  We study group actions on multitrees, which are directed graphs in which
there is at most one directed path between any two vertices. In our main result
we describe a six-term exact sequence in $K$-theory for the reduced crossed
product $C_0(\partial E)\rtimes_r G$ induced from the action of a countable
discrete group $G$ on a row-finite, finitely-aligned multitree $E$ with no
sources. We provide formulas for the $K$-theory of $C_0(\partial E) \rtimes_r
G$ in the case where $G$ acts freely on $E$, and in the case where all vertex
stabilisers are infinite cyclic. We study the action $G\curvearrowright
\partial E$ in a range of settings, and describe minimality, local
contractivity, topological freeness, and amenability in terms of properties of
the underlying data. In an application of our main theorem, we describe a
six-term exact sequence in $K$-theory for the crossed product induced from a
group acting on the boundary of an undirected tree.
