The point of planting a tree, writing a book and having a child I don't believe were meant to be taken as literal accomplishments that once those goals are fullfilled you have lived 'Life'. I believe that it is HOW you do these things and what you learn from them....that is the meaning of life. The 'plant a tree' analogy symbolizes your connection to the earth and natural beauty that we are part of. 'Write a book', the concepts of inner searching and discipline. And procreation, gives the gift of unconditional love.
It's not so much WHAT you accomplish that is the meaning of life but more so the path you travel to get there.