Wireless sensor network (WSN) can reduce human labour and monitoring difficulties in forest fire prevention and environment monitoring due to its low power consumption, small size, self-organising, flexible setting, and unattended operation. However, the limited WSN hardware resources and power capacity, the practical application requirements such as real-time monitoring and timely warning of emergencies, pose higher challenges to WSN on its lifespan, reliability, and real-time performance. This paper proposes a QoS adaptive and energy aware cross-layer opportunistic routing protocol (QE-COR). The protocol sets an adaptive QoS function Q
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comprehensively considering multiple metrics, through which the best forwarding node can be selected, and the QoS of transmission can adaptively adjust according to the data's needs. The protocol can also select a standby node to start retransmission when forwarding errors occur. A RTS-QACK and an ASS-DATA-SACK response mechanisms are designed to improve the transmission efficiency and avoid message conflicts. The sleep mechanism of MAC layer related with node's working state enables dynamical switching between active and sleep modes, which improves the energy utilisation and the flexibility of the network. Compared with similar opportunistic routing protocols, QE-COR can improve energy utilisation and extend network lifetime, while ensuring transmission reliability and end-to-end delay performance.