Think outside the box!
Split the trees into 3 groups and do the following in each group:
Tie 7 camels to one tree.
Tie 6 camels to each of the other two trees.
Tie 1 camel to both of these two trees.
These two trees each have 7 camels attached to them, but 1 camel is shared between the trees by being tied to each.
So in each group we have 13+7=20 camels. In 3 groups we have 60 camels.
Therefore, we have tied 60 camels to 9 trees.

The picture shows schematically the arrangement in just one of the three groups. The points represent camels and the circles represent trees.