Agreed that nine years is probably (not knowing all the details) disproportionate.

But the Wikipedia article mentions the tree is sacred to the Comanche. Compared to what he would have endured at their hands hundreds of years ago, had he been caught by them, he's lucky :)

The article mentions that he used Velpar. A large part of his sentence may stem from him having contaminated ground water (in centuries past, a crime only punishable by death most cruel).