Would that not be the opposite of an explosion or blast ?
OK, let's make an attempt to understand what it could be.
Obviously, on finding young trees that were lightly burned, Ivanov started to look for traces of possible explosion, as he talks about epicenter. But he failed to find one. This matches his another comment that there was no pattern in how the trees had been burned. If a classic bomb is detonated, the epicenter is the bomb's body at the moment of explosion. Depending on what is bomb's body position towards a tree, the tree will be burned more at the bomb's side and less at the opposite side. The same is true if a bomb is detonated above a tree -- lower branches will be affected less than upper ones. But in Ivanov's case, as he says "no pattern", it was not possible to detect from which direction flames came. All that made him think about "heat ray or completely unknown to us energy acting selectively".
My conclusion is quite different from Ivanov's. "No pattern" and randomly scattered burns from all sides of the trees probably mean that some flamable substance was dispersed in the air unevenly. Then someting ignited it. And concentration of that substance was low, since the young trees were burned just lightly while other trees did not have visible burns. In this case, indeed, there was no epicenter, or if you like, there were many small epicenters in the air in the area, where the young trees were located.