A Perky Sentry unit attaches to a fence post w/a infrared (IR) sensor pointed at the ground. Should a vegetation fire move past, the heat will be detected and reported to a home automation hub which can relay the alarm to an external messaging gateway.
The sensor package can be augmented with additional instrumentation such as orientation sensor to warn if a fence post has fallen over (ask anybody with horses).
Commercial Sentry unit should be solar powered and be reliable for years.
Distances are frequently too far for reliable WiFi, a better communications solution would be a mesh network which would permit the sensors to relay observations for each other.