The maximum depth metric, measures the maximal path (in edges), between the tips and the root.
max_depth(phy, normalization = "none")
Maximum depth (in number of edges)
phylo object or ltable
"none" or "tips", in which case the resulting statistic is divided by the number of tips in the tree.
C. Colijn and J. Gardy. Phylogenetic tree shapes resolve disease transmission patterns. Evolution, Medicine, and Public Health, 2014(1):96-108, 2014. ISSN 2050-6201. doi: 10.1093/emph/eou018.