A map showing vegetation zones of eastern North America at the end of the last glaciation. Look closely, and you’ll notice that sea level has dropped, allowing forests to extend about 80 miles south into the Gulf of Mexico. But up around the border of what is now Tennessee and Alabama, you’ll see the old frontline of North America’s deciduous forest.
I hope you appreciate how contrary Alabama’s climate is. We’re the wettest state in the country. Our springs start months earlier and our falls continue months later than springs and falls in most of the United States. And our humidity …well…
But what’s more important to our work here in Paint Rock, and what’s becoming increasingly important to all of North America, is that Alabama’s climate (along with neighboring areas in Georgia, northwest Florida, southern Tennessee and Mississippi) has always been exceptionally contrary, for hundreds of thousands and perhaps millions of years, through changing climate, multiple ice ages and glacial bulldozers. The last of those glacial explosions was actually surprisingly recent – about 50 to 75 forests ago, 18,000 years before the present. The Wisconsin glacial event, as it’s called, snuffed out almost all life north of Washington, D.C. Forests didn’t start over from scratch after those glaciers retreated. Some place – somewhere contrary and much, much warmer – had to be preserving those forests and sharing them with the rest of the country when the glaciers retreated.
What’s clearly made a difference in Alabama’s climate over hundreds of thousands of years is that churning ball of warmth, the Gulf of Mexico. Because big bodies of water lose and gain heat much more slowly than land or air, they tend to stabilize climate. And the Gulf/Caribbean ocean is a massive flywheel, collecting heat from the equatorial regions and spinning it up through the subtropics to the continental U.S.
It now produces so much heat, it exports a great deal of it via the Gulf Stream, which keeps England, Scotland and Ireland from becoming the Arctic countries they are adjacent to. But as the last ice age was at its peak, there’s evidence that the circulation of the Gulf Stream was cut short, so that the difference in temperature then between the coasts of Maryland and Georgia was greater than the difference in temperature between Maine and Georgia now.
And all that extra heat that didn’t escape? It likely stoked the relatively balmy conditions in the southernmost U.S., even as glaciers were crushing the world north of Maryland.
Understanding what that forest south of the glaciers was like, and determining what exactly was where, isn’t easy. There’s no controversy about forests in New England, Pennsylvania, Indiana, Illinois, Wisconsin, Michigan, most of Ohio: They simply didn’t exist. Under the glaciers, there was nothing but dirt, ground up rocks, and perhaps some hardy bacteria.
But imagining the forests south of the glaciers is much harder. The first and still the most important attempt to document the history of early southeastern forests was undertaken by the Delcourts – who reconstructed the forest history of North America by looking carefully at fossil pollen at multiple sites, including Goshen Springs in south Alabama, and Anderson Pond and Mingo Pond just above Paint Rock in Tennessee.
Don’t try to imagine forests that look exactly like the ones we have now. The much higher carbon dioxide levels of the past few hundred years makes it easier for leaves to freeze, for example. CO2 levels were less than half what they are now at the height of the last glaciation, which may help explain why trees we now consider freeze sensitive appeared to live side by side with trees we now know only from very cold areas. (And yes, it’s reasonable to wonder what happens to cold tolerance in plants as CO2 levels continue to rise – something that’s going to be increasingly important as changes in climate produce less predictable spring and fall frosts, as is happening this year throughout eastern North America.)
What’s increasingly clear, however, is that the world immediately south of those glaciers was very different than what we see now. The higher areas of the Appalachians were comparable to Alaskan tundra, and the lower Appalachians were boreal forest like you’d expect to find in northern Canada. And this, I should remind you again, was only about 50 forests ago.
Research by the Delcourts, and those who’ve followed, indicate that the there was a fairly distinct transition zone between those arctic conditions in the central Appalachians and the warmer forests stretching from about Birmingham and continuing about 80 miles south of Mobile Bay, as the growing glaciers dropped sea level dramatically.
The colder part of that transition zone was in central Tennessee, and appeared to be a strange mixture of deciduous forest trees and spruce and fir forests. But as Hazel Delcourt notes:
Full-glacial refuges for deciduous forest species may have also existed in south-facing gorges of the Cumberland Plateau and southern Appalachian Mountains in addition to bluffs along major streams in the southeastern United States.
If you’re wondering what a south-facing gorge on the edge of the Cumberlands and the Appalachians might look like, three prominent examples come to mind: Tallulah Gorge in Georgia, Little River Canyon in Dekalb County, and Paint Rock Valley.
Interestingly, the Delcourts and others also point out the potential importance of “sinkholes” as refuges for species during the last ice age. These sinkholes not only buffered the local temperature extremes. They also would have been important moisture refuges during multiple extended dry periods, from the peak of glaciation to the present. If you think sinkholes are insignificant, let me introduce you to Callaway Sinks, our two-mile wide and beautifully forested sinkhole that seems to be a refuge within a refuge.
So it’s no surprise that the pollen evidence from middle Tennessee suggests that the first post-glaciation forests resembled the moist forests of Paint Rock now, with ash, butternut, ironwood, moisture-loving hickories. The abundance of elm pollen, for example, provides still more evidence that the Tennessee/Alabama border seems to be the cradle of elm diversity in North America. A few of those elms developed special genetic characteristics that helped them survive extreme cold and follow the retreating glaciers northward. But many more species stayed behind in the Southern Cumberlands – the beautiful September elm that was once a signature tree of Birmingham and Huntsville; the newly discovered American elm population that is likely the disease-resistant ancestor of the disease-susceptible elms that went up north; the genetically distinct butternut that survives in spite of the fact that butternut has been lost almost everywhere else in North America. It’s as if they were waiting for a warmer world to spread their much richer genetic heritage.
You may have noticed, that warmer world is here, and getting warmer by the year. That’s why we’re here, and it’s why we’re here, in northeast Alabama, on what has been the frontline of forest survival over millions of years of climate changes.
Bill Finch Paintrock.org