Invasive species have been listed as the second highest threat to biodiversity in the United States surpassed only by habitat destruction (Bir, 2000; Morin, 1999; U.S. Department of the Interior, 1999; Wilcove et al., 1998). The projected economic losses due to invasive species are more than $138 billion each year in the United States. About 5000 invasive plant species exist in the Unites States (Pimentel et al., 1999) which have invaded and disturbed habitats, altered ecosystems, and threatened endangered species (D'Antonio and Vitousek, 1992).
One way that invasive plants displace natives and reduce biodiversity of forest ecosystems is by producing many seeds that are capable of germination (Lehrer and Brand, 2003). Heavy flower production is required on the plant to yield high seed quantities (Augspurger, 1983). It is generally understood that the larger the mature plant, the higher the yield (Dieringer, 1991; Ollerton and Lack, 1998).
Norway maple is one species that has been identified as potentially invasive to natural areas (Munger, 2003; Randall and Marinelli, 1996; Swearingen et al., 2002; Webb and Kaunzinger, 1993; Webb et al., 2001; Wyckoff and Webb, 1996). Based on observations, the norway maple produces high quantities of seed, shade-tolerant seedlings, and dense canopies that further reduce light and moisture reaching the understory (Gresham, 2000; Randall, 1996). Ecological researchers have reported that these affects on a forest community alter its structure by decreasing native species abundance, diversity (Munger, 2003), and richness (Martin, 1999; Webb et al., 2001; Wyckoff and Webb, 1996).
It is logical to assume that less invasive or noninvasive cultivars of the invasive species may exist among the present or previously popular cultivars employed in the green industry (Lehrer and Brand, 2003). Few studies have addressed flower and seed yields along with other biologic traits in determining the invasive potential of cultivars for known invasive species. Of the studies reported, all have focused on shrub (Anisko and Im, 2001; Knox and Wilson, 2006; Lehrer et al., 2006; Lovinger and Anisko, 2004; Wilson et al., 2004a, 2004b), perennial grasses (Meyer and Tchida, 1999), or herbaceous species (Wilson and Mecca, 2003). The immediate value of cultivar evaluation for seed production is the ability to advise the green industry of problem cultivars and possible replacements (Wheeler and Starrett, 2001). The replacements may include reintroducing older, less prolific cultivars that have been replaced by new cultivars. With any cultivar evaluation, the process begins with defining flower and seed productivity.
The purpose of this study was to determine the flower and seed productivity of the norway maple and its cultivars. This is the first report in which norway maple cultivars have been evaluated using seed productivity to determine each cultivars invasive potential.
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