The issue of supporting mobility in Wireless Sensor Networks is recently attracting increasing attention within the research community as new-born application scenarios require the deployment of hybrid network architectures composed of fixed and mobile sensor nodes. In this work, we address the problem of mobility management in WSNs by proposing a tree-based routing protocol able to support mobile sensor nodes. Distinctive features of the proposed routing solution are the provision of bi-directional uplink/dowlink connectivity to mobile sensor nodes, the use of proactive procedures to speed up the association/reassociation phase, a reduced impact of the signalling overhead to manage node mobility by resorting to ”local” handover management procedures. The routing protocol has been implemented on commercial hardware and thoroughly evaluated in terms of reactiveness and overhead through testbed experiments, as well as detailed simulations