Weighing the impact of the Emerald Ash Borer on Boulder’s ash tree population
2020 Trends | Urban Land Cover
Since its arrival in the 1990s, the Emerald Ash Borer (EAB) has spread rapidly throughout the U.S. and Canada, killing more than 100 million ash trees and wreaking havoc on local municipalities in the form of costly mitigation. The EAB spread to Boulder in 2013 and has since killed thousands of Ash trees, with thousands more waiting for it. Here we explore Boulder’s tree inventory data to better understand the impact of EAB infestations in Boulder today and into the future.
The Emerald Ash Borer (Agrilus planipennis) is an iridescent green beetle, native to Asia, that has made its way to the United States, most likely through the transport of wood-based shipping materials. The Emerald Ash Borer (EAB) was first found in southeast Michigan in 2002, but the USDA estimates that EAB had been here since the 1990s based on the size of the infestation.
The consequences of this insect pest have been devastating. The Emerald Ash Borer (EAB) has killed more than 100 million ash trees from Massachusetts to Colorado and has another 8 billion or so waiting for it. The costs of the EAB hover around $1 billion each year in the U.S. That figure comes from studies by the U.S. Forest Service, and it only takes into account the removal, treatment and replacement of trees in urban areas. Another estimate, from a team of U.S. and Canadian researchers, placed annual damages at $1.6 billion including the loss of residential property values and timber. EAB infestation is almost always fatal to infested ash trees, unless treated, and infested trees will be dead within approximately four years. EAB is the most destructive forest pest in recorded history.
The EAB was first identified in Boulder in September 2013. This incidence marked the western-most extent of the EAB infestation in North America. As of 2018, EAB has only been found in Boulder County, although it has been found in several cities outside the city of Boulder.
Emerald Ash Borer / City of Grand Prairie
According to Boulder’s Urban Forest Strategic Plan (2018), tree cover in the city is approximately 16%, as measured in 2013. Ash tree losses due to EAB are expected to reduce that cover by 25% in the coming years. The city is continually working to monitor EAB infestations through visual surveys, traps, and destructive sampling.
The urban tree inventory managed by the city of Boulder provides information on where trees occur on public property throughout the city. It also provides information on where EAB treatments and removals have taken place. Here we explore the tree inventory data to better understand the impact of EAB infestations in Boulder today and into the future.
explore the data
Trees infected with Emerald Ash Borer in the city of Boulder in 2019
Figure 1. Ash trees on public lands in the city of Boulder, including those with EAB.
Green points represent the locations of ash trees managed by the city of Boulder. Brown circles represent ash trees with an EAB infestation.
Source: Data provided by the city of Boulder’s urban tree inventory and offers the best available knowledge of tree species located on public lands. The inventory accounts for about 20% of Boulder’s tree canopy, although for ash tree species, the inventory includes a much larger percentage. The inventory does not include trees located on private lands.
Ash trees removed from the city of Boulder inventory before 2019
Figure 2. Ash trees in the city of Boulder, 2019. Green dots represent ash trees in the 2019 urban tree inventory labeled "active" and gray dots represent green dots in the 2019 urban tree inventory labeled "removed". There are 4921 green dots and 204 gray dots. Note: we have no way of knowing why trees were removed from the inventory - only that they were removed, i.e. just because an ash tree was removed does not mean that it had EAB.
Source: Data provided by the city of Boulder’s urban tree inventory and offers the best available knowledge of tree species located on public lands. The inventory accounts for about 20% of Boulder’s tree canopy, although for ash tree species, the inventory includes a much larger percentage. The inventory does not include trees located on private lands.
Why is the EAB so destructive?
Unfortunately, EAB infestations aren't usually visible until the trees are already half-dead. The borers are smaller. They lay their eggs covertly in cracks and crevasses in the bark of ash trees. Their larvae feed exclusively on the phloem, the part of a tree that carries nutrients from the leaves, killing their hosts with uncommon speed. And they spread fast, unpredictably.
Lab tests show they're capable of flying as much as three miles in a day, although most adults fly less than ½-mile from their emergence tree. Larvae carried along in firewood can go much farther, which is probably how the EAB reached Boulder, which is hundreds of miles from any previously discovered infestation.
Management strategies and preventing transmission
The city of Boulder’s Forestry division and partner agencies have continually worked on detection methods through visual surveys, traps and destructive sampling to determine and monitor the extent of the EAB infestation. The interagency Colorado Emerald Ash Borer Response Team, comprised of nine agencies and organizations, is working with communities to help manage the spread and impacts of EAB. Starting in 2013, the EAB Response Team and partners worked to complete an initial survey to determine the extent of spread of EAB in Colorado, and the team continues working with local governments to determine and map the extent of infestation.
According to Boulder’s Urban Forest Strategic Plan (2018), tree cover in the city is approximately 16%, as measured in 2013. Ash tree losses due to EAB are expected to reduce that cover down to 12%, with anticipated losses of hundreds to thousands of ash trees. Because of these expected losses and continued climate change stressors, the city of Boulder’s forestry department has set a goal to restore and maintain the the current tree canopy cover around 16 percent.
Learn more about the Urban Forest Stategic Plan with this video:
Figure 3. The common names for the most frequently occurring trees in Boulder, sized by number of occurrences. Larger letters in the word cloud indicate a higher number of that tree species in the tree inventory. Source: Occurrence is based on data from the city of Boulder’s tree inventory and offers the best available knowledge of tree species located on public lands. The inventory accounts for 20% of Boulder’s tree canopy, although for ash tree species, the inventory includes a much larger percentage. The inventory does not include trees located on private lands.
recommendations
Managing the spread of the invasive EAB to protect surrounding cities and counties.
Taking climate change into consideration is important when determining the potential impacts of the Emerald Ash Borer and the spread-risk to nearby counties. When an ash tree dies or is removed, we also need to determine proper replacements. With this essential tree cover shrinking, we will see reductions in the ecosystem services provided by urban tree canopies. By diversifying tree cover in urban areas, we can also provide some immunity to trees farther away from highly infected zones, as these insects aren’t as successful at spreading without some form of human-based mode of transport.
By using preemptive strategies to target susceptible trees and strict monitoring programs for the transport of potentially infected wood products, we can prevent the spread to areas that still have healthy ash tree populations.
If your neighborhood is concerned about the impacts of the Emerald Ash Borer on your local trees, talk with an arborist in your area to develop an action plan.
Boulder County’s Emerald Ash Borer Management Plan is another great resource to use when considering the management of vulnerable ash trees in your area. Most ash trees are located on privately owned land, so the residents of the city and county of Boulder have the potential to make a huge impact on this initiative to save ash tree populations in Colorado.
Get involved as a Tree Tender! The Tree Trust is a program led by the PLAY Boulder Foundation to work with Boulder residents to plant and care for trees, as well as to support a robust urban canopy and the ecosystem services that trees provide.
