In this paper, we propose an adaptive transmission scheme for wireless sensor networks over slow fading channels. In the proposed scheme, the fusion center adaptively selects between distributed beamforming and distributed quantization transmission schemes such that the advantages of both of them are exploited. As a result, the distortion performance of the proposed scheme is significantly improved. Simulations show that about 2 dB power gain can be achieved in case the number of sensors is equal to the number of quantization bits at the distortion floor level. A larger power gain will be obtained with a relatively large number of sensors. The significant power gain is very useful for battery-operated wireless sensor network application where replacing batteries is very difficult or impossible.