"So, TP in dryer conditions decomposes faster than in wet areas? Boy is that ever counter intuitive."
It's not the water per se but, rather, that excessive water displaces the oxygen necessary for rapid aerobic decomposition. Anaerobic decomposition proceeds more slowly and the faecal matter/TP will persist in the environment for a longer period of time..
"Seriously though, up above 10,500' or so in the Sierra, the soil seems so lifeless. I doubt things would break down quickly."
There are bacteria, lots of them, everywhere, including the Sierra above 10,500'. I've been interested in how faecal matter and TP break down up in the Sierra for 30 odd years now, precisely because I was concerned about leaving traces.
There is an area in the Upper Kern Basin, elevation ~10,700', that I have visited at least once a year during that period and I have checked sites I used on previous visits. No trace of TP and only a small, dessicated pellet unrecognizable as faecal matter are the most that I have ever found. Part of the reason for this, I hypothesize, is that I burn as much of the TP as I can in the hole and then urinate on it, both to quench embers and to add additional nutrients to the mix. It seems to work. TP is mostly cellulose and lignins, which do not break down as rapidly as, say, leaves but, in the presence of adequate nitrogen(think urine and faeces) and oxygen, the resident bacteria will get the job done.
"Add to that the typical rockiness of the Sierra high country -- there just aren't that many places you can make your, uh, "deposit."
The rockiness of the Sierra provides countless "safe deposit boxes" for your convenience. Just find a secluded place where the slope will not cause spring runoff to flow into a water source, lift a fairly large, partially embedded rock to one side, and voila!, you have a prefabricated cathole/safe deposit box. Simply make your deposit, burn the TP in the hole, pee on it, and replace the rock. Simple as that. Talus slopes are another matter. Best do your business before or after negotiating the talus. Clearly there are countless other options, but this is one that I have found to work well and it definitely leaves no trace that humans or animals are likely to detect. Within one year, there is no trace, period. I probably should urinate on this post to avoid igniting another TP flame war. :)