Yopo anybody know why there leaves fall off in the winter?

I have been growing this tropical plant from south america called a yopo tree. It's a ferny tree that grew so far about 4ft tall. Where I live I thought it would be pretty easy to grow. I have been growing this tree for three years now and every winter all the leaves fall off. :eek: but they come back once it starts to warm up the first year i thought i had killed it until the leaves started to grow back again. I know that tropical plants aren't spose to lose there leaves. I tried researching the plant but there is very limited information on it I thought maybe someone might be able to answer my question.