Much of the forest land on the peninsula is privately owned, not by individuals, but by conservation organizations.
> The main problem is SFH
Except that people like their yards and pools.
All else being equal, denser housing is better. But all else is rarely equal. Almost invariably, increased density comes with increased population. The SFH it replaces doesn't get turned back into open space, it remains SFH. Or it turns into more MFH, so now you have N times as many people living in the same space. That is the problem.
Immigration is not a problem. It is an opportunity.
N people live in a place. K people want to move there. You can either find ways to accommodate N+K people, or you can choose not to and have the richest people drive out the poorest (or the middle class if you start building affordable housing). Only one of those two options is classist and xenophobic. Take your pick.
But redwood forests are a distraction. The main problem is SFH which can and would be replaced with duplexes and fourplexes if the market were allowed to work.