Climate and Culture
Historical Geography of Medieval America:
Whether we want to admit it or not, humans are animals, so history is often an ecological science. This is especially true of the medieval period, when cultures had very little control over nature, and they were forced to adapt themselves to the environment more often than vice versa. The map of cultural zones in Medieval America would therefore closely correspond to the map of environmental zones.
Map Sources: Hammond Ambassador World Atlas (1985)
Audobon Society Nature Guide: Eastern Forests (1986)
Generally speaking, if there's enough rainfall to support a forest, then there's enough to support agriculture without any special effort. Beyond the edge of the forest, however, irrigation becomes necessary. The differences between irrigated and non-irrigated cultures, however, goes beyond the simple differences in the technology of growing food. Building and maintaining the canals, dams, reservoirs, pumps and valves of an irrigation system requires an enormous collective effort, and a society which depends on irrigation needs to support a rather large Department of Public Works in order to survive. Unfortunately, the man with his hand on the water valve has the absolute power of life and death over his neighbors, and these societies tend to become an extreme type of absolute monarchy known to historians as hydraulic empires, of which Egypt is the best example.
Prime sites for hydraulic empires in Medieval America would be along the rivers of the desert -- the Rio Grande in New Mexico, the Columbia in Idaho, the Gila in Arizona -- and also in the foothills of Utah. Here we would see rigidly organized societies with an underclass of oppressed peasants under an exteremly centralized bureaucracy. The rulers of these places would have total authority to enforce spiritual and secular uniformity on their empires.
Where water falls freely from the sky, however, rulers have less control over their subordinates. In the forest zone, oppressed peasants and ambitious barons can rise up in rebellion without jeopardizing their water supply. Because they are not hemmed in by the surrounding desert, they can also just leave and find equally good opportunities elsewhere. The tax burden in forest zones is generally milder than in the desert because it is only necessary to support the king and his entourage, not the king, his entourage and massive engineering projects. Traditionally, forest zones such as India and Europe have been broken into a bewildering mishmash of feudal states and diverse cultures because there's no central authority to impose uniformity.
Seasonal variation shapes the nature of the forests and the cultures that live there. In the far north, winters are harsh and summers brief, and a special kind of tree - the evergreen conifer - predominates. This type of forest is called the taiga, and life here is specially adapted to handle the severe and almost perpetual winter. Unfortunately, farmers have never really been able to adapt to this climate, and the southern limit of the taiga has traditionally been the northern limit of agriculture.
Farther south, there is a greater equilibrium between the seasons, and the deciduous tree has evolved to adjust to the variation. In the spring it grows broad leaves to catch the sun, but it sheds them before they can be damaged by the onset of winter. Humans too adjust to the seasons in this zone. They plant in the spring, harvest in the fall, and hunker down to wait out the winter. Relatively speaking, however, only a few crops can survive this cycle -- grains like wheat and rye, fruits like apples, vegetables like carrots and cabbage and fibers like hemp and flax. On the plus side, the annual deposit of dead leaves has made the land under the deciduous forests thick and fertile.
Even farther south, the winters grow milder, and the trees can remain evergreen to take advantage of the longer growing season. A greater diversity of live can thrive in this climate, and the farmers here have more crops to choose from. Instead of simple subsistance farming, they can grow subtropical cash crops like cotton, tobacco and sugar. Unfortunately, this greater diversity of life also produces a wider variety of parasites and pathogenic microorganisms. Southern homes are riddled with termites and roaches, while southern populations are cut down by malaria and yellow fever and weakened by intestinal worms. In the Medieval Era, hot, wet regions such as India and Sub-Sahara Africa had too much energy drained away by disease to leave much of a surplus for taxation, and therefore, subtropical regions rarely produced large, organized kingdoms.
Grasses are specially adapted to survive in regions of light rainfall. They have deep roots to search out water, and they recover quickly from the wildfires that regularly sweep over the dry plains. Grasses are also tough to digest, yeilding little nourishment, and being recent arrivals in evolutionary biology, only a few animals are adapted to eat them. The most noticeable of these are ruminants -- cloven hooved mammals that chew their cud -- like antelope, bison, cattle, sheep and goats. Across the world, primitive humans have survived on the grasslands by eating the animals that eat the grass.
It's not that farming is utterly impossible on the grasslands. The principle breadbaskets of the world today -- the American prairie, Argentina, Ukraine -- were originally grasslands, but in the medieval period, nomadic herdsmen acted like wildfire. They would sweep over the prairie, destroying the farms and irrigation canals, and it would be the herdsmen who would benefit by the return of the wild grasses. In fact, in order to restore the pasture for his herds, Genghis Khan (and others like him) made the destruction of villages one of the major priorities in his conquest of the transitional climates at the edge of his native grasslands.
In the desert scrubland, however, grazing is scarce and widely scattered, so the herdsmen were often too few to overwhelm settled communities. Therefore, as we see a further decrease in rainfall, we also see a return of agriculture. Although scrubland like Morocco and Syria saw a great deal of conflict between the herdsmen and the farmers (the nomadic Israelites' conquests of Canaan, for example), the size of the invading force was usually much smaller than that which could flood out of the Central Asian steppe, so the devastation was correspondingly less. In Egypt, possibly humankind's most arid habitat, agriculture flourished and produced unparallelled population densities largely because it was so completely isolated from neighboring nomads.
There are only two authentically barren areas in the continental U.S. which would be utterly incapable of supporting human existence: the Mohave-Death Valley region of California and the Bonneville region of Utah.
Sources, or at least a few good books which say a few of the things that I've said above:
(c) Matthew White 1996