At least in north America, the transition usually goes from desert to grasslands to open scrub forests of oak and juniper, then to forests of ponderosa pine, then finally aspen and firs.
In Arizona, you can find all those transitions going up a single mountain.
The setting of your book really depends on the specific route your characters take…as the transition would be quite different in the mid-section of the continent as opposed to travel in the intermountain west, or Appalachia, or the coastal areas.
At least in north America, the transition usually goes from desert to grasslands to open scrub forests of oak and juniper, then to forests of ponderosa pine, then finally aspen and firs.
In Arizona, you can find all those transitions going up a single mountain.
The setting of your book really depends on the specific route your characters take…as the transition would be quite different in the mid-section of the continent as opposed to travel in the intermountain west, or Appalachia, or the coastal areas.