Summer heat...or more specifically, higher overnight temperatures thanks to high humidity and the lack of respite that they'd get in more northern states (the heat waves 'break' on a fairly regular basis). I wouldn't say that you can't grow spruce and fir where you are...it's just that the native species are no longer competitive at your elevation.

Great summary, Smivies. I have no idea if that's true, but I'm sure you do and there is an elegant logic to it.

Definitely evening heat/humidity. They hate that.

Thanks all. Makes sense. We are indeed very humid here. It sometimes makes me less competitive as well.

Haha, good one! Hey, I would urge you and all to not lump all fir and spruce together. The Rocky Mountain species are certainly not at home in the humid east, and for the some of the reasons already offered. But as you have noted, the Norway spruce don't suffer so. So there's a difference. Cats and dogs both have four legs, two eyes, one nose, and a mouth full of teeth. Yet I'm pretty sure that if a veterinarian therefore decided that they were the "same", the treatments offered for one or the other would fail badly. These organisms have all evolved under differing conditions, and that is why they struggle when placed into regions having markedly different conditions.