We consider a set of measures on the real line and the corresponding system of multiple orthogonal polynomials (MOPs) of the first and second type. Under some very mild assumptions, which are satisfied by Angelesco systems, we define self-adjoint Jacobi matrices on certain rooted trees. We express their Green's functions and the matrix elements in terms of MOPs. This provides a generalization of the well-known connection between the theory of polynomials orthogonal on the real line and Jacobi matrices on Z + to higher dimension. We illustrate importance of this connection by proving ratio asymptotics for MOPs using methods of operator theory. Contents arXiv:1806.10531v1 [math.CA] 27 Jun 2018where Y (p) has d children each corresponding to the index i ∈ {1, . . . , d}.First, we claim that J κ, N → J κ, c in the strong operator sense, i.e.,for every fixed f ∈ 2 (V). Indeed, let χ |X|<ρ be the characteristic function of the ball in T with center at O and radius ρ. Given any > 0, there is ρ such thatSince coefficients {a n,j } and {b n,j } are uniformly bounded, we haveuniformly in N . Having and ρ fixed, we getby our assumptions (4.19). This proves our claim. Next, the Second Resolvent Identity from perturbation theory of operators givesfor z ∈ C\R. Since (J ej , c − z) −1 | Im z| −1 by the Spectral Theorem, we can take | N | → ∞ and use the above claim to obtain (4.21) lim