Black Poplar (Populus nigra) agg.

The Black Poplar, Populus nigra, is a pioneer tree species of riparian forests and floodplain woodlands. (Riparian = living or located along the banks of natural watercourses). It colonises bare soil along the riverbank, has spreading branches that arch downwards and may reach 35 m in height. Populus belongs to the family Salicaceae, the willow family. This tree is dioecious, with separate male and female trees. In the British isles, at least, most trees are male, like the one pictured above. The form native to the British isles is Populus nigra subspecies betulifolia. This species is rare in the British Isles due to over-management and development of watercourses and floodplains. The Black Poplar exhibits a range of interesting adaptations to its riparian habitat. As we shall see, Black Poplar is an aggregate of varieties or subspecies and the species also hybridises with certain other poplar species. it can be difficult to obtain an exact determination.

Flowers and pollen dispersal

Above and below: details of the male catkins. Black Poplars put out their flowers in early spring before the leaves emerge. The male catkins are pendulous cylindrical structures, 2 to 3 inches (5 to 7.5 cm) in length and each is an inflorescence or flowering shoot. Each flower has a bract at its base which forms the visible catkin-scales which are irregularly laciniate (bordered by a fringe) at their apex. The floral scales (sepals and petals or perianth) of each flower form a cuplike disc; 12-20 stamens with distinct filaments are borne on this disc. The anthers are red.

Above: the twig of Black Poplar are medium to golden-brown in colour. The alternate and pointed buds have hairless, shining scales (bracts) that are sticky to the touch. The buds have been used to make combustible wax and from their sap a soothing ointment used to be made.

Above: a close-up of the anthers. A catkin scale with its fringed tip is just visible in the top left.Notice the disc or cup-like nature of the flower with all but the male organs reduced. Below: old dehisces anthers showing pollen. The flowers rely on wind to disperse their pollen (anemophilous pollination).

The female catkins (illustrated below) are borne on short stalks and upright in flower and 1 to 1.5 inches (2.5 to about 4 cm) in length but elongating to 4 to 6 inches (10 to 15 cm) in fruit and then pendulous. Each female flower consists of a cup-like reduced perianth (petals and sepals) bearing a single ovary (which has a single cell containing many ovules) with a short style and two very thick stigmas that spread or curve backwards and typically branch at their tips, and also sometimes at the bases, so as to appear 4 or 8 in number (sources differ). On the fruit catkins, each fertilised flower produces a green conical capsule that opens by two valves to release numerous seeds. Each seed is clothed in silky white hairs which originate from placental tissue at the base of the ovary (at least in Populus tomentosa, Ye et al. 2014).

Above: Populus nigra. A: male twig, B: female twig; C: leaf-bearing twig; 1. Male flower, showing bract, perianth cup and red anthers; 2. stamen (anther and filament); 3. stamen with dehisced anther having shed its pollen; 4. cross-section through anther; 5. female flower showing bract, perianth cup and ovary; 6. detail of female flower showing ovary; 7. fruit capsule beginning to open by two valves; 8. opened fruit capsule showing downy seeds; and 9. seed having shed its hairs. (From: Thome, O.W. Flora von Deutschland, Osterreich und der Scweiz, 1885, Gera, Germany. Available on Wikimidia Commons). Note the characteristic leaf shape.

Varieties

The form native to the British Isles and also widespread in France and parts of Germany is the variety betulifolia (subspecies betulifolia) which has also been imported into North America. P. nigra aggregate is also found in other parts of Europe, the Mediterranean and extends as far east as Kazakhstan). The 'fastigiate' tall narrow poplars often planted alongside roads in the British Isles belong to var. italica which is also known as the Lombardy Poplar since it is thought to have arisen in northern Italy around the 17th or early 18th centuries (Meikle, 1984). This form is widely planted because the narrowness of its crown means that it drops less litter onto roads.

Seed Dispersal, Germination and Vegetative Reproduction

Initial dispersal of the seeds is by wind, making use of the the white cottony hairs. Seeds landing in water (or taken up by flood water) float (aided by the hydrophobic hairs) to be distributed by water (such plants are said to be hydrochorrous), but eventually shed the hairs and imbibe water and germinate. The seeds are small (there are about 1450 seeds per gram) and exhibit no period of dormancy and only remain viable for about two weeks. There is no soil seed bank. The hypocotyls (embryonic shoots bearing the embryonic leaves or cotyledons) develop 6 to 8 hours after moisture is imbibed.

Seeds may disperse about 100 km downstream. However, Populus nigra also shows extensive asexual vegetative reproduction. The branches and twigs are quite fragile (though perhaps not to the extent shown by Crack Willow, Salix fragilis, another riparian species in the same family). Twigs and branches readily root and this has been shown to be an important mode of reproduction. Mature trees are also shallowly rooted and a tree uprooted by floodwater may be carried downstream, shedding its branches as it goes, facilitating vegetative reproduction. By these means the same genetic individual (genotype) may spread several km, though natural stands are polyclonal, that is consisting of a number of different genotypes.

