A combinatorial proof of Postnikov's identity and a generalized   enumeration of labeled trees
  In this paper, we give a simple combinatorial explanation of a formula of A. Postnikov relating bicolored rooted trees to bicolored binary trees. We also present generalized formulas for the number of labeled k-ary trees, rooted labeled trees, and labeled plane trees. 
Seunghyun Seo (Brandeis University)