the residuals against the fitted values, I see a clear 
positive trend (meaning that the variance increases with the mean).

I tried to solve this issue using weights=varPower(), but it doesn´t 
change the residual plot at all.

How would you implement such a positive trend in the variance? I´ve 
tried glmmPQL (which works great with poisson errors), but using glmmPQL 
I can´t do model simplification.

Many thanks for your help!

Regards
Chris.