TWO NEW INTERMEDIATE SPECIES BETWEEN SORBUS AUCUPARIA, ROWAN AND S. HYBRIDA ON GOTLAND
Submitted by Tina on 8 February 2019.
TWO NEW INTERMEDIATE SPECIES BETWEEN SORBUS AUCUPARIA, ROWAN AND S. HYBRIDA ON GOTLAND
By Joel Levin, Michael F. Fay, Jaume Pellicer and Mikael Hedrén (authors)
During the last ice age, the Baltic Sea basin was covered by the thick Weichselian ice sheet, which finally melted away from the area at ca 17000 years ago. At first, the basin was filled with fresh water, forming the Baltic Ice Lake. Due to land uplift, dry land, including the big islands in the Baltic Sea and bordering lowland regions, became available for plant colonization. The whole flora is therefore quite young. Nevertheless, some genera have a rich diversity in the region. One of those is the genus Sorbus in the Rose family Rosaceae – rowans and service trees (sw. rönn, oxel). This genus consists of small and medium-sized trees, and some species are common and dominant in the region.
Two of the species are S. aucuparia, rowan (sw. rönn), which grows virtually everywhere, and S. hybrida, Swedish service tree (sw. finnoxel), which is fairly widespread on Gotland and Åland. Sorbus hybrida also has a few scattered occurrences on bordering mainlands, and is distributed along the coast of southern Norway.
During botanical excursions on Gotland we have become interested in trees of intermediate appearance between the two above-mentioned species. Such trees are recognized by a leaf shape that is intermediate between those of S. aucuparia and S. hybrida, suggesting that they have arisen by hybridization. The intermediates are usually scattered and widely separated from other such trees, but in some parts of Gotland they form whole populations. Previous studies, conducted by the lecturer and geneticist Alf Liljefors in the mid 20th century, have shown that some intermediates are sterile, unable to reproduce, while other trees have inherited the capacity for asexual seed production from one of the parents, S. hybrida. As these seeds give rise to offspring with the same leaf shape as the mother trees, they have been treated as a species of their own, S. teodori, Swedish rowan (sw. avarönn), or sometimes included in the Norwegian S. meinichii, which is characterized by a similar leaf shape.
However, we have also observed that the local populations of intermediates differ slightly from each other in leaf shape, stature, and that they grow in somewhat different habitats. Therefore, we performed a detailed study, in which we made careful analyses of leaf shapes, studied molecular differentiation, performed flow cytometry analyses to determine chromosome numbers, and made a growth experiment, in which we checked whether seed parents gave rise to constant offspring. We also sampled more widely, and included material from Åland and the Swedish east coast. Finally, we combined all information and made a synthesis.
Three populations on Gotland differed from each other in leaf shape and in molecular markers, but trees within each of the populations were closely similar to each other in both these aspects. We concluded that the populations have independent origins from separate hybridization events, and that the trees have the capacity to set seed by asexual means. The three populations are therefore best recognized as different species. The population from Fårö is already known as S. teodori Liljef. The other two populations are here described as S. faohraei Hedrén & J.Levin (sw. “Bungerönn”) and S. atrata Hedrén & J.Levin (sw. “Garderönn”), respectively. One additional population on Gotland, and the dispersed population of intermediates on Åland ,apparently consist of sterile hybrids and do not merit taxonomic recognition.
The three intermediate species on Gotland all grow in dry calcareous soils, but in slightly different habitats. The new S. atrata on southeastern Gotland is the most extreme, even found in fissures in open limestone karst. The other two species can be found in open woodlands or at forest edges, and S. teodori is possibly sometimes planted locally.
The intermediates have obviously evolved within the areas they grow today, emphasizing the value of the unique mosaic of dry alvar and open pine forests on Gotland as a natural laboratory for evolution and speciation.
Both S. teodori and S. faohraei develop into beautiful trees in cultivation, as shown by trees kept in the Botanical garden of Lund, and they should be promoted as interesting alternatives to other ornamental trees used in gardens and plantations in our cities.
Selected references
Fåhræus, G. 1980. Sorbus teodori, avarönn, och dess förekomst på Gotland. – Svensk Bot. Tidskr. 74: 377–382.
Fåhræus, G. 1989. Finns även fagerrönn, Sorbus meinichii, på Gotland? – Svensk Bot. Tidskr. 83: 186–190.
Hedlund, T. 1901. Monographie der Gattung Sorbus. – Kungl. Svenska Vetenskaps-Akademiens Handl. 35: 1–147.
Hedrén, M. 1994. En population av avarönn, Sorbus teodori, på östra Gotland. [A population of Sorbus teodori on E Gotland, Sweden.] – Svensk Bot. Tidskr. 88: 13–16.
Högström, S. 2003. Fagerrönn på Gotland. – Svensk Bot. Tidskr. 97: 15–22.
Liljefors, A. 1934. Über normale und apospore Embryosackentwicklung in der Gattung Sorbus, nebst einigen Bemerkungen über die Chromosomenzahlen. – Svensk Bot. Tidskr. 28: 290–299.
Liljefors, A. 1953. Studies on propagation, embryology, and pollination in Sorbus. – Acta Horti Bergiani 16: 277–329.
Liljefors, A. 1955. Cytological studies in Sorbus. – Acta Horti Bergiani 17: 47–113.
Liljefors, A. 1972. Sorbus teodori on Åland. – Svensk Bot. Tidskr. 66: 87–93.
Rich, T.C.G., Houston, L., Robertson, A. and Proctor, M.C.F. 2010. Whitebeams, rowans and service trees of Britain and Ireland. A monograph of British and Irish Sorbus L. – Bot. Soc. British Isles.
Salvesen, P.H. 2011. Rogn nog asal (slekten Sorbus I Arboretet på Milde, 2: Norske spesialiteter [The genus Sorbus at the Milde Arboretum, 2: Norwegian specialties]. – Årringen 15: 77–123.
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https://doi.org/10.1111/njb.02035
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