hierarchical unbalanced design

Hi all!
 
I have a mixed model with timeseries of y and x for populations:
lme(y~x,random=1|Population)
I am wondering if I can make it hierarchical by adding a random effect for year:
lme(y~x,random=1|Population/Year)
However the design is unbalanced: the time-series have different lengths for each population although the time-periods are overlapping in general. 
 
Would it be meaningful to include the Year effect this way? 
 
Thank you!
 
Irene