Identifying Native Black Poplars
The Native Black Poplar (Populus nigra subsp. betulifolia), also known as the Water Poplar, Wild Black Poplar, and Atlantic Poplar is a sub species of Populus nigra restricted to Britain, Ireland, North-West France, and the Low Countries.
The native black poplar was historically planted to indicate field, parish, manor, and county boundaries and as a waymark in Britain. As such many specimens exist in hedgerows, where they can thrive as the native black poplar is a light-demanding species. However, it's native habitat, much of which has been lost, consists of flood plains, river banks, and streams. These damp habitats are critical for regeneration as seed falling in June must land on damp mud which must remain wet during and following germination. The historical loss of suitable habitat and timber harvesting, together with the increasing isolation of remaining native black poplars and propensity for hybridisation, have contributed to the disappearance of populations reproducing from seed. In the early 21st century the population was estimated to be approximately 7000, only ~10% of which were female. Much work mapping and propagating cuttings for planting out followed in the 2000's, which should help to safeguard the species in regions such as Sussex. Today citizen science platforms such as iNaturalist provide an invaluable opportunity to identify and monitor native black poplar trees.
Identifying native black poplars can often be very difficult owing to the close similaritity of many it's features to hybrid poplar (Populus × canadensis, a cross between Populus nigra and Populus deltoides) cultivars. This guide extends the 1997 Botanical Society of Britain and Ireland (BSBI) Populus spp. crib sheet written by D. D. Hobson, providing a useful reference for distinguishing between the native tree and the commonest hybrids as mature trees. Look out for other cultivars (see notes). A mix of character across the two columns is likely to be a backcross.
| Native | Hybrid and hybrid cutivars | |
|---|---|---|
|
Form |
Trunk leaning | Trunk straight or slightly leaning. |
| Bark | Bark knobbly (burry), very fissured 1. | Bark less knobbly, less fissured 1. |
| Branches | Main branches sweep down, twigs sweep up. | More erect branches. |
| Shoots | Shoots round in cross-section. | Shoots square. |
| Unfolding Leaves | Unfolding leaves green or slightly red 2. | Unfolding leaves red/brown. |
| Exudate | Sticky yellow exudate on terminal bud. 4, 5. | No exudate. |
|
Glands |
Leaf - no glands, smooth. | Glands at the base of leaf blade and stalk - feel knobbly if rub finger across. |
|
Leaf margin hairs |
Leaf margin very few hairs. | Margin many hairs. |
| Twigs and petioles | Twigs and petioles hairy 4, 5. | Twigs and petoiles glabrous 5. |
| Leaf margin teeth | Leaf margin no hooks to teeth. | Leaf margin with hooked teeth. |
| Spiral galls | Spiral galls on petioles usually present 3. | Spiral galls absent (usually 3). |
| Mistletoe | No mistletoe. | Occasionally supports mistletoe. |
| Suckering | No suckers unless roots damaged. | Occassionally suckers. |
| Buds | Buds tend to point outwards. | Buds close to stem. |
Notes
1 - A relatively smooth-trunked, boss-less clone of the native exists - clone 34. Much planted on the North Wirral coastal plain and on the dunes of Sefton Coast.
2 - The hybrid characters in the table above are for the commonest clone of mature trees in Cheshire, 'Serotina' (male). Other hybrids have some differences:
'Regenerata' (female) originally common by railways, leaves unfold brown.
'Robusta' (male) has pubescent twigs, leaves unfold orange-brown.
'Marilandica' (female) has leaves opening green.
The amount of red varies with season (environmental factors). Young trees grown from seed may tend to have more red than adult trees - but very few examples have been recorded yet.
3 - Spiral galls (Pemphigus spyrothecae) are much more common on the native, but reports elsewhere in the UK say are not entirely confined to it. The population of spiral galls present varies year to year. We have found the occassional spiral gall on backcrosses. Natives tend to be eaten by invertebrates (such as Willow Weevils, Dorytomus spp.) much more than hybrids or backcrosses!
4 - Pubescent and glabrous twigs can appear on the same tree - look at several.
5 - Hairs on leaves, petioles and even twigs soon drop off (by July).
Leaf shape is very variable within and between trees. Petioles are usually flattened, but can be rounded in some hybrids. Neither of these are good characters to separate a native from hybrids.
Lombardy poplars are a growth form of the native, with identical leaves and usually spiral galls. The hybrid between Lombardy and the American poplar is similar to the hybrids described in the table above, but intermediate in shape.
A hybrid between a Lombardy and native was found at Capenhurst in 2014, in a row of females. It looks native apart from a more ascending shape. Need to be careful where lines of trees have influenced the shape of a black poplar - especially if surrounding trees subsequently felled leaving the native alone! (Rivacre, Wirral).
Several back-crosses between a hybrid and the native have been found in the Wirral. They are mostly Victorian plantings (the only one for which we have a definite date is 1851); impressive large mature trees, intermediate in characters, but with red unfolding leaves. More ascending in shape than the native.
Dr Hilary Ash, Wirral Wildlife
This post is a working document, additional photos for the characteristics will also be added when obtained.
