Problems with St. Augustine's definition of the Past

On page 293, Book 11, Chapter 20: Three Kinds of Time, Saint Augustine says that what is in the past, is only what is in memory, but there are problems with this theory.

Let's say that, somewhere deep in a forest, there is a tree. Now a wind blows and knocks down the tree. No one ever saw the tree fall, so did it really happen since it is not in anyone's memory? You could say that someone saw the tree before and after it fell, and then say that tree had fallen, but what if no one had even known the tree existed? Would tree's life really be in the past, because according to Saint Augustine's theory, what is in the past, is only what is in memory.

Is there a problem with my logic?

~Hannah