Plants parts such as leaves, bark, needles, branches, sterms and twigs, are all active part of the plant the feed on the plant food to grow and develop however, excess food are mainly stored in the root regions especially for repairs, maintenance, flowering and fruiting. Just as most humans store excess as fats around the belly.  

Hi Riccardo,  Plants bearing green embryos (chlorophyll bearing embryos)  do  synthesyse food materials and accumulate in their  cotyledons  independent of their roots  except for minerals and water (Plant Physiology,1990. 92: 408-412). 

in my experiences with potassium ions, major osmotic cation in plants, it is need to study the circulation capacity, but the distribution between roots and leaves is temporarily ( www.iaea.org/inis/collection/.../16/.../16071036.pdf) ;

This is bipolar function of growth hormones : auxins -gibberelins

And for fruits growth & others see www.cliffsnotes.com › Sciences › Plant Biology

Also, keep in mind that root systems are 3 to 4 times larger than the tree to hold nutrients that are needed for transport through the trunk, limbs, and leaves. Just visualize the size of the tree underground that extends 3-4 times the circumference of the drip line of the aboveground tree.

Lignotuber is an important structure present in some trees and accumulates starch and other polychaccarides. In my master´s thesis I proved that polysaccharides accumulated in this structure are used for regrowth after fire. The lignotuber is essential for the survival of individuals after a strong impact as fire or drought

Ricardo is right  that shoots  are  more mportant in terms of reserves for their position and size relative to roots in majority of  plants.  Few Exceptions are from from family: Convolvulaceae and Diascoreaceae .  Tree trunk of   certain palms (Corypha ) do reserve a  huge  qty of starch  to the entire trunk before flowering .

Thanks everyone. Indeed, I was referring to fruit trees mostly, and although they are not necessarily tall trees, root systems never reach the size and volume of above-ground woody frame. I am sure there are exceptions, first among all vines (like grapevines), but in the majority of fruit trees I doubt roots may be "by far" the main contributor to food reserves. Besides, the distance between roots and actively growing sinks like young fruits and leaves makes me doubt on the real importance of carbohydrates coming from down below, especially at bloom for deciduous species. I think it is a matter of space and time, in addition to size.

Riccardo, as you probably know the major resources stored in deciduous trees are mostly C and N.  Both are stored in above and below ground structures.  N can be stored as amino acids, but vegetative storage proteins account for the majority of N storage reserves.  C is more complex in that is usually accumulates during the summer as starch which is hydrolyzed during early fall and winter to soluble sugars.  This is probably a temperature mediated response.  I above ground tissue most N is stored in phloem parenchyma and xylem ray cells.   N is also stored in roots but is less studied.  The utilization of this N is dependent on sink development during regrowth (vegetative or reproductive).  A number of studies using 15N demonstrate the importance of these reserves during spring growth.  For example, MIllard has shown that in poplar that new growth relied on stored N for up to 40 days in the spring.  Since N remobilization is sink regulated I suspect that root N reserves are likely to be partitioned to root sinks while above ground N reserves are partitioned to shoot growth. 

In most tree crops, majority of the reserves are accumulated in the stems and roots. Leaves and twigs age and fall off or are shed seasonally and are as such not good storage organs. They are also very active in respiration. Dry matter analysis shows that most readily available reserves (starch) are stored in stems and roots of tree crops.

Dear Gary, thanks for your input. Indeed, this is what I was trying to say. There is C and N stored in trunk and limbs, as well as in roots; yet, sink activity, size and proximity should regulate reserve movement; as such, I think above ground structures are the main contributors to shoot and fruit growth, while woody roots sustain growth of fine roots. As a result, writing that most of tree reserves are located in the root system, to me, is incorrect and should be avoided in modern hort books.