Abstract
Blue honeysuckle (Lonicera caerulea) is a circumpolar species complex with representatives in Europe, Asia, and North America. Although honeysuckles (Lonicera spp.) from Eurasia have a history of invasiveness in North America, farmers and homeowners are interested in growing nonnative blue honeysuckle hybrids because of their edible blue fruits. To assess whether these cultivars and closely related native blue honeysuckles (Lonicera caerulea subsp. villosa) might have similar growth and fecundity, we planted five nonnative cultivars of blue honeysuckle and five native genotypes in a common garden in Orono, ME, USA, along with invasive red-fruited honeysuckles [Tatarian honeysuckle (Lonicera tatarica) and European fly honeysuckle (Lonicera xylosteum)] for comparison. Rooted cuttings were planted into a field plot in Jun 2016 and fully maintained during the first season; thereafter, maintenance consisted of weeding once annually. Seventy-three percent of native blue honeysuckle plants survived to the end of the study, whereas survival and growth of nonnative cultivars were more robust. In 2021, nonnative cultivars had an average height of 81 cm and width of 86 cm, which were 2.8 times the height and 2.9 times the width of surviving native plants. The estimated canopy volumes of nonnative blue honeysuckles were an average of 20 times those of their native counterparts. The bloom periods of native and nonnative blue honeysuckles overlapped considerably. However, only seven of the 22 living native plants produced fruits in 2021, with an average of three fruits per plant among them. In contrast, nearly all plants of the nonnative cultivars produced fruits, with an average of 616 fruits per plant. In comparison, the red-fruited invasives had an average of 9739 fruits per plant. Native blue honeysuckles produced very few seeds, whereas nonnative cultivars had an average of 13,918 seeds per plant, which was approximately one-fourth the number produced by invasive red-fruited honeysuckles. We concluded that native and nonnative genotypes of blue honeysuckle differ strikingly in survival, growth, and production of fruits and seeds. However, invasive red-fruited honeysuckles grew faster with higher fecundity than nonnative blue honeysuckles in our full-sun landscape. Because bloom times overlapped substantially between native and nonnative blue honeysuckles, the potential for gene flow to occur from planted cultivars into native populations merits consideration. Several possible explanations of differences in performance among blue honeysuckles include hybrid vigor of cultivars or shallow rooting or poor adaptability of native genotypes to the environment of the common-garden trial. Our results, which demonstrated that nonnative blue honeysuckles are likely to be distinct from their native relatives in terms of competitiveness and fecundity, suggest that caution is warranted during the introduction and cultivation of agricultural genotypes.
Cultivars of blue honeysuckle (Lonicera caerulea) have gained interest from the North American market, where they are sold in the horticulture trade and farmed for their blue fruits as a northern berry crop. Breeding programs have released numerous cultivars developed from germplasm that originated from mainland Asia and Japan (Gerbrandt et al. 2017) and marketed under the common names of honeyberry, haskap, and sweetberry honeysuckle. In contrast to the commercial development of nonnative blue honeysuckle, the eastern North American member of the blue honeysuckle species complex, mountain fly honeysuckle (Lonicera caerulea subsp. villosa; hereafter referred to as native blue honeysuckle), has never been commercialized.
The accepted taxonomy for the blue honeysuckle complex has evolved over time. Hooker (1840) held the view that North American representatives of the complex were indistinguishable from several Eurasian specimens of blue honeysuckle. Subsequently, Fernald (1925) suggested that the North American representatives differed enough from the Eurasian representatives and from one another to elevate them to two distinct species: Lonicera villosa in north-central to eastern North America and Lonicera cauriana in western North America. Interestingly, Fernald (1925) found common traits between the North American and Japanese blue honeysuckles that convinced him of a close biogeographical relationship between the two. Modern classifications often place the entire complex into a single circumpolar, polymorphic species with numerous subspecies across Europe, Asia, and North America. No one taxonomy is universally accepted, perhaps because systematic comparisons of genetic and morphological diversity across the entire complex are absent (Holubec et al. 2015; Hummer et al. 2012; Naugžemys et al. 2011, 2014). To date, no infraspecific comparisons of growth between North American and Eurasian representatives have been conducted to clarify the extent to which they are competitively or reproductively interchangeable in a common garden.
