Thanks for the input to my posting. Now to answer the ultimate question, what caused the fire to occur. Because of the information provided, I was able to eliminate the prism effect, spontanous combustion, leaking motor oil onto hay/rags and a few other suggestions which were posted and sent privately.

After arriving on scene this morning, I noticed a burned area in the grass behind the barn that did not match the others. We followed it down hill for about 20 yards and found where a dead tree had fallen onto the fence which included one (1) stran of wire for an electric fence. The fence including the electric fence wire were lying on the ground. In the center of the burned grass was the stran of wiring for the electric fence, which burned through a section of tree bark, igniting it. The resulting fire traveled uphill and then into the adjacent shed/barn. Case closed.