The city of Northfield recently published a press release confirming that emerald ash
borers have started to infest Northfield ash trees (see northfieldmn.gov). This is bad news for
the campus arboretum, as green ash (Fraxinus pennsylvanica) makes up 20-30% of the trees in
the lower Arb.
Green ash is found in floodplains and moist upland forest (see the Cowling Arboretum
website for more information), making the Lower Arb the perfect habitat. A green ash can
normally survive an initial emerald ash borer infestation, but, after that, the tree almost inevitably
perishes. The beetles feed on the tree’s cambial tissue, which is the growth tissue found
underneath the bark.
A telltale sign of infestation is the presence of woodpeckers. Woodpeckers enjoy eating
the larvae of emerald ash borers; therefore, multiple woodpecker holes in an ash suggest that the
tree is infested. Another sign of infestation is the presence of cracks in bark, which occur due to
tunneling larvae splitting the bark open.
It is sad to envision the Lower Arb depleted of its most common tree, but all is not
lost: there are already ideas for the ash’s replacement. The swamp white oak and sycamore are
two species that have the potential to replace green ash. Swamp white oaks (Quercus bicolor)
are a good option both because they are native to southeastern Minnesota and because they
can tolerate heavy and wet soil (see umn.edu). Sycamores (Platanus occidentalis) also do well
in floodplains and are both easy to propagate and quick to grow (see iastate.edu).
The unfortunate decimation of ash trees serves as a lesson for future reforestation
efforts. Without a diversity of tree species, disease can easily wipe out entire swaths of forest.
—Ella Daniels-Koch ’25 for the Cole Student Naturalists
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