Effects of herbicide on the invasive biennial Alliaria petiolata (garlic mustard) and initial responses of native plants in a southwestern Ohio forest

Summary

Garlic mustard Alliaria petiolata, native to Europe, is a prevalent invasive herb in forests of eastern North America. It now poses a severe threat to native plants and animals in forest communities in much of the eastern and midwestern USA by outcompeting native plants through monopolization of light, moisture, nutrients, soil and space. In one of the best remaining stands of old-growth American beech Fagus grandifolia – sugar maple Acer saccharum forests in the midwestern US, field trials were undertaken to investigate the effects of applying a glyphosate herbicide (Round-up) to Alliaria, and the response of the forest floor plant community to herbicide application.

Study site: This study took place in the ODNR State Nature Preserve in Hueston Woods State Park, Butler County, southwestern Ohio, USA. Garlic mustard was first reported at this site in 1977 and by 2000 was present throughout the Preserve, but occurred at greater densities along trails, in moist areas and in tree-fall gaps. Two treatment sites were a 20 ha area of an old-growth stand dominated by American beech Fagus grandifolia and sugar maple Acer saccharum, and a 16 ha area of secondary-growth dominated by tulip poplar Liriodendron tulipifera.

Experimental design & herbicide application: In mid-May 2000, 50, 1 x 1 m plots in each stand (100 plots in total) were established. Each plot had high garlic mustard abundance, was located under a closed canopy on reasonably level ground, at least 8 m away from trails, and 5 m or more from neighboring plots. Each plot was randomly assigned to be sprayed or unsprayed (control), giving a total of 25 plots per treatment per stand. On 6 November 2000 and 7 November 2001, 1% glyphosate (prepared by dilution of Roundup PRO [41% glyphosate]) was spot-applied to garlic mustard plants within 2 m of the centre of each treatment plot using a backpack sprayer. To prevent repeat spraying, the herbicide contained a blue dye which highlighted sprayed plants.

Herbicide effects on garlic mustard: To elucidate the herbicide effects on garlic mustard, the number of individual leaf rosettes and adults in each plot in May, June, August and October 2000, February, May, June, August and October 2001, and May 2002, were counted.

Herbicide effects on 'wintergreen' herbs': The effects of application on each wintergreen species (i.e. herbs in leaf in October 2000) were investigated by counting individuals in each plot in that month and subsequently in February, May and October 2001, and May 2002.

Community effects: Community effects were determined by the percentage cover of each plant up to 85 cm tall (including herbs, vines, tree seedlings and shrubs) in each plot during mid-May, late June, and mid-August 2000 and 2001 by point frame sampling. For each plot, peak percentage cover of each species and species richness (total species detected in cover sampling) was determined each year.

Effects on demography of native species: To test whether the reduction in Alliaria enhanced the survival, growth, or reproduction of individual plants of native species, individuals of species representing different growth forms and/or functional groups, and abundance in plots early in the growing season of 2000 was monitored. If a plot had only a single individual of a study species, that individual was marked. If a plot had several individuals of a species, the plot was divided into four quadrants and the individual closest to the centre of each quadrant and the centre of the entire plot was monitored. Thus, a maximum of five individuals per plot (but usually fewer) of each species were marked. These plants were censused every 2–3 weeks during each growing season for survival, growth and reproduction.

Effect of herbicide on garlic mustard: Monitoring in May 2001 revealed that in comparison to control plots, spot application of Roundup in autumn 2000 reduced the density of Alliaria by 85% in the old-growth stand and 44% in the second-growth stand. Old-growth stand sprayed plots averaged 1.3 adults whereas control plots averaged 8.3. In the second-growth stand, sprayed plots averaged 3.4 and control plots averaged 6.0. Treatment did not affect the initial density of Alliaria plants that germinated in the spring of 2001 (visible as leaf rosettes), old-growth-sprayed plots averaged 85 versus 92 in control plots, whilst second-growth-sprayed plots averaged 118 versus 101 in controls. After spraying, when this Alliaria cohort reached the adult stage (May 2002), densities were extremely low in both treatments in both stands.

In contrast to herbicide application in 2000, application of herbicide in 2001 only reduced Alliaria in the old-growth stand, and this effect was only marginally significant. However, this weak effect appears due to the extremely low density of Alliaria adults in both treatments in May 2002. Treatment effect can be ruled out because the same trend occurred in both control and sprayed plots. It is likely that the low density of adults in control plots in May 2002 was due to mortality caused by low precipitation in June 2001 (both autumn rosette and spring adult densities correlated strongly with precipitation the previous June). However, after the second herbicide application (2001), lower densities of seedlings were found in sprayed plots, this decline was probably due to herbicide induced reduction of the 2000 plants, which presumably produced the majority of the seeds that germinated in 2002.

Effect on wintergreen herbs: Before spraying (October 2000), there was no difference in total density of all wintergreen herb species between treatments in either the old-growth stand or the second-growth stand. Lower wintergreen herb density was apparent in sprayed plots in the old-growth stand in February 2001 and February 2002, but not in October 2001. No such treatment effect occurred in the second-growth stand, where increased reproduction of the late-summer perennial, lopseed Phryma leptostachya, was apparent.

In the second growth stand the commonest wintergreen herbs were the winter annuals common chickweed Stellaria media and cleavers Galium aparine, both considered weeds in North America (S.media is non-native, G.aparine has native and introduced populations). The effect of the herbicide probably explains the lower May 2001 cover of Stellaria in sprayed versus control plots of the old-growth stand. Other herbs in both stands occurred at very low cover before treatment, so could generally be avoided during spraying and autumn leaf fall also probably reduced non-target effects, covering and protecting many native herbs.

Community effects: Control and sprayed plots did not differ in community composition before spraying (2000 growing season) in either stand. Minor community differences were found in the old-growth stand after Alliaria reduction, i.e. greater cover of spring ephemerals in the herbicide treatment. There was no significant treatment effect on species richness either before spraying in 2000 or after spraying in 2001. Although some community effects were apparent, it is speculated that removal experiments require 3–5 years of treatment before community changes really become evident.

Effects on demography of native species: In the old-growth stand, the annual pale touch-me-not Impatiens pallida was not significantly impacted by spray treatment in terms of either growth or reproduction (fruit number). Similarly, no treatment effect was found for either of the two monitored spring ephemerals: spring beauty Claytonia virginica and squirrel corn Dicentra canadensis. However, fruit production of the perennial P.leptostachya in the second-growth stand tended to increase (from 2000 to 2001) in the spray treatment.

Conclusions: The results indicate that autumn treatment with glyphosate significantly reduces Alliaria density, does not negatively impact native species and some species respond positively to a single-year reduction of Alliaria. However it only resulted in very modest responses of the native plant community. The modest responses may be due in part to the short duration of this study and longer term effects of Alliaria reduction through annual herbicide application are being investigated.


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