Maples, ashes and other hardwood trees are being harmed by an invasive weed that indirectly slows their growth to about one-tenth the normal speed, scientists say.
The researchers, who report in Tuesday’s issue of the journal Public Library of Science Biology, studied how a tree-stifling weed called garlic mustard impacts native hardwoods in Ontario.
The lead scientist warned that the weed’s ultimate effect could be to stifle dominant trees in the forest and even change the type of trees that can grow in the habitat.
Until now, few studies have looked at how invading plants change the delicate ecological balance of native species.
Researchers focused on garlic mustard, which invaded the United States in the 1860s from Europe and has since spread to 30 states and Canada.
Weed harms fungus that aids tree growth
The weed harms hardwood trees such as sugar maples by releasing chemicals that harm a soil fungus that the trees need to grow and survive, said the study by Canadian, American and German researchers.
The fungi have long filaments that penetrate the roots of plants, forming a network to extend the reach of a tree’s roots.
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In exchange for providing seedlings with nutrients, the fungi receive energy from the plants.
Plant population biologist Kristina Stinson of Harvard University and her colleagues tested how well native hardwoods in Ontario were able to form the fungal relationships after garlic mustard invaded.
Sugar maple, red maple and white ash showed less fungal colonization and grew only about one-tenth as fast in soil infested with the weeds, they say in their study.
Seedlings grown in soil free of both the weed and the fungi also showed similar reductions, which supports the idea that tree growth suffers without the fungi.
Only hardwoods affected
Adding extracts of garlic mustard to soil also impaired fungal colonization and seedling growth. Studies on 16 other species of seedlings showed only hardwoods were harmed by garlic mustards.
“This suggests garlic mustard invades the understorey of mature forests by poisoning the allies of its main competitors,” Stinson said in a release.