I don't know if this applies to apples as the study was done on pine, but
for what its worth, I have read that small pine trees get their moisture in
winter mostly via their roots and even extract moisture from unfrozen soil
pockets if available.
Larger pine trees get most of their moisture stored in their sapwood.
Still puzzling to me though is how the water is transported through the
xylem if its 0 degrees outside. I believe it happens during warm spells.
Most water is unavailable from the soil by the way, in our area from
December through March. The hardest time for trees to obtain water in winter
is late winter when the sun is warming up the upper tree and the soil is
still frozen.
This is another reason that snow cover is great, not only to protect the
roots from damage but for water supply.