As the day warms, sap flow begins. Environmental conditions can affect the yield of sugar. Trees grown with adequate moisture and fertilizer produce higher yields than trees in infertile soil and dry conditions. Sap yield is also lowered if leaves are defoliated in the previous season. The cause of maple sap flow is complex and not completely understood.
Plants can often generate sizable root pressures that can play a role in water movement.
The root pressure then increases the stem pressure which results in sap flow. The crucial factor is apparently related to the age-old observation that sap flow requires warm days and cool nights. Stems must experience a freeze-thaw cycle for sap flow. When pieces of maple stems are given a source of water and then placed in a freeze-thaw cycle, they exhibit sap flow. As the temperature cools, gases in the xylem dissolve and the pressure decreases.