Hedlundia

Hedlundia is a genus of plants in the rose family (of Rosaceae). They are shrubs or small trees that have a hybrid origin involving crosses between Aria and Sorbus sensu stricto. There are about 48 species are distributed across central, western and southern Europe, Scandinavia, Turkey, the Caucasus, Crimea, and also central Asia. The term Hedlundia was published in 2017.

Description
Hedlundia species are small trees or shrubs, with simple leaves, pinnatilobate (having lobes arranged in a pinnate manner) or basally pinnate with 1–2(–3) leaflets. They are white- or greenish-grey-tomentose (covered with dense, matted, woolly hairs) beneath, with 7–15 pairs of lateral veins, with small to prominent, long, sub-acute to obtuse lobes with a variable number of teeth.

They have flowers with white petals and 2–3 styles. The fruit is medium-sized, orange-red to crimson in colour, with few to sparse small lenticels (porous tissue consisting of cells). (Sennikov & Kurtto 2017; Rushforth 2018).

History
They appear to have arisen via hybridization events between the clades Aria (Pers.) Host and Sorbus (L.).

The hybrid term ×Ariosorbus was used by Mezhenskyj et al., 2012.

Majeský et al. (2017) reviewed the taxonomic treatments of apomictic taxa in the Asteraceae and Roseceae and provided 5 criteria to recognise an apomictic species.

The rules around plant naming say that a genus of hybrid origin needs to have a new name to distinguish it from the two genera that have created it. In other words, the species that arose from a cross between Sorbus and Aria needed a new genus name.

Rushforth in 2018 agreed with Sennikov & Kurtto (2017) changes. European and West Asian hybrid species involving Aria edulis (all) with variously Sorbus aucuparia, Torminalis clusii and rarely Chamaemespilus alpina are treated in Sennikov & Kurtto (2017) under the genera Hedlundia (Aria × Sorbus), Borkhausenia  (Aria × Sorbus × Torminalis), Karpatiosorbus  (Aria × Torminalis), Majovskya  (Aria × Chamaemespilus) and Normeyera  (Aria × Chamaemespilus × Sorbus).

In Rosaceae subtribe Malinae, many taxa in nature and in horticulture are the result of hybridisation between two (or more) genera. This situation results in the use of nothogenera (hybrid name), e.g. ×Sorbaronia (= Sorbus × Aronia ), ×Pyraria  (Aria × Pyrus). ×Sorbocotoneaster (= Sorbus × Cotoneaster) or ×Amelasorbus  (Amelanchier × Sorbus). If narrower genera are used, notably in Sorbus sensu lato, some authors (e.g. Sennikov & Kurtto, 2017; Kurtto et al., 2018; Sennikov, 2018), have treated some of the resulting nothogenera as genera in their own right, and this can lead to the use of other names, including Hedlundia [Aria  × Sorbus sensu stricto] and Scandosorbus  [= Aria × Sorbus sensu stricto × Torminalis, replacing the illegitimate Borkhausenia ; see Sennikov, 2018].

Even in Britain, the use of separate generic names for the hybridogenous taxa by Kurtto et al. (2018), and Sennikov & Kurtto (2017), however, fails to address the full complexity of the situation, as some of the species result from more than one hybridisation event. For example, Robertson et al. (2004) showed that Sorbus arranensis (Arran Whitebeam) was the result of hybridisation between S. aucuparia and S. rupicola. (Rock Whitebeam); in the system of Sennikov & Kurtto (2017), this was treated as Hedlundia arranensis. While, Sorbus pseudofennica (Arran Service Tree), was treated as Hedlundia pseudofennica, is in turn the result of back-crossing of Hedlundia arranensis onto Sorbus aucuparia (Robertson et al., 2004). This shake up of the taxonomy has changed the names of the three microspecies that are unique to the island of Arran on the west coast of Scotland. Arran whitebeam (Hedlundia arranensis), Arran service-tree (Hedlundia pseudofennica) and the Catacol whitebeam (Hedlundia pseudomeinichii) where all previously Sorbus species and have now been moved to the newly created genus Hedlundia as they are the product of inter-generic crossing involving rowan (Sorbus aucuparia) and rock whitebeam (Aria rupicola).

