  In this paper, we characterize a mixed graph $F$ which contains $k$ edge and
arc disjoint spanning mixed arborescences $F_{1}, \ldots, F_{k}$, such that for
each $v \in V(F)$, the cardinality of $\{i \in [k]: v \text{ is the root of }
F_{i}\}$ lies in some prescribed interval. This generalizes both Nash-Williams
and Tutte's theorem on spanning tree packing for undirected graphs and the
previous characterization on digraphs which was given by Cai [in: Arc-disjoint
arborescences of digraphs, J. Graph Theory 7(2) (1983), 235-240] and Frank [in:
On disjoint trees and arborescences, Algebraic Methods in Graph Theory,
Colloquia Mathematica Soc. J. Bolyai, Vol. 25 (North-Holland, Amsterdam)
(1978), 159-169].