Despite increasing interest in nonnative cultivars of blue honeysuckle in North America and the well-documented invasive potential of Lonicera spp., the invasive potential of nonnative cultivars has received little attention. Peterson et al. (2018) summarized what was known at the time about the invasive potential within the species complex: Eurasian blue honeysuckle ranks among Norway’s most ecologically impactful invasive organisms on the Norwegian Biodiversity Information Center’s Black List (Gederaas et al. 2012), where it earned a severity classification that exceeds that of Tatarian honeysuckle (Lonicera tatarica) and Amur honeysuckle (Lonicera maackii); it is among the few invasive taxa that have successfully invaded intact forest communities in Sweden (Tyler et al. 2015); a naturalized population was discovered near Duluth, MN, USA (Schimpf et al. 2011) and revisited by Peterson et al. (2018), who documented seedling recruitment and probable natural layering; and the evaluation of nonnative blue honeysuckles using checklist-based screening tools (Conser et al. 2015; Koop et al. 2012) has indicated a credible risk of invasion in North America. In 2022, blue honeysuckle was added to the Species Watch List for Maine, which includes plants that could become invasive but do not presently meet all the criteria for inclusion on the state’s list of plants prohibited from sale or import (Maine Department of Agriculture, Conservation, and Forestry 2022).
Hayes and Peterson (2020) compared the growth of native and nonnative blue honeysuckles cultivated in containers. In a moisture study, plants of nonnative blue honeysuckle produced substantially more growth than their native congeners, with measures of growth similar to those of two red-fruited invasives, Tatarian honeysuckle and European fly honeysuckle (Lonicera xylosteum). Likewise, plants of nonnative blue honeysuckle grown across a range of fertilizer rates consistently produced twice the root and shoot dry masses of their native relatives, but invasive red-fruited honeysuckles produced multiple-times the dry masses of nonnative blue honeysuckles and were more responsive to increasing fertilizer rates. Collectively, the results to date do not definitively illustrate the invasive potential of nonnative genotypes, but they cannot not rule it out. Indigenous populations of mountain fly honeysuckle are typically diminutive in stature, represent only a minor component of their plant communities, and are not especially fruitful. The extent to which native and nonnative congeners may differ in their competitive ability and fecundity in the landscape remains uncertain. Taken together, prior observations undercut the view that native and nonnative members of the blue honeysuckle species complex are morphologically, physiologically, or ecologically interchangeable.
The effects of nonnative, invasive plants on natural ecosystems are well-documented. Many invasive species and genotypes are adaptable to a range of environments, grow quickly, and have high fecundity. During invasion, nonnative species may also hybridize with closely related indigenous species, potentially swamping small local gene pools or causing outbreeding depression. Through gene flow, the traits of an invader may even replace traits of the indigenous relative within the natural landscape (Ellstrand and Rieseberg 2016). Moreover, hybridization of nonnative genotypes with local native genotypes or multiple nonnative genotypes with one another can lead to evolution benefitting an invasive species (Ellstrand and Shierenbeck 2000; Shi et al. 2018). The benefits of hybridization for greater adaptability, increased growth rate, and greater fruit yield are well-demonstrated by plant breeders, who may deliberately cross diverse genotypes to produce new cultivars with improved productivity.
To date, no published research has compared landscape performance and fecundity of nonnative cultivars of blue honeysuckle and their North American native congeners. Moreover, no landscape trials have compared these traits between red-fruited honeysuckles that broadly invade North America and introduced cultivars of blue honeysuckle. Our objectives were to evaluate survival and growth among native and nonnative blue honeysuckles and compare them to invasive red-fruited honeysuckles, evaluate the potential for overlapping bloom times between native and nonnative genotypes of blue honeysuckle that could facilitate hybridization between them, and record fruit yield and estimate seed production per plant among native and nonnative blue honeysuckles and invasive red-fruited honeysuckles. The results provide information regarding the competitive and reproductive similarity of introduced and native genotypes of blue honeysuckle to inform expectations about invasive potential.
