Research Article |
Corresponding author: Zigmantas Gudžinskas ( zigmantas.gudzinskas@gamtc.lt ) Academic editor: Peter Bruyns
© 2019 Zigmantas Gudžinskas, Lukas Petrulaitis, Egidijus Žalneravičius.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Gudžinskas Z, Petrulaitis L, Žalneravičius E (2019) Asclepias speciosa (Apocynaceae, Asclepiadoideae): a rare or unrecognized alien species in Europe? PhytoKeys 121: 29-41. https://doi.org/10.3897/phytokeys.121.33573
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Studies on populations of Asclepias syriaca L. in Lithuania revealed the occurrence of a new alien plant species, the North American native Asclepias speciosa Torr. (Apocynaceae, Asclepiadoideae), in southern parts of Lithuania – the first report of the latter species in Europe. Interestingly, a thorough analysis of herbarium specimens revealed that A. speciosa had first been collected in Lithuania in 1962, but the specimen was misidentified at the time as A. syriaca. The newly discovered population of A. speciosa occupies mesic grasslands, tall-herb fringe communities and arable field habitats. Sexual reproduction of this species was not recorded; it spreads locally by means of vegetative reproduction. We present here an exhaustive analysis of morphological characteristics and differences between A. speciosa and A. syriaca and other species of the genus, as well as a key for identification of alien Asclepias species in Europe. We predict that the effect of A. speciosa on native habitats and communities, and its economic impact, are comparable to those of the highly invasive A. syriaca. Although A. speciosa currently occurs very rarely as an alien species in Europe, its existence in other regions of Europe is highly probable.
alien species, ecology, identification, invasiveness, morphology, naturalization, reproduction
The genus Asclepias L. s. str. (Apocynaceae, Asclepiadoideae) includes about 120 species native to the Western Hemisphere. Most of the species are distributed in North America and the Caribbean, ten species occur in South America (
In Europe, three alien species of the genus Asclepias s. str. had been reported so far: Asclepias curassavica L., Asclepias incarnata L., and Asclepias syriaca L. (
In an attempt to implement the requirements of Regulation EU 1143/2014, we set out with an extensive study of the distribution, habitats, population structure, and impact of A. syriaca on native plant communities in Lithuania, because available information on this species was only fragmentary. Over several decades, A. syriaca had been reported to occur at several localities in southern Lithuania, and it has been recognized as a naturalized species (
The aim of this study was to identify reliable morphological characters for distinguishing A. syriaca and A. speciosa at the vegetative stage and during flowering, to study habitats of A. speciosa, and to evaluate the possibilities of its further spread and invasion.
The population of A. speciosa was studied in the environs of the village of Liepakojai, Alytus district, South Lithuania (54°28.51'N, 23°40.86'E). Research on morphological characteristics, flowering, and habitats was performed on 26–28 June and 20 September 2018. For comparison of morphological features, we selected a population of A. syriaca located in a similar habitat in the village of Meškučiai, Kaišiadorys district (54°44.88'N, 24°10.09'E). In each population, we studied characters of 30 flowering shoots.
Plants for the study were selected randomly from all over the colony, with a distance of at least 2 m between sampled plants. Stems were cut at ground level with garden shears. Stem height was measured from the soil level to the apex with a precision of 1 cm, using a measuring tape. The number of leaf pairs on the stem was counted, including wilted and fallen leaves. In cases where some of the lower leaves were fallen, their number was determined by leaf scars on the stem. Leaf measurements with a precision of 0.1 cm were taken from the leaf that was situated closest to the middle of the stem. The length and width of the leaf blade, length of the petiole, and diameter of the lowermost inflorescence were measured using a ruler. The length of the leaf blade was measured from the leaf base at the junction with the petiole to its tip, whereas the width of the leaf was measured at its widest point. The number of developed inflorescences on the stem was counted. The diameters of the inflorescences were measured at their middle part and the number of flowers was counted.
We studied the herbarium specimens of Asclepias species stored at the Herbarium of the Institute of Botany of the Nature Research Centre (BILAS), Vilnius, Lithuania. Specimens collected during this research were also deposited at the same herbarium.
