Answer: I would change the mulch you are using from stone to an organic mulch, to start with. As you know, clematis likes cool, moist roots. A 3-4" layer of organic mulch like bark chips would provide better insulation and moisture retention than the stone. (Just pull the mulch an inch or two away from the trunk.) Also, the plant needs adequate moisture, so if mother nature doesn't provide at least 1" of rain a week, then you'll need to supplement with watering. Water stress could well be contributing to the yellow lower leaves. Have you provided the plant with any fertilizer lately. Usually, a spring application of compost mulch is adequate, but for large plants a monthly feeding with a balanced soluble fertilizer wouldn't hurt.
If the top of the plant is beautiful and blooming well, I wouldn't worry yet that the plant is dying. It is not uncommon for the plant to lose some of its lower foliage. Have you considered planting some low shrubs or perennials around the plant, to hide the lower vine and provide some protection and shade?
If you start to see problems with the upper part of the vine, then I would worry about the health of the plant.
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