Tim,
Yes I was surprised with the study too. When you think of those crevices though it is most degraded leaves and other organic matter trapped in the crevices. I had them planted in similar crevices in the huge banded gneiss boulders on my parents' property in MA. They looked great in it. Most of the "soil" was degraded pine needles and oak leaves so I'm sure it was very acidic too.
You are also right that semps grow in the wild where other plants don't. They couldn't compete with grasses but they can in a crevice in the rocks. They LOOK much better given good garden conditions. The Swiss horticulturist Correvon sort of rejected this lush growth as "unnatural" and grew some of his plants in austere conditions. In my opinion, you wouldn't starve your other plants, so why semps? I want mine to be HAPPY.
Kevin
