Ten-mile transition

A reader identified that mysterious (to me) tree in my last post as being a Betula nigra, more commonly known as river birch. 

Following this bit of edification, I was still perplexed, because I just don't remember ever seeing them around here before.   And I would have noticed: I really miss seeing birch trees of any species - birch are ubiquitous in Canada. 

So I looked 'em up, and what I found is that Montgomery County pretty much represents the absolute southern extent of their natural range. 

Excerpted from:
http://en.wikipedia.org/wiki/File:Betula_nigra_map.png

So I'm not necessarily brain-dead on this one: yes, it IS possible that I have spent WEEKS in Lake Houston Wilderness Park never noticing a single one of these, and yet I drive just ten miles further north to a spot along the SAME CREEK, and the land is fulla them.

One wonders what controls this: what makes the tree grow on THIS spot along Peach Creek but not THAT one nearby?  I am used to seeing microclimatic variations here near the coast, but there are no perceptible differences to me in this particular case.  Is it one degree cooler on average ten miles further north?  One inch less annual rainfall?  The trees may be the only living things that can parse the answer to why they grow where they do, and they ain't talkin'.
:-)