The people of the Most Ancient Church had begun wanting to lead themselves and think from themselves instead of from the Lord. The Lord knew it would be their downfall, and sought to fulfill them through the spiritual gifts they already had in their celestial state.

Here we see the Lord showing them the presence in themselves (the ground) of all the beautiful things of their state: affections springing from love to the Lord (the beasts of the field), thoughts inspired by mutual love (fowl of the air) and all other spiritual activity filled with life from the Lord (living creatures). And theses things were gifts; the fact that the people were invited to name the animals means they were shown the nature of all those wonderful things.

(références: Arcana Coelestia 142, 143, 144, 145)

In general, trees represent the deepest and most significant intellectual concepts: the ones that come to us most directly from the Lord. This varies depending on us and our states: the people of the Most Ancient Church, who were in a state of loving the Lord, understood truth automatically and internally through what the Writings call "perception"; people in lower states (including most of us) have to work a little harder to learn it from the Word and by willing to follow the Lord. In other parts of the Bible, especially in the prophets and New Testament parables, the meaning of "tree" is broader, meaning not just a person's intellectual concepts but the whole person.

'Trees,' as in Joel 1:10-12, signify knowledges.