See the One-Tonne Carbon Tree
Ecometrica, a consultancy who undertake Green House Gas accounting, have produced a useful illustrated guide showing which parts of the tree store carbon and in what proportion, based on a tree which stores a tonne of carbon (‘a One-Tonne Carbon Tree’). Ecometrica say the leaves of the tree store 1% of the carbon, the branches 11%, the stem 62%, and the roots 26%. Yes, that really does add up to 100%! Of course, while a tree may store carbon, it's not in itself, a permanent store.
Ecometrica have produced a 'comic' and a PDF document of their One Tonne Carbon Tree. Visit the page on the Ecometrica website: http://www.ecometrica.co.uk/ecometrica-press/webcomics/one-tonne-carbon-tree/