Complex sentence: The fire towers fell out of use by the 1970s as fire spotting from airplanes had become more effective and efficient, so the fire towers were decommissioned; the Hunter Mountain Fire Tower was the last to be taken out of service in 1990.
Split into simpler sentences: The fire towers fell out of use by the 1970s as fire spotting from airplanes had become more effective and efficient, so the fire towers were decommissioned. The Hunter Mountain Fire Tower was the last to be taken out of service in 1990.