Californian climate refugee here. The issue isn't that they can't plant more trees. The issue (dependent on species, of course) is that the trees have stopped sprouting at the same time that they've been under a massive decades long assault by an invasive species, bark beetles:

Bark beetles are most commonly recognized by their impact on the lumber industry. Massive outbreaks of mountain pine beetles in western North America after about 2005 have killed millions of acres of forest from New Mexico to British Columbia.[20] Bark beetles enter trees by boring holes in the bark of the tree, sometimes using the lenticels, or the pores plants use for gas exchange, to pass through the bark of the tree.[3] As the larvae consume the inner tissues of the tree, they often consume enough of the phloem to girdle the tree, cutting off the spread of water and nutrients. Ambrosia beetles are also known to aid in the spread of pathogens, such as diseases that can cause cankers, further damaging the trees they infect.[21] Like many other insects, Scolytinae emit pheromones to attract conspecifics, which are thus drawn to trees already colonized by bark beetles. This can result in heavy infestations and eventually death of the tree.[3][22] Many are also attracted to ethanol produced as a byproduct of microbial growth in the dead woody tissues.[23] Increases in international trade, as well as the use of wood containers for storage, has aided numerous species of bark beetle in spreading across the world.[24] They are also extremely adaptable and able to quickly spread through new environments, as seen in France with eleven different species.[25] Bark beetle infestations are also predicted to increase with global warming, meaning infestations will most likely increase in frequency as temperatures rise.[26][27] Besides the fact that these rising temperatures provide the optimal conditions for larval growth, the development time that the larvae need to become an adult also drops, from 8–9 weeks to 6–7 weeks. As a third the result of global warming, the breeding season of the bark beetle is extended, meaning that number of generations per year will increase.[28] All these factors contribute to an increasing amount of bark beetles and will thus likely result in an increasing frequency of infestations. In the past, fire has been suggested as potential mechanism for controlling bark beetle populations; however, most studies of wildfire after beetle outbreaks have found no effect of beetle-caused tree mortality on wildfire size or severity.

In the Napa Valley, the hillsides were covered in Douglas Fir. About 20 years ago, they began to die as the bark beetles spread along the ridgelines, leaving swathes of dead, hollowed out tree trunks. It was bad, but manageable at first - the fir were still sprouting seedlings.

But after things continued to get hotter, the seedling growth slowed. The forests got drier, filled with dead timber, and between 2017 - 2021, about half of the western hills and most of the eastern ridge was burnt. And when I say burnt, I do not mean your standard burn - it was razed to the ground by fire tornadoes 90' tall, in places that hadn't burnt in over 200 years.

Five years later, the forest is coming back - but not the same. There are bay laurel and oak seedlings now about 5' tall - they're adapted for hotter weather. Normally, the Douglas Fir would be there as well, but there are no seedlings at all.

It's become too hot for them to germinate in the Napa Valley. The seeds won't sprout.

If there's one small silver lining, it's that the big trees will eventually come back - just not the Douglas Fir. The more heat resistant Ponderosa has begun showing up, and in 20 - 30 years will likely become the dominant big tree on the ridgelines.

20 years ago, the Napa Valley was like Bordeaux. Now the climate is like Sicily, and the deep forest is gone.

Visit your local forest now. Savor what it is NOW. Because it's very likely you will live to see it burn, and once it burns, it will never come back the same way again.