In a forest landscape where some native trees are succumbing to invasive plants and pests, could a non-native species provide an ecological benefit? That’s a question Calvin Ritter raised in a study of bird species diversity in Norway spruce plantations. In 2020, Ritter, then a master of science candidate in environmental conservation at UMass Amherst, published the results of research he conducted under UMass wildlife biologist David King. According to Ritter’s observations, stands of non-native Norway spruce in his western Massachusetts study area hosted a similar number and diversity of native forest birds as did nearby native eastern hemlock stands. Although the scope of this research was limited, it raised uncomfortable questions about if and when – for the greater good of maintaining ecosystems – non-native species could serve as substitutes for declining native species.
“The reviewers expressed reluctance,” said King. “We had to stress we weren’t suggesting that more spruce be planted, only that we observed hemlock-associated birds using them at parallel rates.”
Anyone who has wrestled a backyard knotweed patch, battled garlic mustard, or observed the inexorable spread of emerald ash borer understands there isn’t enough sweat and money for total invasive eradication. Emma Marris, whose 2011 book The Rambunctious Garden was a live grenade in some conservation circles, advocates for a more adaptive approach to protecting ecosystems, one that accepts the presence of some non-native species.
“As the climate warms and humans remake biomes, ecosystems adapt,” she said. “When managers try to hew to the ‘pristine’ state, they impede other species from adapting and thriving.”
King likewise cautioned that the notion of a pristine state is itself illusory. “Today’s forests are anthropogenic, artifacts of arbitrary land use,” he said. “That doesn’t mean let every exotic run wild, but we should understand that what we’ve inherited isn’t natural. These are human forests.”
Norway spruce has been part of the Northeast’s “human forests” for almost a century. Many stands date to the 1930s, when the Civilian Conservation Corps planted the species extensively. Norway spruce is also a popular specimen tree in yards and parks. Although prevalent and often considered invasive, it does not spread as aggressively as more notorious non-native tree species such as autumn olive and buckthorn. Instead, it shares many traits with eastern hemlock. Like hemlock, Norway spruce’s growth form is tall and densely foliated, trapping heat beneath its boughs in winter and creating deep shade in summer. In areas of heavy hemlock wooly adelgid (HWA) infestation, Norway spruce may offer critical habitat for birds and other species that might otherwise decline as hemlock stands dwindle.
Ritter and King’s work confirmed that blue-headed vireos, Blackburnian warblers, golden-crowned kinglets, red-breasted nuthatches, and black-throated green warblers all nest and forage in Norway spruce stands. “Branch and foliage structure likely have a lot to do with it,” King said. He pointed to a study in Maine that showed how black-throated green warblers’ foot and leg anatomy is specifically adapted to perching in needled trees. “There it was red and white spruce,” he said. “But black-throated greens love hemlocks, and Norway spruce have roughly the same structure.”
Other ecologists have noticed examples of hemlock-associated birds using Norway spruce. For example, David Foster, a former director of Massachusetts’ Harvard Forest who now serves as the co-coordinator of Wildlands, Woodlands, Farmlands & Communities, has observed birds inhabiting Norway spruce stands on Martha’s Vineyard. “The Manuel F. Correllus Forest’s Norway spruce stands are magnificent,” he said. “Hemlocks aren’t found there, but the hemlock bird suite now nests in those spruce.”
While hemlocks are unlikely to go extinct, HWA has already altered ecologies from New England to Tennessee, and biocontrol efforts to stem the insect’s spread have not yet been effective. “It’s still unfolding, but I’m skeptical biocontrol can be effective at scale,” Foster said. Given that grim reality, land managers’ tolerance of Norway spruce – if not active encouragement of the species – may help maintain bird diversity in some HWA-infested areas. This kind of selective tolerance of introduced species tracks with Emma Marris’ perspective that some non-native species can play a positive role in local ecosystems. “Every case is unique,” she said. “A species’ origin shouldn’t predetermine what managers do. The goals should come first, then each species assessed in that light, making origin beside the point.”