The Lady of Whispering Valley — Dean Wesley Smith

Buckey's best friend loves to recite limericks and can travel in time. Not bad for an oak tree named Fred.
But Fred must help Buckey and the love of his life find a way to live together. Not an easy problem considering Mary lives a hundred years in the past.
But an oak tree that recites limericks knows tricks of time.
Can true love be blocked by the simple passing of time?
And can Fred come up with a limerick that fits the situation in time?