Materials and Methods
In 2015, we collected cuttings in early summer from four indigenous populations of blue honeysuckle in Penobscot, Aroostook, Kennebec, and Washington Counties in Maine, and one population in Sawyer County, WI, USA. We also collected four genotypes of invasive red-fruited honeysuckles that grow in Old Town and Orono, ME, USA, including three of Tatarian honeysuckle (two of L. tatarica and one of L. tatarica var. morrowii) and one of European fly honeysuckle. Rooted cuttings of five cultivars of nonnative blue honeysuckle that we purchased from a commercial orchard in Manitoba, Canada, arrived in Oct 2015. Nonnative cultivars of blue honeysuckle included Berry Blue, Borealis, Honeybee, Svetlana, and Tundra.
Cuttings of native blue honeysuckles and invasive red-fruited honeysuckles were rooted over Summer 2015 under intermittent overhead mist. In Oct 2015, rooted cuttings of all genotypes were transplanted into a substrate of 1:1 peat:perlite (v/v) in plastic pots (510-mL square or 8.9 × 8.9 cm; Dillen-ITML, Middlefield, OH, USA). Cuttings were grown for several weeks in a glass-glazed greenhouse to enhance root systems, overwintered in a cold storage room at 4 °C, and transferred to the greenhouse in Apr 2016. In Jun 2016, when honeysuckles had a height of ∼8 to 12 cm, they were planted in a field plot in the Lyle E. Littlefield Ornamentals Trial Garden in Orono, ME, USA, on a grid layout with plants spaced 1.5 m on center. Within each of six randomized complete blocks, we planted one plant per genotype to a total of five plants of native blue honeysuckles, five plants of nonnative blue honeysuckles, and four plants of red-fruited honeysuckles per block. Because we planted six blocks, the total number of plants in the study was 84. Before planting, the plot was solarized with clear plastic sheeting, tilled, and raked. The field soil consisted of 47% sand, 41% silt, and 12% clay, with a pH of 6.1. After planting, each plant was fertilized with 1 tablespoon of controlled-release fertilizer (Osmocote Pro 17N–2.2P–9.1K; Everris, Dublin, OH, USA), and the ground was mulched with aged hemlock bark to a depth of 3 inches. The plot was weeded twice and irrigated by hand several times over Summer 2016 and Fall 2016. The only annual maintenance we performed after 2016 was annual weeding of the plots. In 2019, we rolled out and pinned woven nursery groundcover between the rows to reduce the area that required weeding around each plant. For the duration of the experiment, the common garden site received full sun exposure with natural photoperiods, temperatures, and precipitation.
To determine the degree of temporal overlap in anthesis between native and nonnative blue honeysuckles, we recorded the bloom times of all individual plants in Spring 2019, when plants were in their fourth season of field growth. A plant was considered to be in bloom if we could find at least one flower open on the plant. Of the six plants included in the field plot for each blue honeysuckle genotype, we observed flowering daily on all plants that were still alive until all plants had completed flowering.
In Spring 2021, when plants were in their sixth growing season in the field, we collected all fruits from native and nonnative blue honeysuckles and three of the four genotypes of invasive red-fruited honeysuckles. Fruits from each plant were collected into plastic bags to prevent water loss from each sample and weighed on a laboratory scale to obtain the total fruit weight per plant. To obtain the average weight per fruit, a representative sample of fruits from each plant was weighed on an analytical balance, and the total fruit mass was divided by the size of the sample. Samples included all the fruits of each plant when plants had fewer than 100 fruits or a representative set of 100 fruits when plants had more than 100 fruits.
