female Eastern Swallowtail on an Oak

Arbor Day and Native Trees

We recently received an envelope from the Arbor Day Foundation with the boldfaced title  “2019 Maine Tree Survey.” I was excited to fill it out until I skimmed the first few questions: “Have you ever climbed a tree?” “When you were a child, did you ever play under or amongst trees?” “Did you ever collect leaves, acorns, or pine cones for a school project — or just for fun?”

So the survey is apparently an exercise in nostalgia, intended to provoke an emotional reaction, not collect meaningful data. You’re supposed to skim these questions, fill in the bubbles, and realize that yeah, trees are pretty great. (“Do you ever relax in the shade of a tree?” it later wonders). But this isn’t the survey’s only objective. At the end of the document you can elect to receive some free seedlings: 10 “Fast-Growing Norway Spruce” trees (Picea abies) and two “Fragrant Lilacs.” (The latter isn’t the name of any particular species of lilac, so I’m guessing they mean the common garden variety, Syringa vulgaris.)

I’m not sure why the Arbor Day Foundation selected these two species, but there are far better choices for North America. If the intent is to encourage people to cultivate some sort of plant, then Norway Spruces and lilac do fit the bill. But if Arbor Day hopes to make a positive ecological impact with their distribution program (and why not?), they could be far more discerning. Setting aside the practical matter that the Norway Spruce is too enormous for many yards (mature specimens can grow over 100 feet tall) the issue here is that neither the lilac or Norway Spruce is native to this continent. They’re imports from Asia and Europe. And this means that many of our insects can’t eat them.

I’ve discussed the importance of supporting insect species with plants before, but here’s a quick summary. Insect populations are declining dramatically across the globe. This is bad news. Insects not only fulfill numerous ecological roles, they’re also food for many other organisms on the planet. (See the 2018 New York Times piece, “The Insect Apocalypse is Here: What Does it Mean for the Rest of Life on Earth?”) Part of the reason for this dramatic insect decline is loss of habitat. Insects depend on the presence of certain plant species in order to complete their life cycles. And where these specific plants are missing, the number of insects drops. (We’re losing monarch butterflies in part because they require milkweed for rearing young caterpillars.)

As gardeners, we typically think we’re supporting insects by providing pollen in the form of flowers. But we also need to grow food plants for the larval insects that don’t eat pollen. You can provide all the lilac nectar in the world for monarch butterflies, but the species will still die out without the milkweed stalks and leaves that feed the caterpillars. Other insects, like the female Eastern Tiger Swallowtail butterfly (pictured above), prefer oaks and other native trees as hosts. We need to maintain and improve the populations of these native plants because, as entomologist Doug Tallamy has argued, it can take centuries for an insect to adapt to eating the chemicals in a novel tree or plant species. So not just anything you buy at the local nursery or hardware store will do.

Fortunately, there are some new tools to help us make better choices. Consider the National Wildlife Federation’s website, Native Plants Finder (still in beta). You can use this website to find species appropriate to your region and even sort them according to the number of insects that use that genus/plant to host their young. For instance, goldenrod (Solidago spp.) apparently supports 124 species of butterfly and moth caterpillars. Oaks (Quercus spp.) like the one pictured above can be eaten by 431 different kinds of moth and butterfly larvae.

I don’t know for certain whether any larval insects in North America have adapted to eating the Norway Spruce, which has naturalized in Maine and other states, but 177 different caterpillars use our native red (Picea rubens) and white (P. glauca) spruce as a host plant. So why not choose the plants that we know support life? We don’t need to dig up our lilacs (I certainly won’t), but underplanting them with  milkweed would create a feeding station for young and mature monarchs alike. If the Arbor Day Foundation tailored its free plant offerings to different regions in the United States, offering (for instance) local species of goldenrod or serviceberry in lieu of lilacs and Norway spruce, it could support plant cultivation and make strides towards restoring the ecological health of our country. (And since they’ve distributed native species like the Eastern Redbud (Cercis canadensis) to certain regions in the past, I know this is well within their means.)

It could also improve its survey. A few suggestions:

Are you aware that tree species native to this continent support more insect life than others?

Have you ever watched a caterpillar climb a plant and wondered why it chose that plant?

Have you ever gathered just a few acorns and left the rest to grow into trees?

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