CIRCULATION is a tree's most important system, see the Diagrams
Bottom line: Circulation is everything to a tree.
But it can get blocked...
These diagrams are meant to be an educational tool for homeowners and tree-care professionals alike. Please use it to understand the finer point of tree functions. This shows how trees live--or die without unblocked circulation.
if you need to see these diagrams in a bigger view, you should be able to drag them onto your desktop and then print.
The second diagram shows the healthy "cascade" of circulation and its benefits.