I live in the mediterranean climate of Sacramento, CA. Dry rot, as the term is used, occurs in buildings here frequently more so than in any other climate I have lived in. Actually up in the NW where it rains much more, they get less dry rot in buildings. It is because Sacramento gets extremes in temperature and humidity. In the winter it rains a lot and heats and cools a lot . Summers are blistering, 105F is not uncommon.
I have noticed on the tires on my vehicles develop cracks very quickly here. I replaced the tires on one truck less than 5 years ago and just replaced them again before tread wear was complete because of the cracking. The tire people use the same term, dry rot, the term used for wood decay also.
I think tires, especially the cheap ones, will dry rot regardless of if they are mounted and used if they are exposed to extremes in humidity and temperature. So if they are kept in a non climate controlled warehouse, they are aging just like they would if they were mounted. Especially where climates vary greatly and often in humidity and temperature.