Why is easy. They are 'farming' the aphids which are feeding on your tree. The aphids secrete honeydew which the ants feed on. As to getting rid of the ants, then you need to find the nest and use a propriety ant killer and spray the tree to get rid of the aphids.
You could also try putting a sticky band round the trunk to stop the ants climbing up.

Or, if possible, treat the pear tree for the aphid or scale infestation it has, and the ants will just disappear on their own... but it depends how big the tree is as to whether you can spray it successfully or not.

How big is your pear tree? Mix 50/50 dish-washing soap with water and spray tree. The soap is mild enough so it won't harm the tree, but will kill the aphids which will get rid of the ants. After spraying your pear tree, spray the cherry tree. It will work on both trees.

If you still want to get rid of the ants, mix 50/50 borax with sugar, sprinkle around ant next. This may take a 2nd application.