How to read sub-bands of level 0 and level1 of wptree created using wpdec2?

I have applied wavdec2 two-dimensional wavelet packet analysis function to an image ([t,d] = wpdec2(X,3,'db1'); ). I want to calculate mean of sub-bands at each level. I am able to calculate mean of sub-bands at last level(i.e. level 2 terminal nodes). How to get sub-bands of level 0 and level l so that I can calcuate mean of sub-bands at level 0 and level 1.