Botanically, the fruits of blue honeysuckles are false fruits with a fleshy blue cupule that tightly encloses two distinct ovaries (Rehder 1909). To align the fruit count with gross visual observation, the fruit count per plant was recorded as the number of false fruits (two ovaries each) for blue honeysuckles and the number of true fruits (one ovary each) for red-fruited honeysuckles. All blue honeysuckle fruits were counted by hand; however, fruit counts for red-fruited honeysuckles, which produced numerous fruits that were watery and easily crushed, were estimated by counting a representative sample of fruits, recording their mass on an analytical balance, and dividing the total fruit mass per plant by the average mass per fruit. Fruits were placed in zippered plastic bags and stored in a freezer until seed counts could be completed at a later time.
We estimated seed production of each plant in 2021 by counting seeds in representative fruits. From each blue honeysuckle plant, we sampled five representative false fruits. To sample the same number of ovaries from red-fruited honeysuckles, we included seeds from both ovaries of each pair of berries, which are partially fused at the base in Tatarian honeysuckle and European fly honeysuckle. We wet-screened fruits on a wire mesh kitchen strainer held under running tap water, and performed flotation separation in a glass beaker to remove pulp and unfilled seeds. Cleaned seeds from each false fruit or pair of true fruits were counted. The number of seeds per plant was estimated by multiplying the number of fruits per plant by the average number of seeds per fruit. Because red-fruited honeysuckles produced true fruits with a single ovary, we divided their seed counts by half before we multiplied them by the number of fruits per plant.
In Jul 2021, during their sixth and final growing season, we recorded the survival and size of each plant. A surviving plant was one for which any green tissue was visible on the plant. We measured the height of each plant from the surface of the ground to the tallest point of the plant using a measuring tape. We recorded two widths for each plant by measuring both its widest horizontal dimension and the dimension perpendicular to it. The height-to-width ratio was calculated for each plant. We calculated the canopy volume of each plant as an ellipsoid using the following formula: volume (cm3) = (4/3) × (3.14) × (height/2) × [(width 1)/2] × [(width 2)/2].
We analyzed data using JMP Pro 16.0.0 (SAS Institute, Cary, NC, USA) to determine whether native blue honeysuckles, nonnative blue honeysuckles, and invasive red honeysuckles had differing survival, growth, and fecundity. Crucially, our study was not intended to be a variety trial. Because our study was designed to provide a broad genetic representation of native and nonnative blue honeysuckles and invasive red honeysuckles, replication within each genotype was limited to six plants in favor of adding additional genotypes for generalizability of the results. Before the analysis, we simply pooled genotypes within each of three groups to test variations among the following groups of honeysuckles: nonnative cultivars of blue honeysuckle; native genotypes of blue honeysuckle; and nonnative, invasive genotypes of red-fruited honeysuckles. Although this approach does not allow statistical comparison of cultivars or genotypes within each honeysuckle group, it increases the precision of each group estimate, which is an important consideration when sample sizes (blocks) are low (n = 6) and individual plants in some blocks have missing data because of plant death or failure to fruit. Given the wide range of responses among honeysuckle groups, our data strongly violated parametric assumptions of the normal distribution and equal variance of residuals, regardless of data transformations. Moreover, the 95% confidence intervals calculated based on parametric assumptions sometimes contained negative values, which are biologically impossible outcomes for any variable in the dataset. Therefore, we implemented the Friedman rank test because it is a nonparametric alternative to the one-way analysis of variance that is suitable for experimental designs with randomized complete blocks. For the independent variables with significant Friedman rank test statistics, we implemented the nonparametric Wilcoxon each pair test for separation of means.
Results
Anthesis of native and nonnative blue honeysuckle genotypes overlapped substantially in Spring 2019, followed by anthesis of more distantly related red-fruited honeysuckles. Bloom periods of both native and nonnative blue honeysuckles began in early to mid-May and continued through early June (Table 1). Cultivars began blooming slightly earlier than the native genotypes, but the blooming period for native blue honeysuckles was nested fully within that of nonnative blue honeysuckles. Invasive red-fruited honeysuckles reached anthesis in early June, as the final flowers of blue honeysuckles were spent (Table 1). We did not track the remainder of the blooming period for invasive red-fruited honeysuckles.
