The expressions "on the tree" and "in the tree" both refer to things or actions related to a tree, but they are used in different contexts based on the location or movement relative to the tree.
On the tree
Refers to something that is physically resting on the surface or outside of the tree, typically on branches or leaves.
There are apples on the tree.
Moss has grown on the tree trunk.
There’s a swing hanging on the tree.
The bird is sitting on the tree branch.
She hung the decorations on the tree.
There are beautiful flowers blooming on the tree.
There are ripe oranges on the tree waiting to be picked.
In the tree
Refers to something that is inside or within the structure of the tree, or within the area surrounded by branches and leaves.
There is a nest in the tree.
The squirrel is hiding in the tree.
The kids built a treehouse in the tree.
There’s a family of owls living in the tree.
He climbed in the tree to reach the highest branch.
Usage Points
While both phrases can sometimes be used interchangeably depending on context, they generally convey different spatial relationships to the tree:
The birds are singing in the tree.
It implies that the birds are among the branches or leaves of the tree, often implying a sense of being surrounded or partially hidden.
The birds are singing on the tree.
It implies that the birds are on the branches, you can see them sitting on the outside of the tree.
There are a lot of apples in the tree.
It implies that apples cannot be seen due to foliage.
There are a lot of apples on the tree.
It implies that apples can be seen.
The apple grows on the tree.
Apples grow attached to the surface of the tree, typically on the branches. You can't say that an apple grows in the tree, because the apple itself is not inside the tree's structure.