Kłopotliwe derenie - Cornus alba L. i C. sericea L. (Cornaceae). Dwa gatunki czy jeden?

The history of taxonomy and problems with identification of two very similar and closely related dogwoods Cornus alba and C. sericea are discussed. They were described by Linnaeus on the basis of flowering cultivated specimens of unknown origin. When characterizing their fruits, Linnaeus got the information from the older, “pre-Linnean”, literature and wrote that they were white in C. alba (Linné 1767) and black in C. sericea (Linné 1771). It was soon pointed out that both taxa had white fruits, however, their specific identity has not been questioned for a long time. Cornus alba and C. sericea are considered to be geographically isolated – the former is recorded from Siberia and NE Europe, while the latter from North America, but both dogwoods are often cultivated and naturalize in many places outside their natural ranges. In European dendrological literature, in which both plants are usually mentioned, there are permanent controversies concerning the differences between them. Attempts are still made to distinguish them, mainly on the basis of their stones and leaf shapes. Narrow stones and abruptly narrowed leaf apex have been attributed to C. alba, while broad stones have been said to be characteristic to C. sericea. Our analysis reveals that shapes of stones of discussed taxa are very variable and their ranges of variability overlap to a considerable extent. The similar kind of variability can be observed in the shape of leaves of both dogwoods. In C. alba leaf blades are most often broadly elliptic and abruptly narrowed, while in C. sericea they are most often broadly ovate and gradually narrowed at the apex. It must be said that there are also numerous exceptions to the above scheme. The filigree pattern of cuticle and the wax crystals on the abaxial leaf surface are sometimes useful for distinguishing C. alba and C. sericea. Unfortunately both features are also variable as those characterized above. Taking into consideration the great similarity of discussed dogwoods and difficulties with their identification, in our opinion the broad species concept of C. alba (including C. sericea) is most reliable and practical. However, as it appears from presented results, both taxa are not fully identical, so the rank of subspecies proposed by Wangerin (1910) – C. alba L. subsp. alba and C. alba subsp. stolonifera (Michx.) Wangerin, seems to be most appropriate in their case. It facilitates identification of wild plants of C. alba s.l. both in flowers and fruit as well as in the vegetative state. It also helps avoid controversy with the classification of cultivars of uncertain origin.