I recently went on a several week journey to further investigate the big trees of California. Within the past month I have visited Sequoia/Kings Canyon National Park, Sequoia National Forest, Sierra National Forest, Redwood National Park, and various northern California state parks. Simply put, there is an overcrowding problem, but not of tourists.
Above is a giant sequoia surrounded by dozens of younger trees, all of which are competing for the same resources as this ancient tree, In previous centuries, these younger trees would have been removed by fires set by the local California natives.
Fallen giant sequoias from paludification, along with over competition.
Top dieback is a clear sign of distress in giant sequoias from too many younger trees. Resources, especially water and nutrients, are limited!
The progression of top dieback is as follows. Here is a healthy tree …
In this case it is a fir tree, but this example applies to sequoias and redwoods in decline. When resources become limited, the tree redirects its energies to the apical meristem, the growing tip of the tree, sacrificing those branches just below the top.
However, as over competition continues, mainly due to fire exclusion, the decline progresses and tops of the trees eventually die.
There are entire hillsides of fir trees dying from over competition. Although the conventional explanation is that they are dying from bark beetles and stem cankers, which they are, proximally, but not ultimately.
The coast redwoods are faring no better. Here are photos of ancient coast redwoods dying from over competition …
Only two of the trees in the above photo were present one hundred years ago. Previously, this was an open canopy, park-like forest.
Same with the over-crowded hemlock trees growing near the redwoods …
So when you hear about climate change killing our ancient forests, be aware that the bigger problem is that all these forests, which used to be tended by native peoples, are no longer being tended. And that is the primary reason they are dying.
Sudden Oak Life 
Interesting, thanks! We have similar issues in Australia, with Indigenous cultural burning practices having been suppressed for almost two centuries. Now making a comeback, with the work of people like Victor Steffensen (author of ‘Fire Country.’) And of course, logging, which results in too many new trees regrowing.