Genotypes of Lonicera spp. planted in a common garden field plot in Orono, ME, USA, in Jun 2016, and dates of anthesis in Spring 2019. We planted six individuals of each genotype.
In 2021, nonnative blue honeysuckles produced substantially more fruits and seeds than their native conspecifics. Only approximately one-third of the surviving native blue honeysuckles bore fruits, whereas all of the nonnative blue honeysuckles fruited (Table 2). Nonnative blue honeysuckles produced more than 500 times the fruit weight per plant, 8 times the fresh weight per fruit, and 200 times the total number of fruits per plant compared with their native counterparts. Although seed counts per fruit were on the same order of magnitude among nonnative and native blue honeysuckles, the high counts of fruits of the former yielded an estimated 13,918 seeds per plant, which was nearly 300 times that of the seeds per plant of native blue honeysuckles (Table 2).
Fruiti and seed measurementsii in Spring 2021 and Summer 2021 among genotypes of Lonicera in their sixth year of growth during a common garden experiment in Orono, ME, USA.
Although they were substantially more fecund than their native counterparts, non-native blue honeysuckles produced significantly less fruit weight per plant, fewer fruits per plant, and fewer seeds per plant than invasive red-fruited honeysuckles (Table 2). Red-fruited honeysuckles produced fruits with lower fresh weights that, on average, contained fewer than six seeds, whereas nonnative blue honeysuckles had more than 20 seeds per fruit. Because they produced over 15 times the number of fruits, red-fruited honeysuckles nonetheless produced over four times as many seeds per plant as nonnative blue honeysuckles (Table 2).
The survival rate of nonnative blue honeysuckles in the common garden plot was 97%, whereas only 73% of their native counterparts survived to 2021 (Table 3). Nonnative blue honeysuckles had an average height of 81 cm and width of 86 cm, which were 2.8 times and 2.9 times the height and width of native blue honeysuckles. Although the plant height-to-width ratio was similar for both groups, the average canopy volume calculated for nonnative blue honeysuckles was an average of 20 times that of their native counterparts (Table 3).
Survival and growth measurementsi in Summer 2021 among genotypes of Lonicera in their sixth year of growth in a common garden experiment in Orono, ME, USA.
All of the invasive, red-fruited honeysuckles survived the duration of the field study. Moreover, they grew much larger than nonnative blue honeysuckles, with average plant height, width, and canopy volume that were 2.3, 2.9, and 17 times those of nonnative blue honeysuckles (Table 3). In the full-sun environment of the common garden, invasive red-fruited honeysuckles produced a height-to-width ratio of 0.75, whereas ratios of native and nonnative blue honeysuckles were 0.99 and 1.22, respectively.
Discussion
Cultivars began blooming slightly earlier than the native genotypes, but the blooming period for native blue honeysuckles was nested fully within that of nonnative blue honeysuckles (Table 1). All nonnative blue honeysuckles produced fruit in 2021, with 200 times the fruit counts of their native counterparts and heavier fruits that contained more seeds than fruits of native blue honeysuckles (Table 2). Seed counts of nonnative blue honeysuckles averaged 13,918 seeds per plant, which was nearly 300 times the seed counts of their native counterparts. With a survival rate of 73% and small stature (Table 3), the native blue honeysuckles struggled to thrive in the environment of the common garden study. In contrast, all but one nonnative blue honeysuckle survived and grew much taller and wider, with canopy volumes more than an order of magnitude greater than those of the native counterparts (Table 3). Although nonnative blue honeysuckles plainly exceeded native genotypes in terms of survival, growth, and fecundity in the common garden environment, invasive red-fruited honeysuckles produced substantially more growth, fruits, and seeds per plant than any of their blue-fruited congeners.
