In this paper we present a routing protocol (Modified HWMP, MHWMP) for IEEE 802.11s WLAN mesh networks with smart antennas that incorporates the optimum selection of the PHY-layer transmission/reception mode (multiplexing, beamforming and diversity). MHWMP adaptively selects between spatial multiplexing and beamforming for data transmission according to the wireless channel conditions. We also modified the traditional RTS/CTS mechanism to take into account the different behaviors of both transmission modes at MAC layer: the advantages of diversity are harvested by sending RTS/CTS frames with space time block coding in case of beamforming while standard RTS/CTS frames are used in case of spatial multiplexing. Simulation results illustrate that MHWMP leads to significant better throughput and delay performance in certain situations. Moreover, it enables a high degree of robustness wrt. wireless link failures for stationary mesh networks.