How is it, when millions of native elms died from a virulent disease all across the county, elms in the South pretty much laughed it off?
Dutch elm disease has been catastrophic for American elms trees in
much of the northern tier of the county. And because so much of the
advanced research on trees is focused on northern forests, most folks assumed that American elms in the rest of the country were dying at the same rate.
Only in the last few years did scientists begin to scratch their heads
about the persistence of southern elms, which were thriving in spite of the fact that Dutch elm disease had clearly invaded here as well.
For years, the few scientists who paid attention thought that trees were surviving here because the disease and the insect that carried it were less common in the South. But there were problems with that hypothesis, including the fact that the invasive European beetle that carried the disease was producing three generations per year in the South.
And then there was the curious case of a few horticultural clones of
American elm that thrived and were disease-immune wherever they
were planted, even in the core of the area devastated by Dutch elm
disease. When researchers at the National Arboretum began studying these trees, they discovered they had an odd number of chromosomes. Because these trees were nursery stock, no one was really sure where the parents came from. But it was pretty clear that at least one of the parents of these trees was very different from any American elm ever studied.
And so for the first time – and just a few years ago – USDA researchers began to test elms from farther south. The American elms north and south may have looked alike, but molecular testing showed they were so different genetically, they might as well have been different species.
For decades, scientists assumed that all American elms had an extra set of chromosomes. Chromosomes pretty much drive the genetic inheritance of all living things, and “normal” creatures (including almost all humans) carry only two sets of chromosomes, one from each parent. But all the American elms tested in cities like Boston and Minneapolis had four sets of chromosomes – twice the normal number. It may sound kind of freakish, but this happens to plants and animals. It cansometimes result in a fatal defect. With plants, however, those extra sets of chromosomes can provide some benefits, even helping a plant grow faster or bigger, or withstand competition or severe conditions.
But when scientists finally got around to testing American elms in the South they were in for big surprise. Many of those elms were “normal” diploids, with only two sets of chromosomes. Trees from the Cumberland Plateau – which stretches from Birmingham and Huntsville north through Kentucky and southernmost Ohio – stood out as being consistently diploid. It’s probably no coincidence that these trees were in the heart of elm diversity in North America, and represented an area where DutchElm disease had been much less troublesome.
What was going on?
It doesn’t make sense unless you understand the repeated changes in climate that brought glaciers as far south as Washington, D.C. Those glacial outbreaks occurred frequently over the past several million years, wiping out forests (and everything else) in their path. The most recent was one of the most intense, and it happened only about 50 forest generations ago. Only forests in warmer refuges close to the Gulf Coast or the southern Atlantic survived.
When glaciers retreated as climate warmed again, they left a freshly
plowed and virtually lifeless landscape. Trees like the American elm
rushed in to take advantage of all that fertile mud. The trees that got
there first had a big advantage. Once they took hold, they dominated, and it was more difficult for others to follow. There’s a downside to that. Scientists call it the “founder effect” – the populations of trees that took hold in the northern states typically carried the genes of only a few parents. They were in essence in-bred compared to trees that had avoided the glaciers farther south.
There was another issue that may have been just as important. The
American elms that could best take advantage of the land rush for the northern territory were obviously polyploids – trees that had a defect that caused them to have extra sets of chromosomes. Those extra chromosomes probably helped the trees cope with the frigid conditions on the retreating edges of the glaciers. Those polyploid changes may have even allowed the trees to reproduce and grow more quickly.
Now, however, we know there was a cost. Those polyploid trees up
north had cell walls and vascular systems that seem to have made them more susceptible to Dutch elm disease. Because their genetics were so limited overall by the founder effect, it’s also likely that they didn’t have the genetic bench that would allow them to
adapt to new diseases. And as is the case with many polyploids, their ability to trade genes with other elms was likely compromised, which further limited their ability torespond to disease.
This recent discovery of two distinct types of American elms has only led to more questions.
Are the diploid “American elm” trees from the Cumberlands the “original” American elm that never made it up north? Are they in fact an entirely new species of elm?
Because trees with extra sets of chromosomes can’t easily mate with trees that have the normal number, these two elm groups are sexually isolated – one of the majorprerequisites for determining new species. University of North Carolina taxonomist Alan Weakley notes that these two groups of American elms appear to have been on a different trajectory for some 15 million years.
And if difference in chromosome number is the primary reason for
disease resistance of the Cumberland elms, then why are so many other elms in the south – regardless of species or ploidy levels – apparently resistant to Dutch elm disease?
And by the way, where’s the borderline between the “normal” southern diploids and the polyploids? Do Paint Rock’s American elms belong to the older diploid group, or to the more recent polyploid group that pioneered up north? Or do the forests of Paint Rock actually support both?
The truth is, no one knows. The study of Southern elms – which
apparently represents the foundation of elm inheritance in eastern
North America – is still in its infancy, and only one tree from Alabama has ever been sampled for its DNA. Let me repeat: Only one tree. In the heart of elm diversity.
Given what’s at stake – the future of American elms throughout North America – don’t you reckon it’s time we started working on that? I get excited about the possibility every time I walk up the first hillside in our census plot. Within a hundred yards, four species of elm are common, including American elms that may hold the key to Dutch elm disease resistance nationally. We’ll soon have mapped the precise location of thousands of elm trees on our 150 acre forest dynamics plot.
It’s also important to note that this isn’t just a story about elms. It’s really a story about our poor understanding of species, and even what a species is, and about how the whole country suffers when we undervalue the genetic diversity of the Southern forest. That’s what we’re wrestling with here at the Paint Rock Forest Research Center, and we’ll visit that subject in more detail next time.