\begin{forest}
    [{$S(y, z)$}, parent anchor=-50, anchor=-50
      [{$R(z, a, b)$}, edge=very thick,
          child anchor=120, anchor=120, parent anchor=-40
        [{$T(b, c)$}, child anchor=90, anchor=-35,
        before computing xy={s=(70)}]
        [{$U(b, d, e)$}, edge=very thick, child anchor=120, parent anchor=-35,
          anchor=-35
          [{$V(e, f)$}, edge=very thick ]
        ]
      ]
    ]
  \end{forest}