crowded out or rotted would be my guess too.

Are there voles in Italy? These little rodents burrow right near the surface under snow or mulch and eat plant roots. I have lost many plants, including a couple of clematis to these cursed critters using my garden as their winter buffet under the safety and relative warmth of snow cover.

Huh. I went out and gave the dead stems of the clematis a good tug and they came up easily. Not much left in the way of roots, in fact. Perhaps we do have a critter that eats roots, though it hasn't been an issue in the past. I know that small rodents adore my hyacinth and tulip bulbs: one of the reasons I'm so fond of narcissus.
Buyorsell and nhbabs, thanks for the suggestions.
Melissa

was there any smell of rot?

No smell of rot. I've encountered root rot once or twice with different plants, and don't see anything like that here.