Debs, pH difference is a possibility, but with low probabilities in my opinion. I think this because pH levels strongly interfere in the nutrient uptake by the roots and, apparently, little in the direction they go. Shade also wouldn't be my guess, because in that case, going down, sneaking through the lava rock (following the gravity) would most likely be the way roots would take. On the other hand, I think you are right when you rise the moisture option, even if the moss stays wetter little longer than the rock.

can this moss also be used in mounts? i tend to water a lot and sphagnum stays wet for too long.

Chids Chids!!!! We're getting off the subject here!! :nono: Tsk Tsk!!!

My personal preference is 'junglesque'. Sounds kinder French, don't it? :blushing:

Anyhoo, here's some pictures. I don't know if you can tell or not but the moss is only on half of the surface or scattered.

Lava rock does not hold moisture effectively, especially the exposed surfaces at the top of a planting container. The moss holds moisture much longer. Even it it isn't wet, as in having been recently watered, it is slow to dry out entirely. In a humid environment, it can also absorb some moisture from the air.

The Cattleya roots are growing under it because it is providing a better environment for them than exposed lava rock. It's holding moisture or humidity and is evidently sufficiently ventilated also.

One further advantage could be shade, depending on your growing conditions. In direct sunlight, lava rock can become too warm for the roots.