Wireless sensor networks (WSNs) may consist of hundreds or thousands of low-cost sensors, which makes it difficult for a central fusion center to oversee every sensor node. In this paper, we are interested in the problem of signal detection using WSNs. We propose a distributed framework for cross-layer design which is not only suitable to such application, but scalable with respect to the network size and able to greatly reduce the dependence on the central fusion center. This framework assumes a clustering infrastructure as the basis, and consists of a distributed scheduling protocol and a distributed routing protocol. These two protocols coordinate the intra-cluster and inter-cluster communications, respectively, while enabling the sensor nodes to make their own decisions about energy-efficient information transmissions, without requiring global network information. Thus, this framework can attain energy efficiency at different network layers in a distributed manner.