The overlapping bloom times of native and nonnative blue honeysuckles suggested the potential for hybridization between planted cultivars and native relatives if the two are interfertile. Especially when native populations are small, hybridization of native plants with nonnative cultivars or with naturalized populations descended from nonnative cultivars could reduce the fitness of native populations by outbreeding depression. Conversely, hybridization between local and nonnative genotypes of blue honeysuckles could increase the fitness of local populations through hybrid vigor. Although the naturalized population near Duluth, MN, USA, represents the sole published instance of nonnative blue honeysuckles naturalizing in North America, their existence demonstrates the potential for these genotypes to reproduce by seed within understory environments in their introduced range (Schimpf et al. 2011).
Ploidy varies within and among taxa in the blue honeysuckle complex. However, both Eurasian and North American blue honeysuckles include diploid (2n = 2x = 18) and tetraploid (2n = 4x = 36) individuals (Solov’eva and Plekhanova 2003). During an analysis of genome size, Fridley and Craddock (2015) found that blue honeysuckles indigenous to New York, USA, were diploid. Lkhagvasuren (2022) reported that approximately 99% of sampled genotypes of blue honeysuckles from Canada were diploid. A mismatch in ploidy between North American genotypes and nonnative cultivars seems likely to prevent most gene flow between nonnative and native genotypes through postzygotic barriers to reproduction (Brown et al. 2023) and sterile offspring (Vallejo-Marin and Lye 2013) common to crosses between diploids and tetraploids. Although natural hybridization of diploid and tetraploid genotypes has been reported for many taxa, most gene flow across ploidies are expected to be from diploid individuals to tetraploid populations by unreduced gametes in diploid parents (Schmickl and Yant 2021). Therefore, gene flow from tetraploid cultivars of blue honeysuckle to diploid native populations seems unlikely, despite the overlapping bloom times. Although beyond the scope of this project, characterizing the genome size and genotyping seeds from openly pollinated blocks that include both native and nonnative genotypes would clarify the potential for gene flow from introduced to native populations, and vice versa.
Several authors have suggested that hybridizing nonnative and native blue honeysuckles could produce a superior crop. Erksine (1967) speculated that hybridization might produce more adaptable blue honeysuckles by combining the adaptability of nonnative genotypes with the superior fruits of native genotypes. However, cultivars with superior fruits have since been bred without the need for native North American germplasm. After nearly half a century without reported attempts, Bors (2015) was the first to report crossing native Canadian germplasm with nonnative genotypes in the Haskap breeding program at the University of Saskatchewan in Canada. However, pollen incompatibility may be a limitation to hybridizing North American native and nonnative genotypes because crossing combinations often produced few seeds (Bors 2015). Plekhanova (2000) reported that diploid–tetraploid crosses among blue honeysuckles in Asia are possible, but the success rate is less than 8%, and it is often close to zero (translated from Russian and cited by Lkhagvasuren 2022). In addition to creating potential challenges for breeding programs, incompatibility between native and nonnative genotypes would strongly limit the potential for nonnative cultivars to genetically replace native gene pools through hybridization, introgression, and genetic swamping.
We showed that the fitness of native genotypes is substantially lower than that of introduced cultivars in the full-sun environment of this minimally maintained common garden study on mineral field soil. In Maine, native blue honeysuckles tend to be restricted to bogs, fens, or cool, moist forests with organic soils, and the failure of native genotypes to thrive in our field site suggested poor competitive ability outside these indigenous habitats. It is unknown why nonnative cultivars have a greater capacity to excel in our field site, but inherent differences in adaptability between native and nonnative subspecies or possible hybrid vigor of cultivars produced from diverse germplasm are two potential explanations. Interestingly, Erksine (1967) suggested that the nonnative L. caerulea subsp. edulis is far more adaptable than native genotypes, which he viewed as unable to thrive outside of the acidic soils of their natural habitat. In our study, nearly 30% of native genotypes died (Table 3), and many that survived produced relatively small plants with few to no fruits. During our annual weeding, we had to be careful not to uproot native blue honeysuckles because they produced relatively poor, shallow root systems in our field soil. In contrast, nonnative blue honeysuckles produced much more robust root systems, as did invasive red-fruited honeysuckles. Bors (2019) demonstrated that cultivars of blue honeysuckle can grow well in neutral soils or slightly basic soils. Lkhagvasuren (2022) found that the pH of soils that support several populations of native blue honeysuckles in Canada ranged from 5.2 to 6.5, but that plants grown hydroponically responded to solution pH in a manner broadly comparable to that of nonnative genotypes. Therefore, pH adaptability alone may be unlikely to account for the differences in the root system and plant vigor observed during our study.
