At night there is little sap flow. 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.