Dry wood and leaves burn, fireballs burn REALLY hot. . .the flash should ignite all the dry tinder available.
It's a fast flash of fire, unlike say Wall of fire, so it's extremely unlikely to set actual trees on fire. (Though it might set some of the leaves on the trees on fire)
That said, if the area is right, it's certainly more than tossing a cigarette out of a car, which is one of the leading causes of wildfires in the real world.
If it hasn't rained in more than a week, and the area is wilderness, then likely the area is full of dry tinder. In more inhabited areas, the people haul off all the wood on the ground for firewood, and tend to cut out dead trees for the same purpose. . .but if the land isn't in regular use, or is relitively new growth, there may be a lot of low brush.
Low brush and dead wood would likely ignite with a fireball. . .dry leaves and dry tinder small branches definitely would. . .green and live wood would likely char and smoke a bit but not flame . .and likely all the green leaves would curl up and die. Pine trees. . .well, I've seen scary things happen with all the dry tinder you get inside the green cone of live needles still on the tree. . .a fireball into a pine tree might have a rather dramatic effect. . .
If you get one started, they tend not to just go out. . . .if he's got some cold or water spells you could probably put the fire out.
