A new distance metric for rooted trees is presented which is based on the largest common forest of two rooted trees. The new measure is superior to previous measures based on tree edit distance, because no particular tree edit operations together with their costs or weights need to be defined. The metric can be computed in expected time linear in the number of nodes, on rooted trees of unbounded degree, either unordered or ordered, labeled or unlabeled. An algorithm for computing the metric is given which is based on a simple and efficient bottom-up algorithm for finding all common rooted subtrees in a forest.