Q: Why was the tree excited about the future?
A: It was ready to turn over a new leaf!

Explanation:  “To turn over a new leaf” means to start anew, to start again, fresh.  Someone who has had problems might turn over a new leaf and begin again (study more, learn something new, stop a bad habit, start a new good habit such as exercising, …).

A tree has many leaves (one leaf, two leaves).  Do the leaves turn over?  Not really but maybe before a storm.

This joke plays with the word leaf and makes me think about the future.  What are your plans?  Do you need to turn over a new leaf?