Convolvulus silvaticus (syn.: Calystegia silvatica), widely overlooked in Belgium?
First of all, a word about the generic circumscription of Calystegia and Convolvulus. Despite Calystegia being morphologically well separated from Convolvulus, molecular data show that it is in fact nested in Convolvulus (Stefanovic & al. 2003). It should therefore be included in the latter or Convolvulus be further divided in several additional genera. Modern floras now increasingly adopt this taxonomic viewpoint, also in western Europe (see for instance van der Meijden 2005). We here apply it as well.
Convolvulus sepium (syn.: Calystegia sepium) belongs to a taxonomically difficult species complex in which hybridisation and introgression frequently occurs. Several species of this complex are cultivated in Europe. In addition to native Convolvulus sepium at least four others are usually involved of which Convolvulus dubius (syn.: Calystegia pulchra) and Convolvulus silvaticus probably are the most widespread (see Hyam 2000). R.K. Brummitt revised the Belgian collections of Convolvulus in 1960. In addition to Convolvulus sepium he only recognised C. dubius from Belgium and it is represented by very few collections. Non-native taxa of Convolvulus were apparently very rarely cultivated in Belgium (or seldom escape), or were simply not collected due to confusion with native C. sepium. Introgression has further blurred the specific limits between these species and several hybrids have been described (Brummitt & Chater 2000). Finally, non-native forms of Convolvulus sepium might have been overlooked as well.
However, in parts of the British Isles Convolvulus silvaticus (and its hybrids) are now more common than native C. sepium (Brummitt 1998)! Therefore, aberrant populations of Convolvulus have been collected and critically assessed in the past years in order to find out whether or not C. silvaticus occurs in Belgium (Verloove, in press).
Indeed, typical plants of Convolvulus silvaticus were soon met with on various occasions (but so far all localities are confined to the province of West-Flanders). At a first glance, they hardly differed from native C. sepium and could therefore easily pass unrecorded. Corollas often are slightly larger but this is not always true. Bracteoles by far provide the best distinguishing feature. In Convolvulus silvaticus bracteoles are more or less rounded to slightly emarginate at apex, usually 18-40 mm wide when flattened, strongly overlapping at edges and largely obscuring the sepals and distinctly saccate at base. Corollas usually exceeds 50 mm in length (see pictures). In Convolvulus sepium, on the contrary, bracteoles are acute at apex, usually 10-18 mm wide when flattened, not or scarcely overlapping at edges, scarcely obscuring sepals and not saccate at base. Corollas rarely exceed 50 mm. Where both species are found together more or less intermediate plants are sometimes found as well.
In most places Convolvulus silvaticus occurs in relative abundance and seems well established. It most likely is largely overlooked in Belgium and probably occurs elsewhere too. It is usually seen in similar habitats like Convolvulus sepium: fences, waste land, as a weed in plantations, etc. Botanists are encouraged to look for it. Voucher specimens and/or photographs may be transmitted to the author for examination.
For convenience, a key for the identification of all Belgian representatives of Convolvulus s.l. (incl. Calystegia) is presented below.
1. Bracteoles ovate, at least partly obscuring sepals. Stigmas swollen. Ovary unilocular (Calystegia) === 2
1. Bracteoles lanceolate to filiform, not obscuring sepals. Stigmas filiform. Ovary two-celled (Convolvulus s.str.) === 5
2. Stem not twining (procumbent). Leaves reniform, thick. Corolla pink with 5 white stripes (native) === Convolvulus soldanella
2. Stem twining. Leaves triangular and sagittate, thin. Corolla white or pink with white stripes === 3
3. Bracteoles acute at apex, usually 10-18 mm wide when flattened, not or scarcely overlapping at edges, scarcely obscuring sepals, not saccate at base. Leaves shiny, leaf sinus V-shaped. Corolla length rarely exceeding 50 mm === C. sepium
3. Bracteoles more or less rounded to slightly emarginate at apex, usually 18-40 mm wide when flattened, strongly overlapping at edges and largely obscuring sepals, distinctly saccate at base. Leaves often dull, leaf sinus square or rounded, not V-shaped. Corolla length usually exceeding 50 mm === 4
4. Bracteoles usually 18-25 mm wide when flattened. Corolla pink, often with 5 white stripes, 50-75 mm. Peduncle (at least some) with a narrow, wavy wing. Peduncle, pedicels and lowerside of young leaves (especially near sinus) sparsely shortly hairy. Sinus of leaves more or less square === Convolvulus dubius
4. Bracteoles usually up to 40 mm wide when flattened. Corolla white (rarely pale pinkish with white stripes), 50-90 mm. Peduncle not winged. Plant glabrous. Sinus of leaves rounded === C. silvaticus
5. Leaves gradually tapered to base, without distinct petiole, entire at base. Corolla pale bluish with white and yellow bands. Stem procumbent to ascending, not twining. Ovary and capsule hairy === C. tricolor
5. Leaves very abruptly narrowed into distinct petiole, hastate to sagittate at base. Corolla white or pinkish. Stem twining. Ovary and capsules glabrous (native) === C. arvensis
More information is available at http://alienplantsbelgium.be/content/convolvulus-silvaticus
Brummitt R.K. (1998) Calystegia. In: Rich T.C.G. & Jermy A.C. (eds.), Plant crib: 231-233. BSBI, London.
Brummitt R.K. & Chater A.O. (2000) Calystegia (Convolvulaceae) hybrids in West Wales. Watsonia 23: 161-165.
Hyam R.D. (2000) Calystegia. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 6. Cambridge University Press, Cambridge: 114-115.
Stefanovic S., Austin D.F. & Olmstead R.G. (2003) Classification of Convolvulaceae: a phylogenetic approach. Syst. Bot. 28: 791-806.
Van der Meijden R. (2005) Heukels’ Flora van Nederland (23e druk). Wolters-Noordhoff, Groningen: 685 p.
Verloove F. (in press) Weinig gekende Calystegia taxa (Convolvulaceae) in België. Dumortiera.