I need a bit more information about your trees. Is the sticky liquid on the ground around the trees? Do you observe any insects on the leaves of the trees? If possible, can you send a photo?
Hello, I haven’t heard back from you about your aspen trees. I believe that your trees may have aphids. For more information on aphids, I recommend this fact sheet from Colorado State University Aphids on Shade Trees and Ornamentals.
If you would like to talk with a Master Gardener about your aspen trees, you can call the Deschutes County OSU Extension Office at 541-548-6088.