The root-class residuality of tree products with central amalgamated subgroups

Given a root class ${\Cal C}$ of groups and a tree product $P$ in which each amalgamated subgroup lies in the centers of the corresponding vertex groups, we point out certain sufficient conditions for the ${\Cal C}$-residuality of $P$. In particular, we show that the tree product of solvable groups of bounded derived length with central amalgamated subgroups is residually solvable.