Trees are quite passive in our world. I am not sure it will still look like trees with active wings.
Let's make them plane.
How could a tree plane ?
Longs boughs with large leaves. Some plants of our world are moving (carnivor plants, mimosa pudica...), your trees could have a slight ability to move leaves and boughs. Like sunflowers, they could turn in front of the sun and grow leaves which form will give the tree a global direction to move. Or it could move withe the wind (a seasonal wind blowing in the same direction several weeks/months for example)
In order to stay in the air, it could use the sun to heat the air under big balloons-like leaves. 731
Trees would look more like a shoal of jellyfish than trees, though.
You could imagine a symbiosis with a micro organism rejecting a light gas that the tree could use to swell its leaves.
How could the tree survive in the air ?
The major issue is that the tree needs nutrients. I think it can find some water in the air. If it uses the sun or another way to warm air up to float in the air, there should be mistiness.
Either the tree floats very low and collects some nutrients from roots lying on the ground (or plankton on the top of the see ?), either the flying part of its life is like winter sleep : the tree is just floating around with minimal activity and does not reproduce, grows flowers nor fruits. It could be an early stage of development of the tree, from a little floating seed to a bunch of balloons-leaves until it reaches the ground and start growing properly (but I feel like a seed needs more nutrients than what the air can give)
Where did evolution find such an idea ?
In a bad ground trees could not survive for long, so they began to move a little, with their roots growing on a side while roots on the other side where dying, making the whole tree move.
Then, nature found that it was far more easy to make a tree move if it was lighter, and leaves began to carry a part of the weight and the trunk became lighter.
Then, flying trees began to slowly walk and float while roots became thiner to absorb nutrients then die and grow elsewhere. Either they are still doing this, either they began to grow big trunks only when finding a good place. 344