Average Rainfall versus Tree Population
Trees need water. The only way that a tree can survive is either the average rainfall is high enough that the tree can find enough ground water, or the area is surrounded by mountains and possible snowmelt to provide water, water flows naturally from high elevations or from rivers and finally gets to the area the tree needs, or water is piped in from less arid regions.
Look at these two maps, and you'll see that I'm right:
[link to inhabitat.com
[link to www.nationalatlas.gov
If I was living in these arid regions, and also expecting a collapse of any duration, then I'd by necessity need agriculture and forestry. I must be able to grow crops, raise animals, and build tools and shelters from lumber. While I can build sod shelters for some things, my animals need water as do my plants.
Soils are rich for two reasons. Either the soil has slowly built up humus from dead or dying plant material and decaying animal manure, or it's chemically added. If there has been a disturbance in the soil by agriculture that doesn't renew the soil, then it won't be fertile very long. That process cannot happen without water.
If I lived in the Great Plains, I'd sure have supplies, perhaps three times the supplies of other preppers/homesteaders....otherwise I'd simply be gambling.