Feature generation at certain y levels

recently, i have been working on a worldgn datapack. i wanted to have 2 types of trees: normal and windswept. The idea is that windswept trees generate above y = 128, and normal ones generate below y = 128. I cant get this to work though, as they just place in the air? i say this with a heavy heart, but at one point i even resorted to chatgpt, but it just made things worse. How can i get this to work?