sap rise in spring

[SCHMID at Butler.EDU]

What's the current theory on the rise of sap in trees before they
leaf out and initiate tension/cohesion/adhesion?  Is "root pressure" 
resulting from release of sugar by stored starch considered enough 
to account for it? I've also heard theories about pressure from
carbon dioxide.  This question is about due to pop up in class 
again....

Kathy Schmid
schmid at butler.edu