By Laurel Haavik
Agrilus planipennis Fairmaire, a shiny green beetle from Asia commonly known as the emerald ash borer (EAB), has taken North America by storm. Assisted mostly by people, but also by its own wings, EAB is rapidly spreading across urban and forested areas alike.
EAB-killed ash trees in urban areas are noticeable and require immediate attention, with either insecticide protection or removal. This maintains safety, aesthetics, and function of the urban forest. Trees that die in natural forests hardly require such vigilance. If dead ash trees aren’t likely to damage property or injure people when they fall, they can often be left alone. Also, in most hardwood forests, ash is relatively less common than other trees such as oak or maple, so the structural and functional loss to the forest canopy may be minimal.
Loss of ash has serious ecological and economic implications, however, as ash fruits and seeds are an important food source for wildlife, white ash wood is used for baseball bats, and black ash is used to make baskets. Given these environmental losses and that this invasive species seems capable of killing nearly every ash tree it encounters, finding out how much damage EAB has incurred in our natural forests is a pressing issue.
In their recent article in Biological Invasions, Randall S. Morin and fellow researchers in the USDA Forest Service, Northern Research Station used a national forest inventory database to measure just how destructive EAB has been so far in the United States.
EAB has yet to invade many forests that do in fact have a sizable amount of ash in them, such as in northern Minnesota, northern Wisconsin, northeast Pennsylvania, and southern New York. As of 2014, EAB occupied only about 18 percent of the counties that contain ash trees in the United States.
The ability to predict how fast ash will die once EAB arrives in these forests will be useful. Morin et al used data from the USDA Forest Service’s Forest Inventory and Analysis to estimate how much ash died and how quickly this has happened in natural forests. The FIA is an extremely useful tool because it provides a systematic, annual, country-wide census of forest health across the landscape, giving researchers the ability to assess changes in forest composition and structure without the limits of space or time imposed by most studies.
Morin et al categorized locations sampled by FIA according to the year that EAB was detected (at the county level) and then measured the number and size (volume) of ash trees that died on an annual basis. They discovered that once EAB was detected in an area, the rate of ash mortality increased by up to 2.7 percent per year, with a corresponding annual decrease in live ash volume of up to 1.8 cubic meters per hectare. In areas that EAB had not yet invaded, there was an annual increase in ash volume of 0.3 cubic meters per hectare, which may represent the normal rate of forest growth.
The most noticeable loss of ash occurred five to 10 years after EAB was discovered in an area. This lag effect may be because, while EAB is not usually found until it has been in an area for years, it also takes some time for local beetle populations to grow large enough to kill trees rapidly.
Ash mortality rates have slowed in areas where EAB was first discovered, prior to 2002, though little ash remains in these forests. Perhaps it takes the beetles 10 to 15 years to eat through all the ash that they can. Future research might investigate if and why the remaining ash trees will die or persist.
Laurel Haavik is a postdoctoral researcher at the University of Kansas, where she studies the interactions between insects and the trees that they eat. Follow her on Twitter at @ljhaavik, and check out her blog Science Shapes Lives.