While including this species in Hedlundia reflected the broad contributions of the parental genomes, extending the splitting approach to its logical conclusion, the Arran service tree could be treated in another nothogenus with a name like ×Sorbohedlundia. To make matters yet more complex again, Sorbus pseudomeinichii (False Rowan) is the result of a further round of back-crossing of the Arran Service Tree (Sorbus pseudofennica) with Sorbus aucuparia (Robertson & Sydes, 2006). Problems still occur, such as the naming of the cross could this be "×Sorbosorbohedlundia" ? Including taxa like S. pseudofennica and S. pseudomeinichii in Hedlundia, as done by Sennikov & Kurtto (2017), means that Hedlundia becomes a name applied to taxa with distinct evolutionary histories.

Taxonomy
The genus name of Hedlundia was chosen by botanists Alexander Nikolaevitsch Sennikov and Arto Kurtto in 2017, "The new genus is dedicated to Johan Teodor Hedlund (1861 - 1953), the renowned Swedish expert in Sorbus, who contributed very much to the early understanding of the Sorbus hybrida aggr. in Scandinavia and Britain".

Johan Hedlund had studied species variation in this group of trees. He had published nearly 50 new names or combinations of Sorbus L. (Rosaceae), in 1901.

The type species is Hedlundia hybrida (L.) Sennikov & Kurtto, Memoranda Soc. Fauna Fl. Fenn. 93: 37 (2017).

They are known as limipihlajat (in Finnish) and rönnoxlar (in Swedish).

The term Hedlundia is not accepted by all botanists, who still refer to the species as Sorbus such as Sorbus austriaca, and GRIN (United States Department of Agriculture and the Agricultural Research Service) only accepts 3 crosses (as of October 2023);×Hedlundia armeniaca, ×Hedlundia persica and ×Hedlundia thuringiaca.

The Finnish rowan (or Finnish whitebeam), (Hedlundia hybrida, formerly Sorbus hybrida) is, as the scientific name suggests, a native of at least the rowan (Sorbus aucuparia) and the German rowan (Aria edulis).

Known species
According to Kew and Plants of the World Online:

Distribution
They are native to most parts of Europe (within Albania, Austria, Baltic States, Bulgaria, Crimea, Czechoslovakia, Denmark, Finland, France, Germany, Great Britain, Greece, Hungary, Ireland, Italy, Norway, Romania, Spain, Sweden, Switzerland and Yugoslavia). As well as parts of central Asia, (within Kazakhstan, Kirghistan, North Caucasus, Tajikistan, Transcaucasus, Turkmenistan and Uzbekistan) and a few regions in the western Asia, (within Iran and Turkey).

Nine of the species occur as natives in the Nordic countries, most of which grow on the west coast of Norway. Species have also been recorded on the island of Sicily. Species, Hedlundia austriaca, Hedlundia hybrida and Hedlundia mougeotii are all found in Finland.

They have been introduced into several countries (and regions) including; Belgium, parts of Central European Russia and parts of USA (within the states of Illinois, New Brunswick, Utah, Vermont and Washington).

Habitat
The various Hedlundia species can grow in a variety of places. Such as Hedlundia anglica is usually found growing on cliffs, quarries and rocky hillsides. While, Hedlundia hybrida grows on rocky meadow banks and in broadleaf woods.

Uses
Several species are grown as ornamental trees, such as a specimen tree of Hedlundia tamamschjanae, which grows in Kew Gardens and was introduced to Britain by the Armenian botanist Dr Eleonora Gabrieljan.

Hedlundia persica is used as a source of firewood in Iran, Kazakhstan, Kyrgyzstan, Tajikistan, Turkey, Turkmenistan and Uzbekistan.

Endangered
Hedlundia schwarziana (formerly Sorbus schwarziana) is listed as Critically Endangered by the IUCN in Germany. It is being threatened by forest management changes which includes forest clearance and shading from spruce plantations.

Species Hedlundia persica is rare and red-listed in Kyrgyzstan (Davletkeldiev 2006).

Hedlundia pseudofennica, which is endemic to the Isle of Arran in Scotland, is threatened by habitat loss.

Hedlundia cuneifolia (syn. Sorbus cuneifolia, Llangollen Whitebeam, ) in Wales, UK has been assessed on the IUCN Red List of Threatened Species as 'Endangered'.

Other sources

 * Kurtto, A., Sennikov, A.N. & Lampinen, R. (2018). Atlas Florae Europaeae. Distribution of vascular plants in Europe 17: 1-132.