Sharp Vaught's conjecture for some classes of partial orders

Matatyahu Rubin has shown that a sharp version of Vaught's conjecture, $I(T,\omega )\in \{0,1,c\}$, holds for each complete theory of linear order $T$. We show that the same is true for each complete theory of partial order having a model in the minimal class of partial orders containing the class of linear orders and which is closed under finite products and finite disjoint unions. The same holds for the extension of the class of rooted trees admitting a finite monomorphic decomposition, obtained in the same way. The sharp version of Vaught's conjecture also holds for the theories of trees which are infinite disjoint unions of linear orders.