For data-collection applications in sensor networks, it is important to ensure base station receives a complete picture about the monitored area. Convergecast is an important communication pattern commonly used to collect continuous data. The prior broadcast trees are not suitable for convergecast, because convergecast is a reverse broadcast process. We point out the load-balancing problem in the current design of sensor networks. A non-load-balancing tree makes some nodes consume energy faster than others. It is important to design a distributed load-balancing solution due to the lack of global knowledge about the network topology. This paper presents a novel convergecast tree protocol and a distributed adjustment algorithm to attain load balancing and to extend network lifetime. The tree protocol constructs an approximate load-balancing convergecast tree. Additionally, the adjustment algorithm dynamically adjusts tree structure to avoid breaking tree link. The tree adjustment only ne...