It can be said that a tree is sacred when it has a special religious significance for a place. Some communities have one species as sacred, other communities have another species.
There are sacred trees like Thor's Oak (Germanic), or the apple tree in ancient Greece.
By worshiping a tree, myths and legends about its characteristics begin to be created. Currently there are very few trees that are considered sacred and that their felling is prohibited.
In Greece the poplar was related to the dead, Heracles when he went down to Hell, he wore a crown made from the leaves of that tree. On The odyssey it is a symbol of the resurrection. On the other hand, the black poplar was dedicated to the goddess of death, Hecate (a nationalized goddess in Mycenaean Greece, related to sorcery and "Queen of Ghosts").
The cypress was sacred since ancient times, in Greece was the tree of death and later it was dedicated to Saturn, later to Pluto.
The Naiad Thyia (one of the naiads from a spring on Mount Parnassus) was the mother of cypress trees. The doors of the Greek and Roman temples were made of cypress wood, like the doors of Saint Peter in the Vatican.
The holm oak was a sacred tree throughout the Mediterranean, it is said that Zeus sat meditating under one of them.
The ash tree was revered as a symbol of divine justice.
Daphne was turned into a laurel to escape from Apollo and when he found out he declared it a sacred tree.
The apple tree was consecrated to Aphrodite.
The myrtle (perfume) and the rose were associated with Aphrodite.
And the olive tree accompanied Athens and its manifestations from its origins.