Inmates already volunteer to do forestry work including firefighting. This has been going on for decades and isn't all that controversial. Expanding to planting trees shouldn't be too difficult.
All the grand projects of the past few centuries were based on it. Why change a winning strategy?
Well, we have better means for automation now. But I'm just fascinated how quickly that solution pops up in such discussions as soon as it's convenient.
I like how we instinctively characterize prisoners as slaves. Seems like it trivializes actual slavery, but I have been out of school for awhile, so the conventional wisdom is probably different now.
If you don't see labour camps as a form of modern slavery, then I'm genuinely asking you to provide the definition you've learned back at school.
Traditionally, prison labor is considered distinct from slavery for the same reason that prison is considered distinct from kidnapping and forced confinement: that it reflects just punishment for which someone is duly convicted. Whether that actually applies to most people in prison today is certainly up for debate, but to tell you'd mostly need to look at the judicial system (or society at large), not at the work camp.