There is not only a Wikipedia article on graph isomorphism (as Space_C0wb0y points out) but also a dedicated article on the graph isomorphism problem. It has a section Solved special cases for which polynomial-time solutions are known. Trees is one of them and it cites the following two references:
- P.J. Kelly, “A congruence theorem for trees” Pacific J. Math., 7 (1957) pp. 961–968
- Aho, Alfred V.; Hopcroft, John; Ullman, Jeffrey D. (1974), The Design and Analysis of Computer Algorithms, Reading, MA: Addison–Wesley .