Pines on the Rocks

Starved Rock State Park really doesn't look like it belongs in Illinois:

White Pines (Pinus strobilus) in French Canyon, Starved Rock State Park, La Salle Co, IL 1/26/2014

The green trees on those cliffs are Eastern White Pines (Pinus strobilus) and this is pretty far south for them. In the Midwest, you only find small patches of them south of Wisconsin and Michigan, whereas they're a major component of the North Woods in the upper Great Lakes. (They do occur well south in the Appalachians, but at higher altitudes than you can find in Illinois.)

The same sort of pattern exists for this tree:

Northern White Cedar (Thuja occidentalis),
Starved Rock State Park, La Salle Co, IL  1/26/2014

Only in spades -- even in the Appalachians, White Cedar* (Thuja occidentalis) only occurs in small patches south of New York. In places where they're common, they're found in wet lowland areas, but they will also grow on cliffs like you find in Starved Rock.

Why do we see these little outlier populations? One possible explanation is that they were planted by people, but it's hard to see why people would be planting White Cedars here. Some sort of odd dispersal event is also possible, but it seems unlikely that two different species would show up together in the same location. That leaves a scenario wherein these plants are relicts of an earlier, more southerly distribution.

Since we know from geological and paleoecological evidence that the upper Midwest was glaciated 11,000 years ago and pines and even spruces reached south-central Illinois during glacial periods (1), this relict scenario has a lot going for it.

But why here and not elsewhere? Those cliffs visible in the first shot and more clearly here give the

answers. First, the little canyons that these cliffs form all face north, which means that they are short on sunlight and therefore stay cooler than surrounding areas. Second, with the rock so close to the surface, soil is at a premium, and therefore many plants can't survive there. Those plants that can have a spot all to themselves.

Both species mentioned above are quite capable of growing in rocky soils, although they both grow better in deeper soil. White Cedar especially is known for growing on cliffs, and when it does so, it grows very slowly and can reach ages of 1000 years. This sort of longevity increases the chances that a population can persist in small pockets for us to find, making the relict population scenario even more likely.

These little relict populations are an important reminder that the natural world isn't static. Communities are always changing, and species distributions grow and shrink for all sorts of reasons. Those changes do accelerate greatly when modern technology gets involved, of course. Still, this concept of an ever-shifting natural world produces an interesting dilemma for conservationists. We want to protect ecosystems that, often enough, have gone from common to very rare through human activity. (Think tallgrass prairie.) Frequently there is so little left that the loss of even one site is truly problematic. But the systems themselves are built on change, and eliminating that change means interrupting some of the very processes that we're interested in preserving.

The pines and cedars at Starved Rock are holdouts from a time when their compatriots held sway over vast areas of the southern Midwest. Those other trees disappeared with the slow warm-up that happened as the Wisconsin glaciers retreated -- a natural process without, as far as we can determine, any anthropogenic basis. There's no real reason to believe that, absent a human presence, they wouldn't be slowly disappearing even now. So how do we preserve them while allowing that natural process to occur?

I don't have an answer to this question, of course. There isn't going to be one single answer; at the very least, it's going to depend upon the specific characteristics of the system in question. In our real world, it's also going to depend upon what we value about each site, and why, and far too often on what we can afford to do with that site. But as always, ignoring the question is just another way to answer it.

* There are a number of trees in North America called cedars. The original Cedar, (Cedrus libani), is in the family Pinaceae and occurs from Lebanon and Israel north into Turkey and east into Syria and Jordan. Three other species of Cedrus grow in the Himalayas (C. deodara), the Atlas Mountains of North Africa (C. atlantica), and the island of Cyprus (C. brevifolia). North of Mexico, species from 5 different genera (Thuja, Juniperus, Chamaecyperus, Calocedrus, and Cupressus) are graced with the name - they're all in the family Cupressaceae, or Cypress family.

(1) Grüger, E. (1972). Pollen and seed studies of Wisconsinan vegetation in Illinois, USA. Geological Society of America Bulletin, 83(9), 2715-2734.