  The Faber-Krahn theorem states that among all bounded domains with the same
volume in ${\mathbb R}^n$ (with the standard Euclidean metric), a ball that has
lowest first Dirichlet eigenvalue. Recently it has been shown that a similar
result holds for (semi-)regular trees. In this article we show that such a
theorem also hold for other classes of (not necessarily non-regular) trees.
However, for these new results no couterparts in the world of the
Laplace-Beltrami-operator on manifolds are known.
