Reproductive potential of the alien species Asclepias Syriaca (Asclepiadaceae) in the rural landscape

Abstract

In the beginning of its intro- duction Common milkweed, Asclepias syriaca L. was used in Hungary as ornamental plant. After escaping from cultivation it has become wide spread during last decades of the 20 th century. Nowadays, it covers large areas especially on san- dy soils, and its further appearance is expected on dry, deforested and degraded areas. Two neglected fields and one abandoned vineyard were selected. For sampling two quad- rates of size 4 m 2 were used at each stand. The stands were characterized by the following vari- ables: density of stems, percentage of reproductive stems, average number of inflorescences, average number of fruits, fruits per all inflorescences on a stem, pods per fruited inflorescences on a stem, average number of seeds per fruit. In the same quadrates the soil seed bank was studied at two depths (0-5 cm and 5-10 cm). The results showed that the density of stand, percentage of reproductive specimens and aver- age number of fruits were significantly higher on the neglected fields than on the abandoned vineyard. In the neglected fields large number of seeds were detected in the upper soil-layer (2.7- 18.6 × 10 3 seeds m -2 ), but in the lower soil-layer much fewer seeds were found and only in one of the fields. Common milkweed seeds were almost completely missing from both soil layers of the abandoned vineyard. However, the seeds recov- ered from the soils practically did not germinat- ed, indicating that seeds of earlier years sets have lost viability, i.e. the studied stands of A. syriaca did not form a persistent soil seed bank. Never- theless, the fresh seed production of the neglected field populations, that can reach 7-10 thousands seeds m -2 , may cause a very strong propagulum load not only in the site but also on the surround- ing areas.