In this area, if you just let some land go that started as grassland, in the first couple of years it will be mostly weeds, then for the next several years it will become overrun by shrubs, such as [1]. At the height of this phase, the land is essentially uncrossable on foot, covered with 10-15 foot tall sharp, spiky plants that are not very hard compared to most wood, but certainly much harder than your skin.

Over the next couple of decades, the trees will start getting a foothold, starting with the faster growing trees, which crowd out all the nasty shrubs but still produce a dense group of small little trees just barely spaced out enough for an adult to get through, then eventually stabilizing on oaks and the other very tall, hard trees, culminating in what one traditionally considers a "forest", something you can walk through because it's mostly very large, mature trees spaced dozens of feet apart. This will take a century or two to progress to the final stable state, depending on how tenacious the smaller trees are.

But if you want to jump straight to that and avoid the middle bit, it takes some maintenance. I assume the promise is a combination of a promise to do that maintenance, and probably to maintain some trails through it.

I could have sworn I learned a term in school for this sort of "biome progression" to what I thought was the "terminal biome" (the final, stable biome that results) but Google does not agree. Anyone who remembers their biology better than I, I'd be much obliged for the term.

[1]: http://en.wikipedia.org/wiki/Elaeagnus_angustifolia