Hi Newguest,
There is a slight difference in the time line between the first 2 sentences.
#1 means that you have spent 1/2 of this day (today) up the tree.
#2 means that you spent 1/2 of some day in the past (yesterday or further back) up the tree.
However, both sentences should use the preposition 'up' or 'in' instead of 'on'. 'Up' or 'in' a tree means you've climbed part way up it. 'On' a tree gives the impression that somehow you're sitting on top of the tree, like it's a chair or table.
For the second pair of sentences, both are equally correct. There's no real practical difference between her calling you and telling you (aside that calling implies she used a telephone).
As for substituting 'yesterday' for 'Monday' again, yes, both work with no difference (aside from the amount of time that's passed).