\begin{tikzpicture}[
  sibling distance=32pt,
  -latex,
  apnode/.style = {
    draw=black,
    dashed,
  },
  ap/.style = {
    draw=black,
    dashed,
  },
]
  \Tree [
    .$r\gets\hash(h_{ab}\concat h_{cd})$ [
      .\node[apnode]{$h_{ab}\gets\hash(h_a\concat h_b)$}; [
        .$h_a\gets\hash(a)$
      ] [
        .$h_b\gets\hash(b)$
      ]
    ] \edge[ap]; [
      .$h_{cd}\gets\hash(h_c\concat h_d)$ [
        .\node[apnode]{$h_c\gets\hash(c)$};
      ] \edge[ap]; [
        .$h_d\gets\hash(d)$
      ]
    ]
  ]   
\end{tikzpicture}