Great pics and story! I would bet $100 this was not radiant heat, but rather that an ember from the fire had floated over and settled into a portion of the rotting trunk at the top of the tree. We live in a small acreage community and a neighbor down the street had to call the fire department when his small leaf and limb cleanup fire sent an ember up into a 50+ year old Oak on the other neighbor's property line. Turned into a full fledged fire at the top of the tree where dry leaves had nestled into a fork, but the FD was able to put it out quickly. Everyone had a good laugh, but burning is serious business, especially with other trees in close proximity. I have verified the travel distance of hot embers while performing burns after dark. Those big fires look an order of magnitude more impressive when their tracer fire can be seen a hundred + feet in the air and a hundred + feet downwind too at night. Thanks for sharing this cautionary tale!