The Multifaceted Botanical Impact of the Invasive Common Milkweed (Asclepias syriaca L.) in a Protected Sandy Grassland in Central Europe

Asclepias syriaca L.is a perennial broad-leaved species native to North America. It has become established in many regions of Europe, and has spread with increasing rapidity in recent decades. Its reproductive behaviour allows this species to proliferate rapidly. The selected grasslands are located in the Carpathian Basin in Hungary, in the area of Kiskunság National Park, near Lake Kolon. In the framework of the research, in two consecutive years (2021 and 2022), and in two different seasons (spring and autumn), we examined the percentage cover of vascular plant species in the stands degraded by A. syriaca and the natural control (without A. syriaca), and their seasonal and interannual dynamics. Between the A. syriaca-degraded and natural control stands, there was no significant difference in the number of species in the spring and autumn of any of the examined years. Surprisingly, in the spring, the degraded stands were somewhat richer in vascular plant species. In autumn, the control stands had more vascular plant species, but to a lesser extent. The Shannon diversity was higher in the A. syriaca-dominated stands than in the control in all recording periods. Simpson diversity showed a similar pattern to Shannon diversity, with one exception in spring 2022. In the case of the social behaviour type, it can be clearly seen that the alien competitor (AC) species dominated in spring and autumn in both years in the stand dominated by A. syriaca. In the natural control stand, specialists (S) and competitors (C) dominated in both years and in both seasons. The negative effect of the invasive species on the number and diversity of species was presumably significantly reduced by the significant drying of the study area experienced in recent years.