It has a tooltip on the loss section that mentions that the young trees need to get big enough to 'achieve canopy closure'

Looking in the area of the map East Texas and eastward toward the east coast, there is a definite blue tinge showing indicating gains in that area. Using the slider to narrow the range makes it even more clear.

I wonder if this is gains from natural regrowth or reforestation efforts like you describe. I was pleasantly surprised to see any positive direction on the map as I have a seeminly pessimistic view on the situation.