Aspen trees do not all change color at the same time in the fall. This can be good or bad news. Good is that there is some flexibility in the timing of fall photo trips to aspen areas. Potentially bad is that there will likely be green or bare aspen trees in your targeted area.
In addition to leaf color, sky cover is a concern for aspen tree photography. While blue skies are beautiful, I much prefer to have photogenic clouds decorating a blue sky (with abundant amounts of sunshine coming through). My reasoning for this preference is probably obvious for images that include those clouds and the sky. But, clouds cast shadows and shadows can greatly contribute to imagery.
On the return hike from Crater Lake on this day, clouds blocked the sun just enough to shade Sievers Mountain while the foreground aspen trees glowed brightly in the sunlight. In the mid-ground was a patch of aspens with only their top-most leaves remaining (these are the last to fall). Also in the sun, these leaves appear as a flame over the trees. While it is not in the limelight, Sievers Mountain, full of character and framed in blue sky with white clouds further separating the sharpest peaks, makes this shot for me.
While a telephoto lens may not have been your first choice for a hike primarily focused on landscape photography, telephoto focal lengths are an integral part of my landscape kit. I often find composing landscape images with a telephoto zoom lens
to be easier than a wide angle lens. The next time you head out to photograph the great outdoors, especially in big mountain areas, make sure that a telephoto zoom lens is in your bag.