If a wind is blowing from a forested area toward a clear area there will be sink over the trees near the transition zone. Since balloons always fly with the wind I was always ready to burn as I approached the transition zone from forest to field when flying close to the tree canopy. My theory is that as the moving air approaches the open field it starts to drop down through the forest canopy as resistance to flow decreases because of the diminishing distance to the edge of the forest. This may snare pilots flying downwind and approaching low over the forest anticipating landing in a small field (they can't hit the burner like I could).
FC