Salix × fragilis

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Salix × fragilis, with the common names crack willow and brittle willow, is a hybrid species of willow native to Europe and Western Asia. It is native to riparian habitats, usually found growing beside rivers and streams, and in marshes and water meadow channels.<ref name=rdm>Meikle, R. D. (1984). Willows and Poplars of Great Britain and Ireland. BSBI Handbook No. 4. Template:ISBN.</ref><ref name=rushforth>Rushforth, K. (1999). Trees of Britain and Europe. Collins Template:ISBN.</ref><ref name="usfs">USFS—United States Forest Service: Salix fragilis — "Weed of the Week" . accessed 1.13.2013</ref> It is a hybrid between Salix euxina and Salix alba, and is very variable, with forms linking both parents.<ref name=Stac19p348/>

Description

Salix × fragilis is a medium-sized to large deciduous tree, which grows rapidly to Template:Convert (rarely to Template:Convert) tall, with a trunk up to Template:Convert diameter, often multi-trunked, and an irregular, often leaning crown. The bark is dark grey-brown, coarsely fissured in older trees. The lanceolate leaves are bright green, 9–15 cm long and 1.5–3 cm wide, with a finely serrated margin; they are very finely hairy at first in spring, but soon become hairless.<ref name="usfs"/>

The flowers are produced in catkins in early spring and are pollinated by insects. They are dioecious, with male and female catkins on separate trees. The male catkins are 4–6 cm long, the female catkins are the same length. The individual flowers have either one or two nectaries.<ref name=rdm/><ref name=rushforth/><ref name=bean4>Bean, W. J. (1980). Trees and Shrubs Hardy in the British Isles 8th ed., vol. 4. John Murray Template:ISBN.</ref> In late spring, the fruit capsules release numerous small cotton-tufted seeds. These are easily distributed by wind and moving water and germinate immediately upon contact with the soil.<ref name="usfs"/>

Taxonomy

Carl Linnaeus first described a willow species as "Salix fragilis" in 1753.<ref name=IPNI_777611-1/> It was later discovered that he was actually describing a species that he had also called Salix pentandra. Since at least the 1920s, botanists have applied Linnaeus's name "Salix fragilis" to both a pure species and to its hybrid with Salix alba.<ref name=Bely09/><ref name=KuzoBely18/><ref name=ChriJons05/> In 2005, it was proposed that "Salix fragilis" should be conserved for the pure species, with the hybrid called "Salix × rubens". The alternative was to conserve "Salix fragilis" for the hybrid, with the pure species requiring a new name.<ref name=ChriJons05/> Following a discussion, the Nomenclature Committee for Vascular Plants rejected the initial proposal in 2009 and opted to conserve the name "Salix fragilis" for the hybrid.<ref name=Brum09/> Irina V. Belyaeva subsequently described the previously unnamed parent species as Salix euxina,<ref name=Bely09/><ref name=KuzoBely18/><ref name=IPNI_77102927-1/> and designated a lectotype for the hybrid.<ref name=Bely09/> The lectotype was shown by molecular evidence to be the hybrid between S. alba and S. euxina,<ref name=KuzoBely18/> the name of which is written as Salix × fragilis to show its hybrid status.<ref name=POWO_777611-1/><ref name=KuzoBely18/>

Varieties

S. × fragilis is very variable, with forms linking both parents. Some formally named varieties include the following.<ref name=POWO_777611-1/><ref name=Stac19p348/> Template:As of, none were accepted at this rank by Plants of the World Online, being treated as synonyms of the species.<ref name=POWO_777611-1/> Clive A. Stace suggested some would be better treated as cultivars.<ref name=Stac19p348/>

S. × fragilis var. decipiens W.D.J.Koch (S. × decipiens Hoffm.) – assigned to S. × fragilis by Plants of the World Online<ref name=POWO_777611-1/> and by Belyaeva,<ref name=Bely09/> but to S. euxina by Stace<ref name=Stac19p348/>
S. × fragilis var. furcata Ser. ex Gaudin – male tree with relatively wide leaves; of cultivated origin<ref name=Stac19p348/>
S. × fragilis var. fragilis – male and female trees known; leaves with even, well spaced teeth<ref name=Stac19p348/>
S. × fragilis var. glauca Spenn.
S. × fragilis var. polyandra Wimm.
S. × fragilis var. rubens (Schrank) P.D.Sell
S. × fragilis var. russelliana (Sm.) W.D.J.Koch – Bedford willow; female tree with long narrow leaves having somewhat uneven teeth; of cultivated origin<ref name=Stac19p348/>

Ecology

The plant is commonly called crack willow or brittle willow because it is highly susceptible to damage from wind, ice and snow.<ref name="usfs"/> The name also derives from the twigs, which break off very easily and cleanly at the base with an audible crack. Broken twigs and branches can readily take root, enabling the species to colonise new areas when they fall into waterways and are carried some distance downstream. The plant is particularly adept at colonising new riverside sandbanks formed after floods. It also spreads by root suckers, forming pure 'groves'.<ref name="usfs"/>

Cultivation

Salix × fragilis is cultivated as a fast-growing ornamental tree. The cultivar 'Russelliana' (syn. S. × fragilis var. russelliana) is by far the most common clone of crack willow in Great Britain and Ireland, very easily propagated by cuttings. It is a vigorous tree commonly reaching Template:Convert tall, with leaves up to 15 cm long. It is a female clone.<ref name=rdm/>

Invasive species

Salix × fragilis has escaped cultivation to become an invasive species in various parts of the world,<ref>Invasive.org Salix fragilis (crack willow) . accessed 1.13.2013</ref> including: all states and territories in Australia,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> New Zealand; the upper half of the United States;,<ref name="usfs"/><ref>U.S. states with invasive S. × fragilis sites include: CN, CO, ID, IL, IN, IO, MA, MI, MN, MO, NB, ND, NJ, OH, OR, NH, NM, NV, NY, PA, RI, SD, UT, VA, VT, WA, WI, WV, and WY. (USFS & invasive.org)</ref> South Africa<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and Brazil.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> In New Zealand it is listed on the National Pest Plant Accord, which means it cannot be sold or distributed. It can replace a habitat's native plant species diversity by forming monospecific stands.<ref name=":0">Template:Cite journal</ref> As only the male plant is present in New Zealand no fruit is formed unless hybridised.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Species spread is facilitated by stem fragmentation which are carried via waterways and nearby adult individuals.<ref>Template:Cite journal</ref> Control and management for habitat restoration projects often uses herbicides.<ref name="usfs"/><ref name=":0" /><ref>Template:Cite journal</ref>