Convergence problem

Hi folks, 

I'm new to nlme and I'm trying to fit a somewhat simple mixed regression model:

PICE.MAR_tree_dbh_model <- lme(log(dbh_tree) ~ Temp+Prec+I(Prec/age_tree)+I(Temp/age_tree), weights = varPower(form =~fitted(.)) , random= ~1|plot_name, data=PICE.MAR_tree, method="REML")


However, convergence is never reached:

  (Error in lme.formula(log(dbh_tree) ~ Temp + Prec + I(Prec/age_tree) +  : 
  maximum number of iterations (lmeControl(maxIter)) reached without convergence)

 Here's the structure of my data. dbh_tree and age_tree is measured on the tree which is nested in plot_name (random effect). Temp and Prec are measured at the plot level. I have also a very unbalanced setup with sometimes only 1 tree per plot and others where there are as much as 150 trees. Maybe the syntax is not right.

Thank you!

Yan Boulanger

Yan Boulanger, Postdoctoral Visiting Fellow
Ressources Naturelles Canada, Canadian Forest Service
Centre de Foresterie des Laurentides
1055, rue du P.E.P.S.
C.P. 10380, succ. Sainte-Foy
Qu?bec (Qu?bec) Canada
G1V 4C7 
Tel. : +1 418 649-6859