The fourth blog post I ever wrote featured the American elm (Ulmus americana). And that seems totally appropriate, as the American elm is probably one of the most exquisitely beautiful trees that we can claim as our own. Most prevalent along the riverbeds in the midwest, the tree has lovely golden-orange fall color, large ovate, toothed leaves (with asymmetrical leaf bases) and beautifully fissured bark on branches which elegantly spread into a v-shaped canopy.
Sadly, too often any remaining specimens left -- especially those in the wild -- look like this, or worse. That's because since the 1960's the population of American elms has been decimated by Dutch elm disease, or various species of the fungus Ophiostoma. The vector (or carrier) for this pathogen is the elm beetle, which first emigrated to the United States via a delivery of Dutch logs shipped to Ohio to use as veneer for furniture. The beetle burrows into the bark of the elm to lay eggs. While borers like this can damage trees simply by this process, more dramatic is the resulting infection of the fungal pathogen. Even in areas where no beetles are present, one infected tree can pass on the fungus to a neighbor if the roots graft underground.
The tree's defense against this infection is to essentially close up shop; the canals of xylem and phloem close, blocking further spread of the disease. Unfortunately, at this point damage has been done and the tree slowly dies back. These photos well demonstrate the appearance of an untreated American elm. Ironically enough, European elms such as the English elm (Ulmus procera) are equally devastated by the disease and first contracted it in a shipment of American timber. (See Wiki for more.)
Land management strategies in some wild populations of elms and the famous allees in Central Park and along the National Mall keep the disease at bay. Frankly, I'm surprised these two trees (near the Yards Park in DC) have remained standing.
