This paper investigates the impact of congestion on the performance of spanning trees in Application-layer Multicast (ALM) environments and proposes using redundant links between adjacent levels to enhance the stability of spanning trees. We analyze this approach by developing a simple statistical link model which describes congestion occurring in spanning trees and by introducing a dependency-degree factor which models the dependence between adjacent links.The numerical simulation results indicate that the statistical link model well describes the congestion occurring in spanning trees for ALM

and that even in case of high congestion probability

the redundant links scheme can significantly enhance spanning trees' resilience to congestion at a low cost of additional network delay.The proposed statistical link model is generic and can be utilized not only to further understand the dynamics in ALM

but also to design algorithms of congestion control for ALM.