Black or Water Poplar

Distribution - The "Native" Black Poplar or Water Poplar (its old English name) Populus nigra var. betulifolia, Britain's rarest timber tree, is now only found as a few thousand old mature trees scattered throughout the English lowlands. Nevertheless, 220 trees have so far been found in Essex, and possibly 100 more are awaiting discovery. Most of the trees are now so old that they are coming to the end of their active lives and many are lost to drought or storms each year. As one of our Biodiversity Action Plan species, we are attempting to locate all the trees in Essex so that measures can be put in place to conserve them as long as possible and to clone each group of them by taking cuttings to maintain the genetic diversity as a barrier to alien poplar diseases. A clone bank has been set up at Daws Hall in north east Essex and another is due to be established in the Lee Valley Park. Once cloned, we are encouraging the planting of cloned trees back in their local areas, if possible in the same vicinity as the original trees and to discourage the mixing up of clones.

Many commercially available trees are of dubious origin and ideally local trees should be recycled through our genuine clone nurseries. The trees are separate male and female, but seldom close enough in location to cross-pollinate so virtually none of the trees are reproducing by seed. Furthermore, there are so many male hybrid trees around (mostly cultivars of the cross Populus nigra Water Poplar x Populus deltoides American Cottonwood) that most of the pollen they get fertilised by is alien.

Identification - this is the only true native subspecies and colonised the British Isles around 7000 years ago. It is a tall broad domed tree with huge arching branches and a heavily burred trunk. It can grow up to 35 metres tall.

It can be difficult to identify a true Water Poplar as they have not only been hybridised with several other alien poplar species, but the crosses are fertile and can back cross with either parent. One particular genetic clone is however planted in vast numbers and that is the Lombardy Poplar (var.italica). It is a mutant of the Water Poplar that has a tall spindle-like shape with branches arising almost vertically around a single trunk forming a spire-shaped tree. Fortunately the problem of identifying a true Water Poplar is all done for us by an aphid that only parasitises this one species. The same aphid species it is also found on the Lombardy, which can be a problem with young trees that have not yet expanded to their final form. The aphid attacks the leaf stalks from about early April onwards and causes them to form a large spiral gall (see diagram below). They occur on all of the trees (unless they are water stressed that year) and are therefore a reliable means of identification. Additional characters to look for are: the male trees have scarlet catkins in early April, the females (only 55 so far in Essex) greenish/yellow catkins, both very similar to the American Hybrids. The leaves however are very different, as the two diagrams depict. Thicker in texture and with very shallow (crenate) teeth, and in the case of the young leaves with distinctly hairy leaf stalks. There are always some diamond shaped leaves to be found on the "native" species, with wedge-shaped bases and long tapering apices - although some of the leaves may be broader and truncate at the base they are never cordate (heart shaped). The hybrids have rounded hooked teeth, usually with a thickened edge. When mature the tree shape is also very different, the "native" having a broad crown with branches that rise up at a shallow angle and then sweep downwards in arch-like curves, whereas the hybrids have sharply ascending branches. Most "native" trees are heavily burred with callus-like knobs bearing numerous short leafy shoots. This may be due to a virus however, as several of our Essex trees are not burred. The bark is generally dark on the main trunk with slit-like splits, whereas most of the hybrids have an ash-grey bark with rounded spaghetti-like bark ridges.

General ecology - Water Poplar are huge trees that in the wild occur thinly scattered along river valley flood plains and on the shingle bars of large braided river systems. They are not woodland species and they are often found planted in rows. It has been used for a variety of domestic purposes since Neolithic times and hence the distribution and planting patterns. In Britain it is believed that all the veteran (old mature) trees have been planted and as they were used largely for farm buildings, carts etc., they tend to occur well away from water on old farmsteads or as amenity plantings, often in quite large numbers, but they also occur quite frequently as isolated trees among groups of hybrids or by themselves alongside rivers.

Threats are from absence of natural habitat, accidental felling, gales, hybridisation, predominant male population and neglect.

Management - Most of the veteran trees have been pollarded at least once, probably originally to encourage several massive branches to develop for timber. They tend not to survive a second pollarding and are ideally left alone. If they fall down, do not remove the trunk or chain saw the stump - They have remarkable powers of regeneration and stumps will regrow into fresh trees from buds in the base of the trunk and fallen trunks will produce numerous vertical shoots that can be used as cuttings in the same way as Cricket-bat Willow sets.

Surveying tips - careful not to record Hybrids!

FEATURE BLACK OR WATER POPLAR LOMBARDY POPLAR and other hybrids
General shape

Large tree with a broad crown and branches that initially rise then sweep down in arch like curves

Tall with sharply ascending branches - spire shaped tree
Bark Dark grey - brown with deep fissures heavily bossed and burred with callus like knobs with numerous short leafy shoots. (may be due to a virus as some Essex trees are not burred) Ash grey bark with rounded spaghetti like bark ridges
Twigs Cylindrical and tapering with ascending tips. Current years growth is pubescent (hairy)  
Leaves Deltoid - ovoid (oval) (see picture) with serrated margins - never heart shaped (cordate). Thick texture with shallow crenate (toothed) leaves.
The leaf stalks are flattened. Young leaf stalks are hairy. Nearly all leaves show a characteristic Spiral gall on the leaf stalk - this confirms that the tree is a black poplar.
Often heart shaped (cordate) with rounded hooked teeth with a thickened edge.
Can have the spiral gall present on leaf stalk but leaf shape is different
Catkins Catkin flowers around April
Male - crimson
Female - lime green
Copious fluffy seed in June
 
 

Black Poplar leaf showing characteristic spiral gall

Hybrid Poplar leaf