Actually it’s just the opposite , when temps are below freezing gases in the tree contract creating a vacuum and sap rises from the roots into the tree . When temps are above freezing gases expand creating pressure inside the tree and then sap will flow out the taphole . Also think of the tree trunk like a straw . When sap runs down the tree during warm spells the lower you tap the tree ( put a hole in a straw ) the more sap you will get . Decent start so far here in Mid Michigan, 3000 gallons collected and 2nd boil tomorrow. Need to get rid of this frost tho . As far as a RO goes , definitely worth every penny . Even super small guys who have difficulty boiling before sap spoils would benefit and make better syrup . Run the ro and freeze the concentrate for later when you have time .
You are of course correct, xylem up and phloem down are reversed in spring but thought that technical for my tongue in cheek response