We have a black cherry (Prunus serotina) growing wild in the fencerow. One noteworthy feature of the black cherry is its extrafloral nectaries.
Ant visiting black cherry extrafloral nectaries
Black cherry has nectar producing glands on the leaf stems.
These "sweet-treat spots" attract ants. The ants patrol the leaves protecting their food source by acting aggressively toward caterpillars or insects they encounter. The extrafloral nectaries are one of the ways the tree gets extra protection from herbivores.