In October, the Quaking Aspens (Populus tremuloides) in the Santa Catalina Mountains near Tucson wear their golden autumn colors.

In autumn, green chlorophyll production stops in the leaves of Quaking Aspens and other deciduous trees. Once the green color due to chlorophyll fades, the underlying yellow color of the leaves is revealed. The yellow color is due to the yellow carotenoids (xanthophyll) which are always present in the leaves.

The leaves of Quaking Aspens and other Populus species like cottonwoods contain large amounts of xanthophyll, which not only colors their autumn leaves golden yellow, but also makes their spring and summer leaves a brighter, warmer shade of green.

To take the above photo, I stood in the middle of a small grove of Quaking Aspens and looked upward at their leaves fluttering in the sunlight. I was dazzled by their golden brilliance. Fall foliage colors may not be widespread in Tucson, but their relative scarcity makes me notice and appreciate them all the more.