Many previous studies propose to use wake-up scheduling to support energy efficient data gathering in a wireless sensor network (WSN). However, these previous works schedule network nodes with regular patterns, and thus can only support regular and periodical data reporting. In this work, we further consider the event data reporting scenario, where the network may randomly have urgent events. We design an event data collection mechanism for ZigBee tree-based WSNs. In the network, nodes are divided into routers and end devices. Routers form the tree backbone of the network, and end devices are connected to routers. Each router will be assigned to a regular slot and several event slots. To support regular data reporting, routers and their child devices periodically wake up in their regular slots to sense and to report. When an event happens, devices nearby the event can wake up more times in their event slots, and routers can collect more event data from child devices and report more frequently. In this paper, we design a tree formation algorithm, slot assignment schemes, and event mode operations for the event data collection scenario. Simulation and implementation results show that our designs can support both regular and event data reporting, and the proposed schemes can effectively increase the amount of report data when there are events.