Giant Sequoias seem to have a very limited range. Why is this and how long have they been restricted to their current range?

Elevation, moisture, topography, and temps all play a role here. As far as I am aware, all of the giant redwoods, including the Sequoias, have only occured on the west coast from mountainous regions in south central California up and into Oregon. Predominately in pockets, with the exception of a much more vast range from Humboldt up into Oregon.

There are only about 5-10% of all giant redwoods left from their historic range. The limiting factor? Man… We have cut nearly all of them down. Here in Oregon, we did cut them all down. Now, they exist exclusively in California.

https://en.wikipedia.org/wiki/Sequoiadendron_giganteum

Historic range

While the present day distribution of this species is limited to a small area of California, it was once much more widely distributed in prehistoric times, and was a reasonably common species in North American and Eurasian coniferous forests until its range was greatly reduced by the last ice age. Older fossil specimens reliably identified as giant sequoia have been found in Cretaceous era sediments from a number of sites in North America and Europe, and even as far afield as New Zealand[15] and Australia.[16]

I believe wildfires are supposed to be part of their life cycle as well. The full grown trees can withstand the fire. The fire is needed to germinate seeds and clear out its competition. We don’t really allow that to happen anymore so whats left is what we got till we quit interupting the cycle.

Redwoods, and I believe this includes the giant sequoias, get most of their water from fog. Their needles are designed to facilitate condensation and then drip the condensation to the ground where their roots soak it up.

I grew up in the redwoods just north of San Francisco. During heavy wind storms, somewhere someone would always have a redwood go down because they have relatively shallow roots since they rely on dripping from the fog rather than some trees that grow deep roots seeking water down in the ground.

This is why above certain elevations in various areas, the trees just stop. Once you get above fog line, the trees don’t do well. If you’ve lived among the redwoods, you may remember walking through them on foggy days and beneath their canopy, it’s raining from the dripping!

Giant sequoias aren’t the same as redwoods, although they are related. I was struck when visiting Sequoia National Park that they only grew above a certain elevation. As you drive up the road, there are no giant sequoias, then suddenly, lots of them. I also found myself wondering why. It’s not like there was a drastic temperature or moisture difference 500 feet lower.