Tracing the introduction history of a potentially invasive ornamental shrub: variation in frost hardiness and climate change

Abstract

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. Through time, increasing prices and the availability of frost‐hardy cultivars were the most significant factors contributing to the sale of I. aquifolium in Denmark. The number of nurseries per area, and hence propagule pressure, was higher in eastern Denmark, outside the natural range of the species. The proportion of very frost‐hardy cultivars increased with milder winter temperatures. The 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.

Explaining naturalization and invasiveness: new insights from historical ornamental plant catalogs

“…To our knowledge, this study is the first to analyze the disappearance phenomenon from a large pool of ornamental plants. It is noteworthy that propagule pressure—as estimated from plant sale data—is often identified as a an important, if not the most important, determinant of naturalization for ornamental or cultivated plants (Dehnen‐Schmutz et al., ; Moodley et al., ; Pemberton & Liu, ; Pyšek et al., , ; Skou et al., ). However, in a cold region such as Québec, not being sold (low propagule pressure) is at least partially dependant on a lack of cold hardiness.…”

“…Two variables were used as indicators of propagule pressure (sensu Lockwood, Cassey, & Blackburn, 2005), that is, the number of catalogs in which the taxon was listed and the number of years elapsed since its first mention in a catalog. We hypothesized that a taxon available from more nurseries and sold for a longer period of time would be more widely planted, thus producing more propagules with the potential to escape from gardens and to contribute to naturalization and/or invasiveness (Dehnen-Schmutz et al, 2007;Pemberton & Liu, 2009;Pyšek, Křivánek, & Jarošík, 2009;Skou et al, 2012).…”

“…The main objective of this study was to identify plant attributes associated with naturalization and invasiveness using century-old nursery catalogs. This is not the first study of this kind (although there are only a few: Dehnen-Schmutz et al, 2007;Pemberton & Liu, 2009;Skou, Pauleit, & Kollmann, 2012), and attempts to link invasiveness with plant attributes are multiplying (for reviews and debates on their relevance, see Pyšek & Richardson, 2007;van Kleunen, Weber, & Fischer, 2010;van Kleunen, Dawson, & Dostal, 2011;Thompson & Davis, 2011;Leffler, James, Monaco, & Sheley, 2014). However, we propose a new approach that takes into account not only plant attributes as explanatory factors, but also propagule pressure, while accounting for the nonindependence of the species analyzed due to their phylogenetic relationships.…”