Interesting question, but i don't think it would work like that here. Actually i know, coz i've seen patches where they have let it go. The problem is that there's been too much man-made interference for it to revert back naturally. The trees were all cut down and non-native grass planted for cows - if you let it go then you'd just get very high grass that would strangle out everything else. There's no way trees could get a foot-hold to compete with the grass. There are patches of it that i've been trying to tackle with the brush-cutter that have been let grow for decades - it's just pure grass about as high as me. The only tree that seems to have any hope of competing is the camphor - which grows on the creek banks - maybe coz the floods periodically flatten the grass out - but it is itself an invasive species here and even poisonous to wildlife apparently.

I did let that block next to me in Taitung go a few times - similar situation. If there are already a few trees in existence to give shade and stop the grass growing so high then they've got some chance, but in an open field it's tough. If you had a few hundred years to play with then the forest might creep down from the mountains where it's established.