A lot of really good points have been raised so far in this conversation. I'd like to toss a couple more into the mix (points anyway...whether they are good is a matter of interpretation): How we treat zone 5 in design, and the building or restoration of prairie soils.
Zone 5, since it's the last one in the list and often the furthest out topologically, tends to occupy less of our collective head space when we do a site survey and design. The role of humans in a landscape has always been to alter it: ever since we became demonstrably "human" with the exploitation of tools and fire, we have actively modified every ecosystem we spread to. Although this has nearly always been done with agency, it wasn't always with regard to longer term consequences. That was learned the hard way and if the end results were too destructive the humans in an over-exploited region would die out or be forced to move on, and the stories would become part of the cultural fabric as cautionary tales and lessons to live by.
Over time, ecosystems and humans have shaped one another and resulted in resilient and productive systems. The Great Plains is (was) perhaps the most dramatic example, both in the speed of its development and the massive biodiversity and underlying fertility it contained. Listen to any of the stories of the people who created and were in turn created by this ecosystem, and you will appreciate how deeply they understood the roles of water, wind, fire, bison, and themselves as inseparable parts of the balance and bounty of the prairie.
This was all prior to the advent of large scale disturbances caused by irrigation of drylands, where large and complex cultures developed comparatively quickly in places like Mesopotamia, Egypt, and Mesoamerica. For the first time, huge surpluses of high-value embodied energy could be stored in the form of grains, and used as tools of power. But in every case except one, the best soils were destroyed by salinisation from irrigation and the cultures either had to conquer others to maintain a productive base, or die off as their crops failed. The exception was Egypt, where the cyclical floods of the Nile not only deposited a fresh dressing of nutrients every time but also carried away the excess salts which built up in the root zone of the fertile lands (and this era came to a close in 1963 when the Aswan High Dam was completed...now Egypt can no longer feed its own people).
The conquering armies and rulers spread grain farming into regions where irrigation was not a requirement, and this led to a period of massive population growth: first in temperate parts of the Old World, and then spreading into the Western Hemisphere. By the time white settlers reached the tallgrass prairie, they ran into an almost unbelievable treasure in the form of biological outputs all resting on -- and at the same time creating -- the fertility of the soil. But that soil was a creation of all the parts of the prairie, and from the first moldboard that sliced into the sod, the first bison shot for sport, and the first band of people hunted off their own lands, that creation was cut off and from then on the soil was mined. In less than a century most of the plains were turned to farmland, and we all know what happened when the western reaches got converted and then a drought came along.
All this is by way of saying that if you're in the Great Plains and you are doing permaculture, you'll probably want to think about how zone 5 relates to what you're doing and put soil creation at the forefront of what you plan to do. Fire just might be off the table unless you've got huge swaths of land and neighbours whose heads are in the same place as yours. Mob grazing is probably one of the best tools in the kit. Perennial grasses are certainly foundational, and there is really interesting work in this area with folks like Wes Jackson. Trees have their part to play, too, and lots of times when we are creating or recreating soil fertility we bring these into the picture even though they aren't dominant in the wild landscape...let's face it, there is almost no virgin prairie left so anything you do at this point is succession planting and not preservation of a climax ecosystem.