The conifers are experiencing fall in their own way. The larch will sprinkle the mountains with their beautiful golden needles, but we are still a few weeks until peak color change. In the meantime, the Douglas fir and ponderosa pine are having a different kind of needle drop. This is the time of year when the
trees get rid of the needles that aren't getting
enough sun, or for other reasons aren't worth defending. Usually the needles towards the interior and underside of the trees turn yellow or reddish-brown and drop off. This is permanent and that spot will not grow new needles but instead will have bare bark. This is the result of the tree growing taller and adding new branch tips each spring. Those new needles end up shading out the older lower needles.
In the first photo you can see a ponderosa pine that has reddish-brown needles on the interior.
The second photo is one individual branch of a ponderosa. You can see green needles near the tip from growth of this year and last. Further from the tip the needles have died off and will soon drop. Further along the branch you can see little nubs where needles fell off in previous years. Eventually as the branch thickens and the bark continues to grow, all signs of needle attachment disappear.
The third and fourth photos are of Douglas fir. You can see that same phenomenon of the dropping of the shaded out needles.
It is through this mechanism that the lower parts of the tree trunks consist of dead branches or eventually a bare trunk as those dead lower branches break off. Those dead lower branches are usually much drier than the branches on the forest floor, and so, they make good fire starter. If those dead branches don't break off they can act as a ladder for wildfires to get into the crown of the tree.