Taxonomy
A Populus nigra specific gall Pemphigus bursarius, Walthamstow Marshes, Essex, August 2007. © F Rumsey
Three sub-species with a broadly geographical as well as morphological basis are recognised in Europe, largely discriminated on the nature and extent of their hairiness, there is however considerable overlap.
The western European subsp. betulifolia (Pursh.) W. Wettst. has somewhat pubescent twigs and young leaves, it is the native form found in the British Isles, although the extent to which its current distribution can be considered natural and its status in many areas is questionable (see distribution). Glabrous plants indistinguishable from the more Central and Eastern European subsp. nigra are also to be found in apparently natural situations in the British Isles. Subsp. caudina (Ten.) Bug. , with more hispid foliage and twigs, occurs in the Mediterranean region (North Africa, Spain, Southern Italy, the Balkans, eastwards to Iran).
Numerous growth-forms have been selected and have been variously taxonomically treated, from species to cultivar level. Historically the most frequently planted is the fastigiate male clone Cv. ‘Italica’ – the Lombardy Poplar, introduced from N. Italy about 1758. A particular clone of subsp. betulifolia grown around Greater Manchester and widely distributed from there is known as the Manchester Poplar – while pollution tolerant it has recently proven to be highly susceptible to Poplar Scab (see Diseases). Recent plantings have tended to be of cultivars derived through hybridisation with North American species, such as P. deltoides and their backcrosses with different forms of the parental taxa. This has made identification and discrimination of “native” Black Poplars particularly challenging.
Genetics
Populus nigra is a diploid with 2n=38 chromosomes. As one of 3 species included within the European Forest Genetic Resources Programme (EUFORGEN) a considerable amount of data has accrued as to the levels and distribution of genetic diversity within this species across Europe (Cottrell et al., 2005), this has recently been supplemented by local initiatives driven by LBAP’s which hopefully will provide a detailed picture across the British Isles (Adams, 2009). The species shows rather low genetic diversity across its wide range but the greatest levels are present south of the Pyrenees, Alps and Balkans indicating that these were refugial areas during the last glaciation and recolonisation of Northern Europe has been accomplished by relatively few clones, naturally and later with human assistance. Probably less than 100 clones exist in the British Isles, within Greater London 6 of the previously identified clones are known to exist in multiple locations, the most extensive being the Manchester Poplar clone. New clones are still being discovered as more intensive sampling is being made; recent work has shown another 9 clones to be present in Greater London, generally as single individuals, on the Thames bank in Kew, Syon Park and at Barnes, where they are threatened by planned renovation work.