Plant size is an important factor for invasiveness because it affects both competitive ability and fecundity. Lkhagvasuren (2022) reported that the stem extension of wild blue honeysuckles in several indigenous sites in Saskatchewan, Canada, was an average of 4.9 to 8 cm. In Maine, USA, we have observed similar shoot extension in wild populations; however, plants in the wild often produce strikingly less shoot growth. Among our observations in Maine, USA, plants tend to complete their shoot extension for the season in a single flush by mid-July, similar to the timing for plants that grow in indigenous habitats in Saskatchewan, Canada (Lkhagvasuren 2022). In Maine, USA, plants are largely confined to bogs and fens and forest understories with moist organic soils, where they are frequently overtopped by woody and herbaceous competitors. The full-sun environment of our common garden study differed from the understory conditions in which North American native genotypes are often found (Lkhagvasuren 2022), and the potential for nonnative cultivars to simply respond more favorably to additional solar radiation remains a possible explanation for our results.
Nonnative cultivars of blue honeysuckle were substantially more fecund than their native counterparts, although they produced lower whole-plant fruit and seed counts than red-fruited invasives. Potential explanations for the especially low fruit counts of native blue honeysuckles are related to their small plant sizes or lower survival rates. Although we did not count flowers, many individuals of native blue honeysuckles produced very few. Because blue honeysuckles are self-incompatible (Bors 2015), fruit set and seed production require pollination from distinct but compatible genotypes. If the native and nonnative genotypes were not compatible, then fecundity of small, native genotypes could have been further reduced in our common garden dominated by pollen from larger, more floriferous nonnative cultivars. Ultimately, we did not track the percentage of ovules borne on each plant that developed into seeds; however, the percentage of ovules that produced seeds within individual fruits was greater for introduced cultivars than for native genotypes (Table 2).
The larger size and greater fecundity of nonnative blue honeysuckles, coupled with the status of related genotypes as high-impact invaders in Northern Europe, indicate a future risk of invasive potential in North America. Although nonnative blue honeysuckles were smaller and produced significantly fewer fruits and seeds per plant than the known invaders Tatarian honeysuckle and European fly honeysuckle (Table 2), cultivars still produced ∼13,000 seeds per plant. In Europe, the fecundity and competitive ability of Eurasian blue honeysuckle were sufficient for it to rapidly invade Norway and Sweden following its introduction to the Scandinavian Peninsula and its naturalization from settlements (Gederaas et al. 2012; Tyler et al. 2015). Although the absolute invasive potential of nonnative blue honeysuckles in landscapes across North America cannot be predicted from this study, our work builds on that of Hayes and Peterson (2020), who showed that nonnative blue honeysuckles cultivated in containers consistently produced greater measures of growth than their native counterparts. By extending the comparison with a common garden study and including measures of fecundity, we concluded that native blue honeysuckle genotypes and nonnative agricultural genotypes are not competitively or reproductively interchangeable. Despite their classification in a single species complex, the uncommon occurrence of native representatives in indigenous populations with a small stature and low fruit set is not evidence that nonnative agricultural genotypes will remain non-invasive. The tendency of nonnative agricultural genotypes toward measures that are broadly more comparable to those of widespread invaders in North America invites further caution.
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