Honeysuckles (Lonicera spp.) introduced from Eurasia have a long history of popularity in the horticultural trade in North America. Tatarian honeysuckle (Lonicera tatarica) from Asia was in widespread horticultural cultivation in Europe and North America by the mid 1700s (Barnes and Cottam, 1974), whereas another popular Asian honeysuckle, Lonicera maackii (Rupr.) Maxim., was introduced into North America by 1896 (Luken and Thieret, 1996). In The Standard Cyclopedia of Horticulture, L.H. Bailey (1919) described honeysuckles as plants “of easy cultivation and propagation . . . quite hardy,” and among “our most popular ornamental shrubs.” The popularity of introduced shrubby honeysuckles continues into the 21st century, despite evidence as early as the 1920s suggesting they often escaped cultivation and naturalized in the wild (Luken and Thieret, 1996). Now, many widely planted shrubby and vining honeysuckles of Eurasian origin are documented invasive pests (Batcher and Stiles, 2000), and recently have become the targets of state-level bans in parts of the United States (U.S. Department of Agriculture-Natural Resources Conservation Service, 2018). Many of the same traits that made Eurasian Lonicera attractive as ornamentals—such as their colorful, bird-dispersed fruits; intermediate shade tolerance; and landscape adaptability—enabled them to become serious invaders in the North American landscape (Conser et al., 2015; Koop et al., 2012). Despite broad concerns about introduced invasive species in general, and the invasive history of Eurasian Lonicera in particular, cultivars of honeysuckle developed from Eurasian taxa, including Lonicera tatarica, Lonicera xylosteum, Lonicera japonica Thunb., and others, continue to be sold in North America.
Recently, many cultivars of the edible blue honeysuckle, Lonicera caerulea, have been developed using germplasm collected from a wide geographic range in mainland Asia and Japan (Gerbrandt et al., 2017), and genotypes from Europe are likewise commercially available in North America. Marketed under the names honeyberry, Haskap, and sweetberry honeysuckle, these cultivars are described as an agricultural berry crop for cold climates, with fruit finding use in value-added products such as wines, jams, and confections (Celli et al., 2014). Clones are sometimes advertised as alternative crops to blueberries (Bors, 2009), and studies indicate the fruit of L. caerulea is high in vitamins and antioxidants (Wang et al., 2016).
Although L. caerulea is a promising shrub for North America, its invasive potential has not been studied experimentally. This species features prominently on the Norwegian Biodiversity Information Center’s Black List, where it ranks among Norway’s invasive organisms with the most severe ecological impact, in a category of severity above other invasive honeysuckles such as L. tatarica and L. maackii (Gederaas et al., 2012). Lonicera caerulea is also listed as an invasive species in Sweden, where it is described as one of the few invasive taxa that have successfully invaded natural forest communities (Tyler et al., 2015). In North America, Schimpf et al. (2011) discovered a naturalized population of honeyberry near Duluth, MN, which was subsequently visited by Peterson et al. (2018), who reported evidence of spread by seedling recruitment and probable natural layering. Application of checklist-based invasive plant screening tools by Peterson et al. (2018) indicated that blue honeysuckle cultivars developed from Eurasian germplasm pose a credible risk of becoming invasive in North America, even with conservative ratings based on limited data available for L. caerulea.
The exact taxonomy of blue honeysuckles is still the subject of debate, and many consider L. caerulea to form a single, circumboreal taxon consisting of numerous Eurasian representatives as well as two North American relatives. However, Fernald (1925) argued that the two North American representatives differed sufficiently from their Eurasian congeners and from one another to elevate them to two distinct species, Lonicera villosa (mountain fly honeysuckle) in north-central and eastern North America, and Lonicera cauriana Fernald (blue fly honeysuckle) in western North America. A universally accepted global taxonomy based on a synthesis of genetic and morphological evaluations is elusive (Holubec et al., 2015; Hummer et al., 2012; Naugžemys et al., 2011, 2014). For brevity, Eurasian L. caerulea is henceforth referred to as L. caerulea, and the mountain fly honeysuckle, commonly accepted as Lonicera caerulea subsp. villosa (Michx.) Á. Löve & D. Löve is referred to simply as L. villosa.
In contrast to several introduced members of the genus that have become widely invasive in North America, the native L. villosa generally forms sparse populations and is a species of conservation concern in portions of its range (Lieurance and Cipollini, 2013; U.S. Department of Agriculture-Natural Resources Conservation Service, 2018). The minor presence of L. villosa in the ecosystems in which it is present has been interpreted by some as evidence that introduced genotypes of L. caerulea are similarly unlikely to become invasive in North America (e.g., Bors et al., 2012). However, the long-standing documentation of morphological differences between Eurasian and North American blue honeysuckles (Fernald, 1925), and the absence of direct comparisons of growth between them, leave uncertain the ecological equivalence of these taxa. In fact, the degree of relatedness and known capacity for hybridization between Eurasian L. caerulea and L. villosa (Bors et al., 2012) may be cause for additional concern, as population genetic consequences of gene flow between local and introduced genotypes have been documented in various taxa (Crispo et al., 2011; Ellstrand and Shierenbeck, 2000; Saltonstall, 2002).
Nutrient and water availability are among the factors that may determine the invasive success of introduced taxa. For example, many invasive plants can rapidly assimilate nutrients and accumulate biomass when transient nutrient enrichment occurs as a result of habitat disturbances (Gioria and Osborne, 2014). Lonicera caerulea has rapidly invaded low-resource environments in boreal forests throughout Norway (Gederaas et al., 2012), suggesting that it may have traits that help it compete for limiting resources in an ecosystem long viewed by conservationists as resistant to invasion. In addition to nutrient availability, water stress also may affect the invasive success of an introduced taxon. Lonicera villosa and L. caerulea are both facultative wetland plants, although the latter also thrives in well-drained field soils. The comparative drought and flooding tolerances of L. villosa, L. caerulea, and honeysuckles already invasive in North America are currently unknown. Notably, L. tatarica is documented to invade habitats with well-drained to poorly drained soils that are acidic to alkaline and high or low in nutrient availability (Batcher and Stiles, 2000).
We conducted two experiments to understand more fully the potential similarities and differences in traits of root and shoot biomass accumulation, average leaf size, specific leaf area, and primary stem length among L. caerulea, L. villosa, and two invasive congeners under different conditions of container substrate fertility and moisture. In Expt. 1, we compared substrate fertility responses of L. caerulea and L. villosa to those of the known invader, L. tatarica. In Expt. 2, we assessed responses of L. caerulea, L. villosa, and the known invaders L. tatarica and L. xylosteum to late-summer moisture conditions. We included known invaders in an effort to put the magnitude of trait variation between L. caerulea and L. villosa into the broader context of honeysuckles that have invaded North America successfully. Fundamentally, our goal was to identify potential functional variation between L. caerulea and L. villosa, rather than to classify either as invasive or not invasive based on comparisons with known invaders.
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