\begin{tikzpicture}[
  -latex,
]
  \Tree [
    .$\hash(h_{01}\concat h_{23})$ [
      .$h_{01}\coloneqq\hash(h_0\concat h_1)$ [
        .$h_0\coloneqq\hash(\mathsf{foo})$
      ] [
        .$h_1\coloneqq\hash(\mathsf{bar})$
      ]
    ] [
      .$h_{23}\coloneqq\hash(h_2\concat h_3)$ [
        .$h_2\coloneqq\hash(\mathsf{baz})$
      ] [
        .$h_3\coloneqq\hash(\mathsf{qux})$
      ]
    ]
  ]   
\end{tikzpicture}