Signs of Fall 12: The Extinction of North American Ash Trees

White ashes on Nature Trail (c. 2000) Photo by D. Sillman

One of my favorite sections of our old, campus nature trail was the first hundred yards, or so, that wound through a volunteer forest of white ash trees. The straight, graceful trunks of the ashes and their deep green, lance-shaped leaves was a wonderful border for the constantly changing forest of the trail. About four years ago, though, I began to see small, D-shaped holes in the bark of these ashes, and about two years ago the trees stopped making leaves. This last winter, storms blew down several of the trees, and the rest are standing dead alongside the trail.

Depending on the relative degree of lumping and splitting of species and subspecies designations, there are between 45 and 65 species of ash trees (genus Fraxinus) around the world. Ashes are found primarily in the northern (temperate) regions of Europe, Asia and North America and range from relatively modest sizes (like the 30 foot tall velvet ash) to trees of substantial heights and girths (like the 120 foot tall white ash). Eight ash species are found in North America where they have historically made up a substantial portion of our deciduous forests (it estimated that there are (or were) 8 billion, wild ash trees in North America). Ashes have also been extensively planted in urban and suburban habitats as ornamental trees and along urban and suburban street as shade trees. Ash lumber is used to make everything from furniture to baseball bats, and ash logs are a highly preferred and commercially attractive form of firewood.

Five of North America’s ash tree species, though, have recently been classified as critically endangered. They are teetering on the brink of potential extinction due to the impact of an exotic, invasive beetle from eastern Asia called the emerald ash borer (Agrilus planipennis). Our nature trail ash trees along with most of the ashes in Western Pennsylvania were some of the victims of these rapidly spreading beetles.

Photo by H. Russell, Wikimedia Commons

The emerald ash borer was first observed in 2001 in Detroit, Michigan. Ash trees in Detroit were mysteriously dying, and a small, iridescent green beetle was collected from the logs of the dead trees. The next year across the Detroit River in Windsor, Ontario these beetles were also collected from dead and dying ashes. Within a few years  the emerald ash borer was collected in forests from Minnesota to New York. Within a few years more, the beetle spread north and east across Canada, east and south to the Atlantic Ocean and the Gulf of Mexico, and as far west as Colorado. Currently thirty-one states and two Canadian provinces report the presence of the emerald ash borer. Researchers speculate that the emerald ash borer had been in North America for more than a decade prior to its 2001 “discovery” in Detroit.

The emerald ash borer probably traveled to North America in ash wood used for shipping crates or as packing material in the cargo holds of ships. From wherever it first landed, it then rapidly began its destructive expansion through our deciduous  forests.

Ash borer eggs, Photo by D. MIller, USDA Forest Service

Adult female emerald ash borers mate within a week of their summer-season (June to August) emergence and may fly ten kilometers or more in their search for a suitable ash tree on which they can lay their eggs. Eggs are stuck into the cracks and crevices of the outer bark of the ash and hatch into larvae in about three weeks. The larvae chew their way into the nutrient rich vascular layer (the “phloem”) just beneath the outer bark and begin to tunnel through and feed on this important tissue. Phloem transport sugars and other nutrients throughout the tree, and as it is destroyed, the tree becomes less and less able to sustain itself. After two or more years of feeding and growing the mature

Photo by D. Herms, Ohio State University, Wikimedia Commons

larva fold themselves up into a pupation chamber just beneath the outer bark where they wait out the winter. In April or May they pupate and transform into adults. Adults chew their way out of the pupation chambers (leaving behind their characteristic D-shaped exit holes in the outer bark) in May or June and quickly mate. Each adult female lays on average 55 eggs. Counting the exit holes on a single ash tree suggests that dozens to hundreds of adult ash borers may be emerging each spring from each infected ash tree.

According to entomologists at Ohio State University, 282 species of native arthropods (insects and spiders) rely on ash trees for their food and shelter, and 44 of these species feed exclusively on ash trees. Other Ohio State scientists have observed that in forests affected by emerald ash borers there are almost no ash seeds in the soil seed bed, and that after the death of the standing ashes, there are no ash seedlings germinating to replace them.  The sun gaps that form from the deaths of the ashes typically are filled up with fast growing, exotic invasive plants like oriental bittersweet, honeysuckle and multiflora rose which shade out and choke out native under-story plants and most of the potential tree seedlings. A dense, shrubby patchwork of invasive plants, then, replaces the destroyed ash forest.

Two dead ash trees, Photo by M. Hunter, Wikimedia Commons

The ash trees of East Asia have evolved mechanisms to control the emerald ash borer. Native, healthy ashes (like the Manchurian ash) are avoided by the adult female ash borer because of the ability of these healthy trees to make inducible, protective chemicals that can kill the borer’s larvae. Emerald ash borers instead seek out sickened, native trees or non-native species of ash that are not able to synthesize these protective chemicals. Some experiments here in North America have involved spraying specific plant alarm chemicals and hormones (like methyl jasmonate) on infected ash trees. Results of these experiments have suggested that these natural plant compounds may help the trees to fight off the emerald ash borer beetles as effectively as insecticides.

The emerald ash borer has been called the “most destructive and economically costly insect ever to invade North America.” It is ravaging the ash components of our wild forests and destroying the graceful ash trees of our cities and neighborhoods. Monitoring programs and attempts at quarantine and control have been ineffective in stopping or even slowing down this pestilence. Many forest scientists have already given up on trying to save these trees. Their extinction will occur, possibly, in the next few years! I mourn the loss of the white ashes of our nature trail and all of the billions of less seen ashes along our ridges and in our valleys.

 

 

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One Response to Signs of Fall 12: The Extinction of North American Ash Trees

  1. Donald Wicks says:

    Thanks for the information, maybe when baseball teams can’t buy any bats some people may be up set. Too late I’m sure.

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