Why should this have anything to do with wind. It would seem to me that he conservation of cross sectional area is because all the capillaries need to go from root to leaf.

If the area above a branching point was more than below then how would those capillaries going up to the leaves be fed?

If the area above a branching point were less than blow then how would the capillaries going down to the roots be fed?

Remember the roots fees the leaves and the leaves feed the roots. So it needs to be a balanced system.