  Self-similarity of random trees is related to the operation of pruning.
Pruning $R$ cuts the leaves and their parental edges and removes the resulting
chains of degree-two nodes from a finite tree. A Horton-Strahler order of a
vertex $v$ and its parental edge is defined as the minimal number of prunings
necessary to eliminate the subtree rooted at $v$. A branch is a group of
neighboring vertices and edges of the same order. The Horton numbers $N_k[K]$
and $N_{ij}[K]$ are defined as the expected number of branches of order $k$,
and the expected number of order-$i$ branches that merged order-$j$ branches,
$j>i$, respectively, in a finite tree of order $K$. The Tokunaga coefficients
are defined as $T_{ij}[K]=N_{ij}[K]/N_j[K]$. The pruning decreases the orders
of tree vertices by unity. A rooted full binary tree is said to be
mean-self-similar if its Tokunaga coefficients are invariant with respect to
pruning: $T_k:=T_{i,i+k}[K]$. We show that for self-similar trees, the
condition $\limsup(T_k)^{1/k}<\infty$ is necessary and sufficient for the
existence of the strong Horton law: $N_k[K]/N_1[K] \rightarrow R^{1-k}$, as $K
\rightarrow \infty$ for some $R>0$ and every $k\geq 1$. This work is a step
toward providing rigorous foundations for the Horton law that, being
omnipresent in natural branching systems, has escaped so far a formal
explanation.
