By Gino Graziano
The snow was deep, which resulted in inappropriate shoes being lost in the snowdrifts, yet the Anchorage students were undeterred as they pored over the trees’ branch tips for their place-based science project. The data, which was checked by myself and trained teachers, proved that the students correctly identified the tree species and the number of moose bites on their branch tips. Determining bite variability and the resulting tree architecture on different species of trees was core to the research question the students were answering: How does the spread of invasive chokecherry trees (Prunus padus and virginiana) in Anchorage forests impact how moose browse native trees and shrubs?
Excessive bites (browsing) by moose can lead to damaging changes to tree architecture that results in the appearance of an upside-down broom and reduces fecundity and survival of the tree. By combining their browse data with that from other schools, my group of students found the invasive chokecherry were not as impacted by moose browse as native trees, and as the density of a chokecherry infestation increased, so did the rate of native trees that resembled upside-down brooms.
Why worry about chokecherry?
In 2017, I gained an interest in the chokecherry invasion of Anchorage forests while at the Alaska Invasive Species Workshop where I saw a poster featuring a youth-led study on chokecherry. The students compared the diversity of understory vegetation in areas with and without chokecherry and found that diversity decreased with the invasion of chokecherry. Later, anecdotal observations of fewer insects on these trees lead to a formal study of impacts on salmon food webs since these insects can make up a significant portion of the diet of young salmon. University of Alaska Fairbanks graduate student David Roon placed small rafts under trees to catch insects falling into streams from native trees and invasive chokecherry on Chester and Campbell creeks in Anchorage. Roon found fewer insects and decreased diversity of the insects falling into streams from the chokecherry than the native tree species.
There are no native prunus species in Alaska. The introduction of chokecherry began in the 1950s as horticulturalists searched for landscape solutions in a challenging environment. The pretty flowers, cold hardy nature, and general resistance to moose made Prunus padus (European bird cherry or May Day tree) and P. virginiana (Canada red variety of common chokecherry) excellent candidates. University Extension Service horticulturalists and other forestry, agriculture, and wildlife professionals promoted planting the tree. The attractive flowers and fruit that birds devoured increased the popularity of these trees. However, by the late 1990s and early 2000s, resource managers noticed these trees were spreading into forests around Anchorage, and Fairbanks. This led to resource managers and invasive plant specialists becoming concerned and seeing a need to control the chokeberry’s spread.
When invasive plant managers and the University Extension Service began educating nurseries, other retailers, and the public about the spread of this species, they were initially met with resistance. Early on, invasive plant managers discussed chokecherry as a species to watch out for but hesitated to call the popular tree invasive. Scientific evidence that chokecherry could impact resources such as salmon continued to mount, and some nurseries stopped selling the tree. Then in the winter of 2010 and 2011, the Alaska Department of Fish and Game confirmed that three calf moose had died after browsing on ornamental chokecherry trees growing in a homeowner’s yard. The toxin cyanide is present in branch tips, and it’s why these trees are resistant to moose; it’s also the likely reason why the Anchorage students found less evidence of browse on chokecherry. Having seen that cyanide can lead to the death of moose, an iconic species of Alaska, the people in Anchorage took notice of the extent that chokecherry had invaded the surrounding forests and increased support for management.
Developing a management strategy
Management of chokecherry in Anchorage and elsewhere in Alaska has challenges. The trees, when cut down, vigorously sprout from exposed trunk and underground roots, which makes herbicide applications necessary for complete removal. Alaska residents, however, are generally opposed to using herbicides on public lands. Invasive species managers initially targeted smaller trees using weed wrenches, a specially made device that can “wrench” small trees from the ground. In Anchorage, the annual Weed Smackdown event draws in anywhere from 50-100 volunteers to wrench chokecherry trees out of forests and parks. Thick infestations, however, are often a network of shoots connected to larger trees and make it impossible to not leave some root fragments behind. Those root fragments lead to quick regeneration, and the public realized that herbicides were needed to manage infestations.
Although the need for herbicides to manage chokecherry infestations was apparent, the community didn’t support broadcast herbicides throughout the parks, and neither did the invasive species and land managers. Direct treatments were used, such as applying herbicide on the cut surface of a stump or squirting small amounts into a cut in the cambium layer. These methods are still used today but have proven inefficient for larger infestations. In these instances, invasive species managers are using injection guns, and basal bark treatments that apply specially formulated herbicides to the bark on the base of a tree. The University’s Cooperative Extension Service is presently studying basal bark treatments to understand efficacy, impacts to native vegetation, and herbicide fate in the environment. All these combined efforts have significantly advanced removal of chokecherry from Anchorage forests.
In Anchorage, efforts are focused on removing large seed-bearing trees from yards and parks to slow the spread, while continuing efforts to address smaller trees before they mature. Dense infestations will require continual monitoring to remove any regeneration.
Other communities are also noticing their own infestations of chokecherry. Seeing what happened in Anchorage, these communities, beginning in Hope, Alaska, have jumped on removal. Residents are removing old mother trees, and events are held to wrench out or treat feral trees with herbicide. Other communities, such as Talkeetna, Homer, Soldotna, and Juneau are beginning these eradication efforts too. These smaller communities have an opportunity to eradicate infestations before they are as difficult to manage as those in Anchorage. In response to community efforts, the Alaska Division of Forestry with support of the US Forest Service, State and Private Forestry is piloting a remove and replace program in select communities.
What we’ve also learned from Anchorage is that we must be vigilant in surveying beyond community green spaces since additional infestations are found along rivers and trails. We have found chokecherry trees in the Chugach State Park and Chugach Mountains around Anchorage. There is also a high potential that chokecherry is already moving up and down rivers because of birds and bears. However, because the public and industry is supportive of eradicating chokeberry from Alaska’s forests, we are hopeful that the spread will be controlled.
Gino Graziano is an invasive plant specialist with the University of Alaska Fairbanks, Cooperative Extension Service. He can be reached at 503-504-5143 or firstname.lastname@example.org.