alba (White Poplar)
Above: White Poplar, Populus alba, a member of the willow family (Salicaceae). Although the leaves of the different poplar species are mostly quite distinctive, White Poplar can be quite similar to Grey Poplar (Populus x canescens) without the leaves to distinguish them: the leaves of White Poplar are palmate with 3-5 distinct and deeply divided lobes (similar to maple) particularly towrads the tip of long/leading shoots (less so at the base and on lateral shoots), whereas the lobes of the leaves of Grey Poplar are much shallower. Furthermore, although the undersides of the leaves in both species are densely hairy on the underside (especially when young) these hairs make the undersides appear distinctly whitish in White Poplar but more grey in Grey Poplar: hence their names. In early spring, however, we have to rely on more subtle distinctions. The similarities between these two species are hardly surprising since Grey Poplar is considered to be the hybrid of White Poplar and Aspen (Populus tremula). Grey Poplar is also a more vigorous tree, often reaching 30 - 35 m in height, whilst White Poplar tends to reach 20 to 25 m in the British Isles where it is a medium-sized tree.
The bark is pale grey or whitish when young and has black diamond-shaped lenticels. The young bark of Grey Poplar is similar. The older bark of White Poplar develops longitudinal (vertical) fissures. The older bark of Grey Poplar is also brown and vertically fissured, though often more obviously so. Both are distinguished from the bark of birch (Betula) can be distinguished by its horizontal slit-like lenticels.
The lenticels of poplars (and willows) are of the simpler type - consisting of suberised cells with air spaces between them. The lenticels may be arranged in horizontal rows in both White and Grey Poplar.
Above: the moss Orthotrichum diaphanum (White-tipped Bristle-moss) growing among a number of lichen species. A number of Orthotrichum species grow as epiphytes on the bark of trees, favouring those with base-rich bark including willows (Salix) and poplars.Several species of orthotrichum may be found growing on a single tree. Characteristic of O. diaphanum is the white (hyaline) awn at the tip of each leaf and the capsule which is at least partially covered by leaves (described as immersed to emergent) due to the short seta (stalk). The outer peristome has 16 narrow teeth that curve outwards when dry. The capsule is lightly furrowed when old and dry.
Above:the twigs of grey and White Poplar are similar, and in both the second-year twigs are borne on short stalks covered in leaf stalks. However, those of Grey Poplar are only slightly hairy and more reddish-brown, whilst those of White Poplar are covered in dense white hairs when young and may turn reddish with maturity. the buds of both are more-or-less egg-shaped and slightly pointed and projecting at an acute angle from the twig. The twigs of Aspen are smooth and hairless and the buds more tapered and closely pressed to the twig.
Above: the catkins can give us further clues. This female catkin is quite young, as the fruit ripen the hairy brown bracts that protected the developing flowers are shed. White and Grey Poplar are both dioecious (with separate male and female trees). In the British Isles White Poplar trees are mostly female, whereas in Grey Poplar they are mostly male. All the trees examined in this locale were female. The catkins emerge in late winter / early spring, well before the leaves. the fruit is a capsule splitting to release seeds with extensive cottony plumes.
The crown of Populus alba is open and broadly spreading.
White Poplars are often planted in parks in the British Isles. The hairs on the underside of its leaves offer protection against pollution, making it suitable as an urban tree (and similarly for Grey Poplar). White Poplar is often found on sand dunes and some types can tolerate considerable salinity. The one seen above is growing near a stream belonging to a network of salt marshes and a tidal estuary. Grey Poplar is often also planted by the coast. White poplar is found naturally in central and southeastern Europe to central Asia; introduced and naturalised in other regions such as the British Isles and planted in temperate North America.
White Poplar readily spreads by vegetative means through suckers (adventitious shoots borne on roots).
Article created: 5 April 2020.