Invading monotypic stands of Phalaris arundinacea: a test of fire, herbicide, and woody and herbaceous native plant groups

Published source details Foster R.D. & Wetzel P.R. (2005) Invading monotypic stands of Phalaris arundinacea: a test of fire, herbicide, and woody and herbaceous native plant groups. Restoration Ecology, 13, 318-324.
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Summary

Reed canary-grass Phalaris arundinacea is an invasive in parts of North America. Although considered native, it has become more widespread and more dominant in wetlands, suppressing other native vegetation. The causes for its spread have been linked to introduction for agricultural purposes, removal of native vegetation, altered hydrology and increased nutrient loads. In this study, experiments were undertaken to assess if other native plants could establish in stands of P.arundinacea by temporarily reducing its biomass by burning or herbicide application. Establishment of three native groups (herbaceous transplants; herbaceous seeds; and woody shrub transplants) were investigated at a site in Tennessee (USA).

Study area: This study was undertaken at a restored wetland (30 ha) in Shady Valley, a former forested wetland and Sphagnum fen peatland. In 1997, The Nature Conservancy raised the water table to former levels.

Treatments: Thirty-six circular plots (3 m radius) were established in dense P.arundinacea stands. Burn, herbicide and control treatments were randomly assigned to each plot. The wetland was burned in spring 2 years prior to to commencment. Fire treatment plots were burned in late March (some again due to poor burning, in April) 2001. Herbicide treatment plots were sprayed with a 1.5% glyphosate solution (Rodeo a herbicide designed for aquatic habitats and surfactant at 0.015 L/m²) in April when fresh P.arundinace was approximately 25 cm tall.

Planting: Plots within each treatment were randomly divided into four groups and planted in May 2001 with one of the following native species groups: herbaceous plants, woody and understory plants, herbaceous seed; and controls (nothing planted).

Data collection: Percent cover of plant species and P.arundinacea litter was estimated in each plot in August 2001 and 2002. P. arundinacea shoot biomass was measured by collecting material from three randomly selected subplots. Piezometers were installed at 28 of the plots and water depth readings taken at least once a month over the 2-years.

Herbicide treatment: Herbicide treatment created a short-term opportunity for native species to establish by reducing P. arundinacea root and shoot biomass for two growing seasons. By the end of the second growing season shoot biomass had nearly recovered to levels in the controls. In the first year (2001), glyphosate application reduced shoot biomass by 37% and root biomass by 19% compared to controls. In the second year, shoot biomass in the herbicide plots increased by 50% but this was still lower than in the controls. Root biomass remained at 2001 levels but was 32% less than that of the controls as root biomass in these plots increased in 2002. The root mat did not re-establish as a distinct layer in the two growing seasons after herbicide treatment, and it included few or no rhizomes at the end of 2002. Litter in herbicide plots was reduced by 30% in 2002 compared to the controls. P.arundinacea cover was reduced by 23% by herbicide treatment in the first year compared to controls but by the second had recovered.

Burn treatment: Fire was ineffective. There was no difference in P.arundinacea root and shoot biomass or cover between burned and control plots, although litter mass in the burned plots was 51% less in 2001 and 33% less in 2002.

Survival of different plant groups: Transplanted monocots had greater survival than dicots. By the second growing season the herbaceous group had the greatest average cover (5%), compared to the woody seedlings (3%) and seed group (0%).

Frost damage: Mortality among both herbs and woody dicot transplants was high in the first winter. Growing-season frosts primarily damaged dicots (many died).

Hydrology: In general, differences in hydrology had little effect on P.arundinacea biomass and cover. Hydrology was only statistically significant for root biomass between the driest (water levels at the soil surface <5% of the time) and wettest (water levels at or above the soil surface >45% of the time) plots.

Conclusions: Spring glyphosate application reduced P.arundinacea shoot and root biomass for two growing seasons thus potentially providing a period for the establishment of native species; fire was ineffective. Plants needed more than two growing seasons to compete with P.arundinacea germinating from the seed bank or resprounting from rhizomes.

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