The significance of differences between the studied characters of A. speciosa and A. syriaca was tested by applying a 2-sample t-test. All calculations were performed using PAST 3.20 (
The studied herbarium specimens revealed that A. speciosa was first collected in Lithuania on 10 August 1962 by M. Vincevičiūtė in the Žuvintas mire (Alytus district, South Lithuania); the specimen was identified by V. Galinis as A. syriaca (BILAS, 92607). This record was also mentioned by
A new locality for A. speciosa was discovered on 24 July 1992 by M. Lapelė in the village of Liepakojai (Alytus district); however, it was also erroneously identified as A. syriaca (BILAS, 63179). Just a few days later, A. speciosa was collected by Ž. Sinkevičius (BILAS, 87421). More than a decade later, in 2004, a specimen of A. speciosa was collected by V. Rašomavičius (BILAS, 76363) and also identified as A. syriaca. Therefore, A. speciosa has been present in southern Lithuania in the vicinity of the Žuvintas Biosphere Reserve for at least 56 years and at its recently identified locality for at least 26 years.
In the tall herb woodland fringe community (Fig.
Asclepias speciosa flowered abundantly in June 2018 in the studied population in the village of Liepakojai; however, no developed fruits were recorded as by September 2018. Therefore, sexual reproduction was probably absent. The colony survives and expands by vegetative renewal, spreading by long rhizomes.
Ascepias speciosa can most easily be distinguished from the closely related A. syriaca at the flowering stage, based on shape and size of flowers (Fig.
Both species also differ in the number of flowers in the inflorescence. Although the diameter of the inflorescence is almost the same in both species (Table
Comparison of some morphological characteristics of Asclepias speciosa and Asclepias syriaca. Mean values are presented with standard deviation and range (minimum and maximum) in parentheses. Different letters denote significant differences of t-test (P < 0.001) between the same characters.
Character | Asclepias speciosa | Asclepias syriaca |
---|---|---|
Mean ± SD (range) | Mean ± SD (range) | |
Stem height (cm) | 117.6 ± 11.9 (86–134) a | 95.7 ± 5.4 (80–105) b |
Number of leaf pairs | 10.3 ± 0.7 (9–12) a | 11.5 ± 0.6 (11–13) b |
Length of petiole (cm) | 0.9 ± 0.2 (0.5–1.2) a | 0.7 ± 0.2 (0.4–1.1) b |
Length of leaf blade (cm) | 17.1 ± 2.3 (10.0–21.1) a | 14.6 ± 1.6 (11.2–18.3) b |
Width of leaf blade (cm) | 10.1 ± 1.0 (8.1–12.6) a | 8.3 ± 0.9 (6.8–10.4) b |
Number of inflorescences | 3.4 ± 0.8 (2–5) a | 3.4 ± 0.7 (2–5) a |
Diameter of inflorescence (cm) | 7.4 ± 0.3 (6.9–7.9) a | 7.1 ± 0.9 (5.2–8.5) a |
Number of flowers in inflorescence | 36.2 ± 7.2 (22–57) a | 61.2 ± 19.5 (30–108) b |
Several characteristics of the leaves are reliable for distinguishing A. speciosa and A. syriaca. The leaf blade of the middle cauline leaves of A. speciosa is broadly ovate or elliptical, widest at its slightly cordate base, whereas the leaf blade of A. syriaca is ovate or elliptical, with a rounded or slightly cuneate base, and is widest near its middle (Fig.
Although stem height, number of leaf pairs, and length of petiole were statistically different between A. speciosa and A. syriaca in the studied populations, these features are not reliable for the identification of these species.
Confusion of A. speciosa with A. curassavica, which is characterized by linear lanceolate or lanceolate leaves and an orange or red corolla and yellow or orange corona, is hardly possible. Confusion is more likely between A. speciosa and A. incarnata. However, the leaves of A. incarnata are narrow lanceolate or lanceolate and the stem is evidently branched. Although the colour of the corona in both species is variable and can be similar, the hoods of A. incarnata have a rounded apex and the horns are longer than the hood. Furthermore, A. incarnata is a clump-forming plant, whereas A. speciosa has long rhizomes and usually forms colonies.
