Before I continue on coast redwoods, let me explain the header image at the top. That is a row of giant sequoia in Beaverton, Oregon. That is the other giant redwood species. That row was planted in 1932, when my mother would have been about 13 years old. Look for me and my Chevy truck in the image. The "footprint" of the trunk's width is equal to the length of my red extended cab pickup to the right of it. That's how well the other species grows in that part of the Pacific NW. I added that one in just for the fun of it.
Back to talking coast redwoods again ...
I've worked in Washington, Oregon and northern California doing tree and landscape work since 1980. But I grew up around forest land from 1963. During all these years, I watched which species would just grow at various properties, and which species would germinate seeds and grow. For example, the vine maple, Acer circinatum is a native plant to Oregon, Washington and California. Not only does it grow, but the seeds germinate in landscape setting and in forests too. Likewise, Japanese maple seeds germinate too, sometimes by the hundreds, but it's indigenous to Japan.
But in all my professional years of horticulture, I do not recall seeing coast redwood seedlings sprouted in residential landscapes, golf courses or municipal parks in all of OR and WA. Doesn't mean it never happened ... but I haven't seen it happen.
So realizing that coast redwoods grow very well throughout much of OR and WA without relying on fog for survival, i began to wonder if coastal northern California has something else worth pin-pointing pertaining to why the coast redwood forest is native to that area. It has to me more than occassional forest fires clearing ground for seeds, because other coastal forests of the PNW also have periodic fires.
Seeds have special needs, and the needs vary by species. If I recall correct from Seeds of the Woody Plants of the United States, the Mt. Ash seeds need about 30 straight days below freezing, twice. That doesn't happen in the lowland of the Willamette Valley, but it will occur half way up Mt. Hood. We can plant and grow Mt. Ash in most west coast cities, but the chance of finding a germinated seedling at low elevation with moderate climate is near zilch.
So with the coast redwood, I throw down the gauntlet for someone to narrow-down the exact requirements of the species survival. Not what it takes for them to get big, because certainly lots of fog and rain achieves that. But why do they reproduce in that area and don't readily reproduce elsewhere? Reproduce and survive.
Back to this coast redwood in the photo. I've seen plenty this big through OR and WA. But this one better than most makes the point what coast redwoods can endure. If you are not familiar with this part of the Rogue Valley, the rainfall is the 19 inches yearly that I mentioned. And summer temps of 85 to 100 degrees are common. About 6 to 8 degrees warmer than Portland, but half the rainfall. This redwood went through two of the driest growing seasons in 2014 to 2016. The winter of 2016 - 2017 saw temperatures down the teens, Farenheit, and a 100 yr. snowfall event that hammered trees from this Jackson county all the way over to Del Norte county. Yet, the folilage of this Sequoia sempervirens is gorgeous.