For comparison he also measured light reflected by an Engelmann spruce. Most conifer trees like Engelmann spruce do not exhibit prominent leaf color changes.
His results (figure above) indicated that the rate of chlorophyll change was influenced by health status of individual aspen trees within a clone. However there was no difference between the young and old healthy aspen trees. The light reflected by Engelmann spruce steadily increased throughout the year and did not show the declining trend in fall season.