Murray McGregor wrote:

> Climate must be a secondary factor as the limit is not a straight
> latitude line, but something akin to seasonality must be at work in this
> case.
>
> Incidentally, Scotland is a country with a relatively mild damp maritime
> climate, a bit like your western seaboard from Northern Oregon up into

I didn't realize that - always thought of Scotland as deep snowdrifts in
winter.

>
>  OR, is the natural, as yet unknown, limiting factor starting to
> kick in and arrest their rapid progress north. I guess that in 10 years
> the line will be relatively stable with seasonal incursions across it.
> Only then will you know.

You make some very good points, and give me pause to reconsider some of my
opinions. You obviously have studied this situation.
Stay in touch, and I may see you down by the loch someday.
     _ John Edwards (of McFarland), Tucson, Arizona