Researcher: Spotted knapweed may suppress natives via root chemicals and fungi

Rick Holloway, a researcher in the University of Montana Division of Biological Sciences, told a public lecture in Missoula that spotted knapweed (Centaurea maculosa) is able to form large monocultures on Mount Jumbo and Sentinel and that multiple research lines point to both direct and indirect mechanisms explaining its competitive success.

Holloway summarized experiments from his lab showing that festuca (Idaho fescue) roots slowed or stopped elongation when they encountered knapweed roots unless activated carbon was added to the soil. "With activated carbon the fescue roots grew much faster," Holloway said, describing the experiment in which activated carbon (added at roughly a 1:100 proportion) reduced the negative effect on fescue root growth. He framed those results as evidence that knapweed roots exude organic compounds that can inhibit native-root growth.

The nut graf: Holloway said allelopathic root exudation is only part of the picture. His team also reported that mycorrhizal fungi — the arbuscular mycorrhizae common on grassland plants — can change outcomes of competition. In paired-plant greenhouse trials, Idaho fescue planted with knapweed and with intact mycorrhizae had sharply reduced biomass compared with similar plants grown without mycorrhizae. Conversely, knapweed often grew better when paired with fescue and with fungi present. Holloway described these results as consistent with an indirect advantage in which knapweed benefits from fungal networks while natives suffer.

Key supporting details: in root-observation boxes with clear backs, Holloway said fescue roots slowed after contacting knapweed roots in plain sand; adding activated carbon to the substrate partially or fully restored root growth. In pot competition trials, fescue biomass was less than half when grown with knapweed without added carbon, and fescue biomass rose toward control levels when carbon was present. Holloway said knapweed biomass tended to decline slightly with carbon added. He cautioned that activated carbon can have confounding effects, but called the pattern "reasonable evidence" that root exudates are involved.

Holloway also described mycorrhizae experiments: knapweed alone showed little growth response to presence or absence of fungal partners, but when knapweed and fescue were grown together the presence of mycorrhizae depressed fescue and tended to improve knapweed performance. He cited earlier published work (Grime, 1989) showing radio-labeled carbon moved from grasses into Centaurea via fungi in microcosm experiments and said his lab's isotopic work similarly suggests resource transfer through fungal networks is possible.

On mechanisms beyond allelopathy and fungal mediation, Holloway said his team observed that knapweed can be a very fast phosphorus scavenger: in a small radiotracer trial a knapweed plant rapidly took up a phosphorus spike and suffered phytotoxic effects from labeled phosphorus, implying high uptake rates. He said that rapid nutrient uptake could be another competitive pathway.

On biological control, Holloway summarized local tests: the root-boring moth Agapeta (deployed widely as a knapweed biocontrol) had modest direct impact in the speaker's plots, while a pathogenic fungus (Sclerotinia) applied correctly killed many knapweed plants in their trial plots. Holloway stressed that Sclerotinia is a consumer fungus (a pathogen) rather than a mycorrhizal mutualist; it worked well under the trial conditions but is sensitive to application timing and environmental conditions. He said about a dozen insect biocontrol agents have been released statewide over the years but none have eradicated knapweed.

Holloway closed by noting ongoing work: additional greenhouse and field trials are being completed (including experiments with Centaurea diffusa and larger fescue competitors) to quantify the relative importance of allelopathy, rapid nutrient uptake, and mycorrhizal-mediated effects. He said those experiments were near completion and that results are being prepared for publication.

Ending: Holloway urged land managers and citizens who will visit Mount Jumbo and Sentinel to expect continued research updates and to consult field botanists such as Pete Lesica for identification and mapping; he said the combination of root chemistry, fungal networks and nutrient uptake likely helps explain why spotted knapweed is a persistent invader in Missoula-area intermountain prairies.