Tracing the introduction history of a potentially invasive ornamental shrub: variation in frost hardiness and climate change Anne-Marie T. Skou, Stephan Pauleit and Johannes Kollmann A.-M. T. Skou (amt@agrotech.dk) and J. Kollmann, Dept of Plant and Environmental Sciences, Univ. of Copenhagen, Rolighedsvej 21, DK-1958 Frederiksberg C, Denmark. AMTS also at: AgroTech, Højbakkegård Alle 21, DK-2630 Taastrup, Denmark. JK also at: Restoration Ecology, Center of Life and Food Sciences Weihenstephan, Technische Univ. München, Emil-Ramann-Straße 6, DE-85350 Freising, Germany. – S. Pauleit, Strategic Landscape Planning and Management, Center of Life and Food Sciences Weihenstephan, Technische Univ. München, Emil-Ramann- Straße 6, DE-85350 Freising, Germany. Nursery catalogues have recently been used as a method for tracing the introduction history of invasive plants. Information on species, cultivars, plant size and price can help quantify historical changes in propagule pressure. Propagule pressure of invasive ornamentals has to be combined with climatic data to understand shifting distribution patterns of (potentially) invasive plants. In this study the spread of Ilex aquifolium L. cultivars was investigated at the edge of its distribution margin in Denmark. Danish nursery catalogues from 1841–2007 were studied to 1) reconstruct the introduction history of potentially invasive ornamental genotypes of I. aquifolium in Denmark; 2) explore potential factors that could explain naturalization of the species, i.e. nursery location, plant size, hardiness, price and year; 3) investigate whether or not marketing has been more common inside the historical range of the species in western Denmark; and 4) test the hypothesis that marketing of this frost-sensitive species has increased parallel with higher temperatures in the past 100 years. Price, plant size, hardiness, location, and year were recorded for a total of 3213 entries. hrough time, increasing prices and the availability of frost-hardy cultivars were the most significant factors contributing to the sale of I. aquifolium in Denmark. he number of nurseries per area, and hence propagule pressure, was higher in eastern Denmark, outside the natural range of the species. he proportion of very frost-hardy cultivars increased with milder winter temperatures. he main conclusion from this combined nursery catalogue and climate study are that nurseries in an indirect way play an important role in increasing the propagule pressure of potentially invasive ornamentals and thus can affect distribution of native species. “he curious, the extraordinary, which the forester detests, is what attracts us (nursery owners). Nature is forever variable and can assume the most wonderful forms, which the nursery owner maintains and prop- agates.” (translated from Danish) – Asger M. Jensen, nursery owner (1947) Propagule pressure is an important component to the suc- cess of biological invasions, with increasing numbers (and frequencies) and higher overall fitness aiding the estab- lishment of the species (Williamson 1996, Mack 2000, Lockwood et al. 2007). his is also the case for invasive ornamental plants that account for about 52% of the naturalized species in Europe (Lambdon et al. 2008), e.g. Fallopia japonica, Veronica filiformis and Petasites fragrans (hompson et al. 1995). For invasive ornamentals, certain traits can have an effect on establishment success, such as inherent heterosis of hybrids (Gray et al. 1991, Pyšek et al. 2003), increased frost hardiness (Friedman et al. 2008, Hanspach et al. 2008), higher fruit (seed) number and size (Ebeling et al. 2008, Ferreras and Galetto 2010) as well as vegetative reproduction, for example adventitious rooting (Pyšek 1997, Reichard 1997, Marco et al. 2010). In addition, the release of invading species from their natural enemies can lead to a competitive advantage over natives, giving rise to increased size, vigour and spread of invasive ornamental plants (Elton 1958, Crawley 1987). Although propagule pressure is a well-known factor in these processes, it is difficult to quantify (Richardson and Pyšek 2006). Nursery catalogues can be used to assess historical changes in propagule pressure and minimum residence time of invasive ornamental plants (Rejmánek et al. 2005). his information has been shown to be important in explaining distribution patterns of invasive alien species (Harvey 1989, Mulvaney 2001, Pyšek and Jarošík 2005, Dehnen-Schmutz et al. 2007, Pemberton and Liu 2009). hese catalogues contain evidence about the number of nurseries selling a specific plant species or cultivar, and indirect evidence of propagule pressure (Mulvaney 2001, Dehnen-Schmutz et al. 2007). he quality and quantity of cultivars offered have a large effect on the plant material that can escape and estab- lish, with regional adaptation of ornamental plants as an additional factor (Ross and Auge 2008). Good, reliable data has been obtained before from seed catalogues, where the Nordic Journal of Botany 30: 739–746, 2012 doi: 10.1111/j.1756-1051.2012.01399.x, © 2012 he Authors. Nordic Journal of Botany © 2012 Nordic Society Oikos Subject Editor: Per Milberg. Accepted 26 July 2012 739