The issue of invasive plants has become a concern to a variety of groups, including environmentalists, policymakers, and nurserymen. Although many surveys of invasive plants have been made, little research on the biology of hybridization has been conducted. Bittersweet (Celastrus) species serve as a good model system to test the effects of interspecific hybridizations since native and introduced species are found in the U.S. The American bittersweet (Celastrus scandens L.) is a deciduous climbing or twining shrub native to eastern and central North America. Although the bark has been used for medicinal purposes, the plant is cultivated as a nursery crop primarily for its bright red berries. In its natural habitat, native bittersweet is also an important source of food and cover for wildlife. Over the past several decades, populations of native bittersweet have declined to such low levels that some states are considering listing it as a threatened species. One reason for the rarity of American bittersweet in the wild is thought to be competition and possibly hybridization with an aggressive introduced species, oriental bittersweet (Celastrus orbiculatus Thunb.), which was introduced from Asia into the U.S. in 1860 as an ornamental. This plant can form dense, tangled, impenetrable thickets or climb small trees to girdle and smother them. It has been seen in at least 21 states since it was first recorded as an escape plant in 1912. Our objective was to determine whether oriental bittersweet can hybridize with native bittersweet, thus contributing to the loss of native populations in the United States. We performed controlled pollinations using C. scandens as the female parent and C. scandens or C. orbiculatus as the male parent. Although the intraspecific pollinations resulted in significantly more germinating seedlings than the interspecific crosses, the seedlings from the interspecific crosses had less seed dormancy and were more vigorous and more quick to vine than the intraspecific seedlings. These results indicate that the decline of the American bittersweet may be due to interspecific hybridizations with the invasive introduced species.
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