Assouad-Nagata dimension of tree-graded spaces

Given a metric space $X$ of finite asymptotic dimension, we consider a quasi-isometric invariant of the space called dimension function. The space is said to have asymptotic Assouad-Nagata dimension less or equal $n$ if there is a linear dimension function in this dimension. We prove that if $X$ is a tree-graded space (as introduced by C. Drutu and M. Sapir) and for some positive integer $n$ a function $f$ serves as an $n$-dimensional dimension function for all pieces of $X$, then the function $300\cdot f$ serves as an $n$-dimensional dimension function for $X$. As a corollary we find a formula for the asymptotic Assouad-Nagata dimension of the free product of finitely generated infinite groups: $asdim_{AN} (G*H)= max\{asdim_{AN} (G), asdim_{AN} (H)\}.$