U.S. forests transformed by series of invading species

By Jeff Mitton

As our world becomes smaller and people become more mobile and commerce becomes more international, species get moved from place to place at an ever-increasing rate.

Quite inadvertently, some are introduced and become established outside their native ranges, where they are referred to as introduced species, or invading species, or weeds.

This is the tragic story of the forests of North America, dramatically altered by a series of epidemics caused by a parade of introduced species.

When European settlers were arriving in New England, American chestnut trees were the largest and most common trees in the eastern deciduous forest. They grew tall and straight, up to 100 feet with trunks 14 feet in diameter.

But around 1904, the chestnut blight was first discovered in the New York Zoological Garden, probably introduced on imported Japanese chestnuts. A fungus that inflicts little harm in Asia causes the disease that was devastating in the stately American chestnuts.

The emerald ash borer, native to Asia and eastern Russia, was accidentally introduced to Michigan in 2002 in wooden shipping crates assembled in Asia. It spread in all directions, and, in 2013, city of Boulder foresters discovered a tree that had been killed by borers. This epidemic has already taken more than 100 million trees.In just 40 years, the blight killed about 4 billion trees, fundamentally and permanently changing the community structure of the eastern deciduous forests.

Dutch elm disease, caused by several fungi spread by several species of bark beetles, was introduced in 1928 in a shipment of lumber from the Netherlands.

The fungus causes little damage in trees in Asia, where it is native, but it caused an epidemic in North America. The disease is still spreading, but it has already killed more than 50 million elms.

White pine blister rust was introduced through the ports of Vancouver and Halifax in shipments of seedlings raised in Europe for foresters in North America.

The rust is a fungus that causes little harm in five-needle pines in Europe, but here it infects and kills most of the western white pines, sugar pines, limber pines, whitebark pines and bristlecone pines.

Since 1908, the blister rust has spread from Vancouver south to California and east to Glacier National Park, where it has already killed 50 percent of the whitebark and limber pines and infected most of the remaining trees.

It was first reported in Colorado in 1998 on limber pines around Red Feather Lakes and subsequently on both limber pines and bristlecone pines on Medano Pass and Mosca Pass in the Sangre de Cristo Mountains. It has been discovered in the last several years along the Peak to Peak Highway near Ward.

The emerald ash borer, native to Asia and eastern Russia, was accidentally introduced to Michigan in 2002 in wooden shipping crates assembled in Asia.

It spread in all directions, and, in 2013, city of Boulder foresters discovered a tree that had been killed by borers. This epidemic has already taken more than 100 million trees.

Adult ash borers feed on the leaves of virtually all species of ash in the genus Fraxinus, but cause little damage. They lay their eggs on bark and larvae tunnel into phloem, where they dig extensive, serpentine galleries, rendering the tree unable to deliver products of photosynthesis to tissues.

Infected trees die in three to seven years.

Vince Aquino, lead arborist at the University of Colorado, showed me infected ash trees on CU's East Campus. Adult borers had eaten the edges of some leaves and chewed holes in others. We saw several of the iridescent green beetles and found one pair mating on a leaf.

Aquino explained that numerous trees on campus were infected. Consequently, some of the large, uninfected trees on the historic quad near Old Main were marked with conspicuous green ribbons to indicate that they were at risk from emerald ash borers and are being watched carefully.

None of the many species of ash are native to Colorado's eastern slope or plains, but approximately 92,000 ash trees have been planted in Boulder, and about 1.45 million in the Denver metro area.

So we have an invading species killing a non-native ornamental. Having said that, we treasure our trees and consequently this epidemic will have a major impact.

The four forest epidemics — chestnut blight, Dutch elm disease, white pine blister rust and emerald ash borer — have much in common. Each was produced by accidental introduction of a species that is fairly innocuous in its native range.

But when these species gain access to naïve hosts that have not had the opportunity to evolve resistance, they kill most of them.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

July 10, 2015