The Loess Zone's North-South
and East-West Gradients

The distance between the Loess Hills' northernmost point at the juncture of the Mississippi and Ohio Rivers near Cairo, Illinois, and the Hills' southernmost point in Louisiana's Tunica Hills, is about 500 miles. This is far enough to accommodate significant climatological differences. One measure of these differences is that the average January temperature at Cairo is 35°, while in the Tunica Hills it's about 52°. These differences are profoundly important to plants and animals.

One way to grasp the significance of the differences between winter in the Cairo area and winter in Louisiana's Tunica Hills is to read Organic Gardening magazine. In January, for instance, the only gardening activity the magazine advises for gardeners in Cairo's Gardening Zone 6 is to start a few things indoors, under lights, toward the end of the month. In contrast, gardeners in the Tunica Hills' Gardening Zone 8 are urged to sow beets, carrots, English peas, herbs, and wildflowers outdoors.

There are rainfall differences, too. Around Cairo, the average annual precipitation is about forty-five inches; in the Tunica Hills, it's about fifty-eight.

If you should travel the Natchez Trace from Nashville to Natchez, you would see that the most conspicuous change in the forest along the road would be from broad-leafed deciduous forests on the Trace's northern end (oak-hickory), to mostly pine forests on the southern end. The transition occurs rather abruptly in northern Mississippi.

If a highway like the Trace ran between the Loess Hills' northern and southern ends, you'd see similar changes, especially if you traveled during the winter. In Kentucky and Tennessee's part of the Loess Hills, the winter forest is an essay of brown and gray. But in Mississippi and Louisiana, Southern Magnolias with their broad, evergreen, glossy leaves appear in the understory. Water Oaks, with their semi-deciduous leaves become common in the south, also adding extra greenness to the winter forest. And, at the southernmost point, graceful Spanish Moss appears festooning the trees. These three species by themselves give the southern section of the Loess Hills' winter forests a considerably less somber appearance than that of the Hills' northern forest.

When you look at distribution maps showing where organisms occur, you notice that species with northern affinities often show range extensions thrusting southward through the loess zone, but species that are predominantly southern hardly ever extend northward through the Loess Hills. Often it seems that the loess zone behaves as a kind of corridor inviting northern species southward. There are reasons for this, which we discuss elsewhere in this site.

East/West gradients also occur in our string-bean-shaped Loess Hills zone. This gradient has a cause other than climate, however; it rests on the fact that all through our loess zone, the loess is thickest (up to 200 feet thick) in the west, and then gradually diminishes toward the east, until it disappears. The deeper the loess, the more profoundly the biology it supports is affected.

In our section on trees, this gradient is very well documented by a study looking at how the forest's composition of tree species changes, depending on how deep the loess is beneath it.

Therefore, as we travel in our Loess Hills, nature is always changing, no matter what direction we take. This is just one more feature that makes traveling in the Hills uncommonly interesting.