lmeGRASS2=lme(GRASS~RODENT*FIRE, weights=varIdent(form=~1|YEAR),
         random=list(~BLOCK-1, ~YEAR-1), data=density)

Random effects:
 Formula: ~1 | YEAR
        (Intercept)
StdDev:    159.8153

 Formula: ~1 | BLOCK %in% YEAR
        (Intercept) Residual
StdDev:    58.42149 79.81666