The trees also put out long and thin horizontal roots that may give off adventitious shoots which grow into new saplings, up to 100 m from the parent plant. Additionally the trees put out stolons, long horizontal shoots that put out adventitious roots and root at intervals, with rooted nodes developing into new saplings. Additionally, the branches are self-layering, particularly when bent by flood waters, putting out roots wherever they contact the ground.

The mature bark of black Poplar is yellow-grey and deeply furrowed on older parts. Old bark is thick, light and corky. The lifespan is typically over 100 years and reach sexually reproductive age by 6 to 10 years of age.

The bases of Black Poplar trunks are characteristically burred. A burr (or burl) is a large nodular outgrowth rich in epicormic buds (epicormic = beneath the bark). Burrs are said to be generally absent on hybrids of Black Poplar and in pure Populus nigra can become extensive. These buds are mostly dormant but can activate and give rise to shoots or 'sprouts' under certain conditions, such as high light levels at the base of the tree, or damage to the upper branches. These branches may be well positioned to detach in flood waters, perhaps facilitating vegetative reproduction.

The wood of Black Poplar is yellow, soft and fibrous and said to never splinter. It has been used to make trays and bowls, etc. and for making clogs and shoes. it can also be useful in construction, having been used for rafters, poles and rails.

Above: in March and below: in early April - new leaves emerging.

Below: young leaves. Young leaves may have a reddish-brown hue and a striking reddish-bronze color of the youngest leaves is indicative of the hybrid Populus deltoides X P. nigra = Populus X canadensis. The latter is said to have truncate (level) or subcordate leaf bases (cordate - curving inwards in the midline like a heart shape; subcordate - less strongly curved inwards) in at least some leaves whereas P. nigra has cuneate (cone-shaped i.e. tapering) leaf-bases where the leaf joins the petiole (leaf-stalk). However, cuneate leaf-bases can be found in the hybrid poplar also. Another distinguishing feature. Another distinguishing feature is that the hybrid often has a few glands where the petiole meets the leaf blade (inherited from P. deltoides) at least on some leaves - none can be seen here though a closer inspection of more leaves might help settle the determination. this trait is variable in the hybrid, however, which may sometimes be without the glands. The hybrid form is much the more common, having been extensively planted and also naturalised. Indeed pure Black Poplar is now rare in the British Isles. In this particular locale there are both planted trees and natural vegetation. The locale being studied here consists of a large park bordering an extensive estuary with salt marshes, so again we can assume nothing. To complicate matters further the hybrid also backcrosses with Populus nigra to give: Populus nigra X P. X canadensis in which the young unfolding leaves are said to be pale yellowish-green. This raises the possibility of an entire spectrum of plants from pure P. nigra to those with only a few P. deltoides genes and I am not entirely convinced that the different forms can always be distinguished. Given the abundance of the hybrid, however, it is perhaps only a matter of time before pure P. nigra disappears from the British Isles. It is also threatened due to habitat loss as river floodplains are increasingly managed and built upon.

As spring progresses more leaf buds open and the leaves expand as they mature and may reach 10 cm in length and 9 cm in width or more.

This specimen is one of the more mature of a stand of poplars along the stream-bank which suggest clonal spread. The branches of the hybrid are generally upward sweeping, whilst the lower branches of pure P. nigra are generally more downward-sweeping. Vegetative propagation can occur by fragments breaking off and rooting downstream, suckers from roots uncovered by floodwaters (suckers are shoots sprouting from roots) and sprouting from buried parental stems that have been washed downstream by floodwaters ('flood-training') and regrowth from damaged plants.

References

Barsoum, N. 2002. Relative contributions of sexual and asexual regeneration strategies in Populus nigra and Salix alba during the first years of establishment on a braided gravel bed river. Evolutionary Ecology 15: 255-279.

Imbert, E. and Lefevre, F. 2003. Dispersal and gene flow of Populus nigra (Salicaceae) along a dynamical river system. J. Ecol. 91: 447-456.

Legionnet, A.; Faivre-Rampant, P.; Villar, M. and Lefevre, F. 1997. Sexual and asexual reproduction in natural stands of Populus nigra.  Bot. Acta 110: 257-263.

English Botany vol 8 3rd ed., Syme, J.T.B. and Lankester, Mrs (eds) 1873. george Bell & Sons (London). Figures by: Sowerby, J.E.; Sowerby, F.L.S.; Sowerby, J. de C. and salter, J.W. Available from the Biodiversity Heritage Library and is in the public domain.

Meikle, R.D. 1984. Willows and poplars of great Britain and Ireland. BSBI Handbook No. 4. Botanical Society of the British Isles (London).

Price, D. and Bersweden 2013. Winter Trees: a photographic guide to common trees and shrubs. FSC Publications (Telford, UK). ISBN: 978 1 908819 11 6.

Ye, M.; Chen, Z.; Su, X.; Ji, L.; Wang, J.; Liao, W.; Ma, H. and An, X. 2014. Study of seed hair growth in Populus tomentosa, an important character of female floral bud development. BMC Genomics 15: 475.

Article created: 26 Mar 2020

Article updated: 17 April 2020, 23 April 2020