Beech Bark Disease: The Oldest "New" Threat to American Beech in the United States
Abstract:Beech bark disease (BBD) has been killing American beech trees in eastern North America since the late 1890s. The disease is initiated by feeding of the beech scale insect, Cryptococcus fagisuga, which leads to the development of small fissures in the bark. Over time, as the population of scale insects builds on the bark, the small wounds provide entryway for fungal infection by one of the species of Neonectria. As the fungus invades, it kills the inner bark tissue and, may completely girdle the tree, leading to death. Cankers may form as the tree attempts to fight the infection, resulting in wood defects and often trees are weakened to the point that they are susceptible to splitting during windy conditions. Large numbers of severely deformed American beech persist in long-affected stands and their propensity for root-sprouting can result in the dense beech "thickets" that prevent other species from establishing, while offering little economic or ecological value. Consequently beech bark disease has the potential to alter the species composition of the forests it occupies. Generally, three phases of beech bark disease have been recognized: the "advancing front", which refers to stands where beech scale is present, but Neonectria infection has not occurred; the "killing front" which represents areas where there are high levels of scale infestation and Neonectria infection is present (typically 3–5 years after scale appears, but can be 20 or more years) leading to heavy beech mortality; and the "aftermath forest" which describes stands that have had heavy mortality in the past and still retain scale and Neonectria populations, but at a lower density. Beech trees remain that are typically of smaller diameter, often of root-sprout origin, and mostly deformed and declining. Damage from this disease complex has been significant in areas throughout the eastern United States where American beech is an important component of mixed hardwood stands. As the disease moved through the New England states, typically 50% of beech trees were killed and many more rendered highly defective from cankering. The loss of beech can have a significant impact on wildlife as beech provides food and habitat for more than 40 species of birds and mammals. The beech scale insect is believed to have been introduced from Europe on an ornamental European beech in the Halifax Public Gardens, Nova Scotia, Canada in the late 1890s. By the 1930s, beech bark disease was well established throughout Nova Scotia and had spread into the United States. In 1932, the disease was reported in Maine and by 1960 most of New England and part of New York were infected. The disease had invaded northeastern Pennsylvania by 1975 and slowly spread into New Jersey, Tennessee, and as far south as North Carolina. The slow movement of this disease can be attributed to the brief phase in the insect's life cycle, immediately after hatching, when it has legs and is mobile. The newly hatched "crawlers" usually move to a different area on the same tree, but they also can be carried further by wind, birds, or animals. Most of the isolated scale outbreaks that have been reported were in scenic areas frequented by campers and tourists, suggesting that humans also play a role in moving the scale long distances. The recently documented infestations in Ohio and Michigan have renewed interest in the disease as previously unaffected forests are now experiencing high levels of mortality. In Ohio, the scale outbreak was first identified at the Holden Arboretum and low numbers of scale were reported in four additional surrounding counties. However, the presence of Neonectria fungi was not confirmed until nineteen years later in 2003. In Michigan, the beech scale infestation and presence of two Neonectria species of fungi were documented simultaneously in 2000. By the time of this discovery, the scale infestation was already well established in seven counties in eastern and northern Michigan. The extent of the infestation and the presence of Neonectria, together with anecdotal evidence indicated that the scale insect had been in Michigan for at least ten years. Beech bark disease, the "new" threat to the forests of Michigan and Ohio, was confirmed more than 100 years after it was first introduced into North America.
Document Type: Research Article
Publication date: 2010-04-01
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