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resource conservation has further spurred this interest ( McDonald, 2001 ). In light of the mounting evidence of ecologically important phenotypic and genetic differences among populations within a species, end users of seeds of native wildflowers are

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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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Native plants are often promoted as an approach for water conservation in urban landscapes. However, information regarding plant water needs is based primarily upon anecdotal observations of plant performance. Direct comparisons between native and introduced species using physiological measures of plant water stress are unavailable to support or refute such recommendations. Ligustrum japonicum and Myrica cerifera, representing an introduced and native species, respectively, were transplanted into a fine sand soil to evaluate establishment rates and growth characteristics under two irrigation regimes. Each species was irrigated either daily or every 3 days and received 1.3 cm of irrigation per event for 8 months after transplant. Predawn, midday, and dusk water potentials were recorded on three consecutive days monthly, with cumulative stress intervals calculated. Height, growth indices, shoot dry mass, root dry mass and leaf area were also recorded. Water potential was significantly influenced by day of water stress level. On days without irrigation, water stress was generally greater and affected growth. Myrica irrigated daily had the greatest growth, yet plants receiving irrigation every 3 days had the least growth and greater leaf drop. In contrast, for Ligustrum there were no differences between irrigation regimes in growth responses except for growth index.

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species management plans. In 1997, a plan to eradicate the american grey squirrel ( Sciurus carolinensis ), an introduced nonnative species rapidly replacing Italy’s native red squirrel ( Sciurus vulgaris ) and damaging trees throughout the region, was

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.g., hydrilla ( Hydrilla verticillata )] introduced species can interfere with the air–water interface, thereby blocking light, reducing oxygen in the water column, and degrading the habitat for native flora and fauna ( Madsen, 2014 ). Dense growth of exotic

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, miscellaneous hardwood/hemlock ( Tsuga sp.) communities, and freshwater intertidal mudflats. Each site has a rich history of introduced exotic plant species in the manicured landscape and exotic crops for agricultural and silvicultural purposes. History of

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, Russia ( Hummer et al., 2011 ; Staudt, 2009 ). Red and white-fruited horticultural forms of F. vesca subsp. vesca were introduced into Asia, the South Pacific Islands, and South America ( Hummer et al., 2011 ), but are not endemic there. This species

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environmental ( Pimentel et al., 2005 ), economical ( Lovell et al., 2006 ; Olson, 2006 ), and management points of view ( Kettenring and Adams, 2011 ). About 25,000 alien plant species have been introduced into the United States. Pyšek (1995) defined an

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used to classify them as invasive is their seed production. High seed production compared with native species is one of the parameters that makes an introduced species invasive ( James et al., 2010 ). The success of many invasive trees is attributed to

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The dominant landscaping practice of recent centuries has been to create landscapes designed with ornamental plants that have been introduced from other countries. This practice has been so pervasive that nonnative plant species now outnumber native

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