After a long time, which you feel are about a few seasons, you can feel heavy fruits hanging on your branches: great, round apples, ready to be picked, a few occasionally falling to the ground. To a human, these juicy fruits would seem most tempting. One day, someone walks by (you know this because you can feel the person leaning against your base) and spots the apples lying on the ground.
"Hmm, wonder if these are good..." He checks to see if the apple isn't rot, and luckily, it is good for eating. Seriously tempted, he takes a large bite out of the apple and happily chews on it. What he doesn't know, though, is that the seed of your transformation is also contained within these apples...and after a few moments, he starts feeling a bit stiff, but continues to eat the apple. Only when he sees his skin turn brown he attempts to run away scared, his arms waving in the air. However, by now his legs have completely turned to wood and he fails to move off his spot, as the rest of his skin turns to wood while he has his arms in the air, now seeming like two large branches. His wooden legs quickly fuse into a base and his feet stretch and shoot into the ground, quickly splitting into roots. Small pores start appearing all over his face, which is quickly covered in bark, from which small leaves start sprouting until his entire head is covered in lush, green foliage. His fingers also quickly shrink, twist and form leaves at the end of his branches. As you now know, he has lost all senses except for feeling and consciousness now.
He has a long way to before he is a big apple tree like you, but it's a good start.