When the supercontinent Pangea started breaking apart 200 million years ago, a number of familiar plant species were already established. This is why there are maples native to Japan as well as Canada, and beeches to be found in both Europe and Eastern North America. Same with magnolias (China, Korea, Japan, America) pines (from the Arctic circle to Guatemala, the West Indies, North Africa, Europe, and America.) The list goes on, but you get the picture.

Once these related plants were separated by thousands of miles, they continued to evolve independently in response to their specific environments. So the maple became a small, draping tree with fine foliage in Japan, while its North American cousins are much taller and straighter. At the same time, pests and pathogens co-evolved with their host trees.

It was only in the early 18th century that these trees, segregated for hundreds of millions of years, began to reunite. Our own John Bartram was a pioneer of the reunification process, sending crates of American seeds to England, where plants like the Sugar Maple, the Tupelo, and the Red Oak had never been seen. He in turn received many foreign species in return. Asian plants had already made it to Europe on trade routes between England and the Far East, and Bartram gave them the final nudge to the New World.

Desegregating our planet's tree species has expanded the gardener's palette many fold, particularly when considering all the hybrids that have been developed from species native to different continents. We live in a world of endless choices regarding the size, color, habit, and type of bloom of our plants.

There's a collateral cost we pay in exchange for our panoply of choices; we're pretty much surrounded by exotic invasive plants. And now, we're beginning to see that there are some other serious downsides of our plants becoming world travelers. Many of our native trees are extremely threatened by exotic pests that co-evolved to bother but not destroy their foreign counterparts.

This has been happening for over a hundred years. It started with the Chestnut Blight, a fungus that co-evolved with the Japanese Chestnut, its natural host. While Japanese and Chinese Chestnut trees have decent resistance to the blight, our poor native chestnut was undefended and was efficiently exterminated from the American landscape.

A similar story can be told about our bygone native Elms, all laid down by Dutch Elm disease. More recent is the grave threat to American Ash species by the Emerald Ash borer, a beetle that hitchhiked from Asia, where it isn't fatal to their ash. Some biologists think that within ten years every ash tree in America could be gone.

It may be that the trees of our future will be those that have only one species, like the Gingko, the Franklinia, and the Sequoia. Because these trees have no living relatives, there is less likely to be a lurking pest or pathogen that has evolved to feed on them.

In the meantime, planting more than one specimen of any tree in your yard may not be the way to go. While the Ash borer is an immediate problem, there are other exotic pests on the horizon, hungry to prey on our unprotected native plants. Having a diversity of species- some native, some not- may be the best line of defense when the next tiny enemy army arrives.