Asclepias purpurascens L. has not been recorded as having escaped from cultivation in Europe; however, it is occasionally planted in gardens and collections. This species is quite similar to A. speciosa, particularly in its vegetative characters. However, the leaf base of A. purpurascens is abruptly cuneate, whereas that of Asclepias speciosa is shallowly cordate or almost rounded, and the petiole is clearly distinct from the leaf blade. Asclepias purpurascens usually has one terminal and 1–2 additional inflorescences in the upper leaf axils, whereas A. speciosa and A. syriaca, being more robust plants, have inflorescences in several lower axils as well. Flower characters are the most reliable for distinguishing A. speciosa, A. syriaca, and A. purpurascens. The hoods of A. purpurascens are of a similar shape to those of A. syriaca, though longer (5–7 mm vs. 3.5–5.0 mm). The hoods of A. speciosa are much longer (10–13 mm) than in both other species and differ in shape, having prolonged acute apices rather than short rounded apices.
1 | Leaves of reproductive shoots linear lanceolate, narrow lanceolate or lanceolate | 2 |
– | Leaves of reproductive shoots elliptic, ovate, or broadly ovate | 3 |
2 | Stems up to 1 m tall, cymes 10–20-flowered; flower corolla red, orange or yellow, corona yellow or orange | Asclepias curassavica L. |
– | Stems 1.0–1.5 m tall, usually evidently branched; cymes 20–40-flowered, flower corolla bright purple, pink carmine, occasionally white, corona carmine or pink | Asclepias incarnata L. |
3 | Leaves of reproductive shoots broadest in basal part, with shallowly cordate or almost rounded base; flower pedicels densely white-tomentose; corolla deep purple or pink, corona pink or pinkish, hoods 10–13 mm long, with a prolonged tongue-like apex | Asclepias speciosa Torr. |
– | Leaves of reproductive shoots broadest at middle of leaf blade, with rounded base; flower pedicels glabrous or sparsely hairy, corolla green-tinged purple or pink, corona lobes pink, pinkish or purple, hoods 3.5–5.0 mm long, with rounded apex | Asclepias syriaca L. |
Analysis of all available information revealed that the first escape of A. speciosa in Lithuania was recorded in 1962 in the district of Alytus. Considering information provided on herbarium labels, this species has been recorded in two separate locations in what is now the Žuvintas Biosphere Reserve and in its vicinity, on the outskirts of the village of Liepakojai. Therefore, the currently studied colony of A. speciosa has existed for at least 26 years and this species can be considered as naturalized in Lithuania. Although the time of its introduction into Lithuania is unknown, we suppose that A. speciosa might have been introduced at the end of the 19th century or the beginning of the 20th century as an ornamental plant at the nearby Riečiai Manor Park (
Fruit set and sexual reproduction of A. speciosa were not recorded in this study. Therefore, it is possible that this population consists of vegetative descendants of one plant and represents a single clone. It is known that most species of the genus Asclepias are primarily or completely self-incompatible (
In its native habitat of Washington State (USA), individual reproductive shoots of Asclepias speciosa produce 2–7 (mean 5) inflorescences (
In its native area in North America, A. speciosa grows in a broad range of moisture conditions. Usually it is found in moderately wet but well-drained soil, though quite frequently it is found in riparian sites and sub-irrigated or occasionally flooded habitats. Occasionally, populations of this species can also be found in very dry sites (
The northern limit of the native distribution of A. speciosa in the southern part of Western Canada (British Columbia, Alberta, Saskatchewan and Manitoba) is at approximately 50°N (
Currently, A. speciosa is a very rare alien species and Lithuania is the first documented country of occurrence in Europe. However, the possibility that this species already occurs in other regions of Europe cannot be excluded. It is possible that this species has been overlooked due to resemblance with the quite variable and similar A. syriaca, particularly when plants are examined at the vegetative stage. Therefore, botanists and ecologists should pay particular attention to species of the genus Asclepias in nature, as well as critically review herbarium specimens. It seems unlikely that A. speciosa has only been introduced into Lithuania.
We wish to thank Mr Theodor CH Cole (Freie Universität Berlin, Germany) for linguistic improvement of the manuscript.