Question

Here's a question for you that has been rattling in my head for the last few days as spring is here. 

How do trees know its spring?

How do they sense the warmth and the fact that there is more sunlight and that the days are getting longer?

What part of the tree senses warmth?

What part that is not a leaf senses daylight?

I don't know the answer to these questions and my searches on google have not been very fruitful in getting me answers to these questions.