  For a digraph $D=(V(D), A(D))$, and a set $S\subseteq V(D)$ with $r\in S$ and
$|S|\geq 2$, an $(S, r)$-tree is an out-tree $T$ rooted at $r$ with $S\subseteq
V(T)$. Two $(S, r)$-trees $T_1$ and $T_2$ are said to be arc-disjoint if
$A(T_1)\cap A(T_2)=\emptyset$. Two arc-disjoint $(S, r)$-trees $T_1$ and $T_2$
are said to be internally disjoint if $V(T_1)\cap V(T_2)=S$. Let
$\kappa_{S,r}(D)$ and $\lambda_{S,r}(D)$ be the maximum number of internally
disjoint and arc-disjoint $(S, r)$-trees in $D$, respectively. The generalized
$k$-vertex-strong connectivity of $D$ is defined as $$\kappa_k(D)= \min
\{\kappa_{S,r}(D)\mid S\subset V(D), |S|=k, r\in S\}.$$ Similarly, the
generalized $k$-arc-strong connectivity of $D$ is defined as $$\lambda_k(D)=
\min \{\lambda_{S,r}(D)\mid S\subset V(D), |S|=k, r\in S\}.$$ The generalized
$k$-vertex-strong connectivity and generalized $k$-arc-strong connectivity are
also called directed tree connectivity which could be seen as a generalization
of classical connectivity of digraphs.
  A digraph $D=(V(D), A(D))$ is called minimally generalized $(k, \ell)$-vertex
(respectively, arc)-strongly connected if $\kappa_k(D)\geq \ell$ (respectively,
$\lambda_k(D)\geq \ell$) but for any arc $e\in A(D)$, $\kappa_k(D-e)\leq
\ell-1$ (respectively, $\lambda_k(D-e)\leq \ell-1$). In this paper, we study
the minimally generalized $(k, \ell)$-vertex (respectively, arc)-strongly
connected digraphs. We compute the minimum and maximum sizes of these digraphs,
and give characterizations of such digraphs for some pairs of $k$ and $\ell$.
