The control of Asclepias syriaca and its effect on native vegetation
In Hungary, large abandoned agricultural areas have been invaded by the non-indigenous common milkweed (Asclepias syriaca L.). This species might hinder the regeneration of sandy oldfields to open sand grassland (Pannonic sand steppes - Annex I 6260 habitat type). Glyphosate application is a cost-effective control method that helps avoid soil surface disturbance and subsequent germination of milkweed seeds in the soil. However, the effect of chemical treatment, but also of milkweed on sand grassland plant species is unknown.
We conducted two studies to we explore the effects of chemical treatment and milkweed on the sandy vegetation. In the first experiment, glyphosate spraying was applied to three oldfields in the Kiskunság National Park to control vigorous stands of A. syriaca in July 2006. Shoot number of A. syriaca and percentage cover of plant species were estimated in 1 m² permanent plots before and after treatment (June 2006 and June 2007). In the second experiment we seeded two native grass species Festuca vaginata and Stipa borysthenica in 12 plots each (a third of the plots was left unseeded). We applied repeated mechanical removal of Asclepias shoots on half of the plots for two growing seasons. The number and above‐ground cover of the two grass seedlings were evaluated for two growing seasons.
Elimination of common milkweed was successful in the short term by chemical application. A significant decrease in sand grassland specialist species and perennials were detected, while weedy annual species increased in their abundance. Whereas we found lower seedling number and cover of Festuca in plots with Asclepias shoot removal in the second year, when a severe summer drought occurred at the study site.
Chemical application disturbs the underlying vegetation so that it relapses succession back to an earlier stage. We did not find any negative effects of the presence of the invasive Asclepias during open sand grassland regeneration in terms of germination and early establishment of the dominant grass species. We even detected a nurse effect of Asclepias on Festuca, under unfavourable summer drought. Based on these results we suggest to avoid chemical application, but instead increase the propagule availability of native species through seed introduction right after land abandonment.
Szitár Katalin, Török Katalin (2008): Short-term effects of herbicide treatment on the vegetation of semiarid sandy oldfields invaded by Asclepias syriaca. L. Extended abstract in the Proceedings of the 6th European Conference on Ecological Restoration, 8-12 September 2008, Ghent, Belgium. 1-4.