Field Transplant Experiment

We are evaluating how non-native Scotch broom effects the establishment of native riparian plant seedlings along the Hoh River, located on the coast of Washington State’s Olympic Peninsula. We hypothesize that where Scotch broom occurs, the combined effects of direct competition and changes to the abiotic and biotic soil environment will reduce the survival and growth of native riparian seedlings. We further predict that the changes to the soil will persist following Scotch broom removal and continue to have suppressive effects on native riparian plant establishment, even in the absence of Scotch broom competition. To test these hypotheses we planted red alder, willow, and black cottonwood into areas where Scotch broom 1) has invaded and remains intact, 2) has invaded and been removed, or 3) does not occur.

In 2019, we installed 180 plots in two riparian sites in the Hoh watershed (N = 60 plots/treatment/site). From within the watershed, we collected red alder, willow, and black cottonwood, seedlings, and planted five of each species into each plot. Each seedling was tagged and measured at the time of planting. In the summer of 2020, we measured seedling survival and growth.

Mycorrhizal Mutualism Disruption Plant-Soil Feedback Experiment

Invasive species can alter the soil microbial communities in ways that inhibit native plants. We hypothesize the Scotch broom may reduce the abundance of ectomycorrhizal fungi that provide important benefit to native riparian plants. To test this idea, we used soils collected from Scotch broom invaded areas and nearby uninvaded areas in a plant-soil feedback greenhouse experiment. We have planted willow and cottonwood stakes and alder seedlings into their own soils and invaded soils, and will compare plant survival, growth, and the abundance of ectomycorrhizal fungi occurring on the riparian plant roots.