my understanding is that carbs/proteins/minerals are supplied from below(mostly from roots) when trees start leafing out in spring, and then at some point-- when the leaves have fully differentiated during growing season, the carbs and protein flow will be reversed. Then next spring, the net flow will be reversed anew
both will generate the callus. Even hard-to-root species will callus “downward” on their own as cuttings, relying on the scant supplies of energy/proteins/ minerals/moisture within the stems. The rootstock is definitely more than able(compared to the scion) to callus in spring considering its ample food storage/moisture within the main trunk, and especially supplied by the roots from previous year’s photosynthesis.
actually just posted proof of “upward” callusing of an internodal segment of a stem(where nodes shouldn’t be present), wherein the stump’s callus didn’t just produce xylem and phloem, but also upped the ante by differentiating into nodes that leafed out.