Taliyah question

Before: not much as far as we can tell. She almost killed Yasuo and seemed incredibly fearful of her powers.

After: It was clearly better than it was. The ground came up to greet her boots, and Yasuo was willing to let her go so she must have become quite skilled.

Trees: Pretty sure she bathed the Ionian Valley in sap.

Based on the story, she had very little control at first, and had to seriously concentrate to get anything done.

As for when she was training with Yasuo, I'd say that maybe atleast two trees died in the process.