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01G1

G. Forests

Riverside poplar-willow forests

Relationships with habitat classifications. EUNIS: G1.1112 Eastern European poplar-willow forests; PAL. CLASS.: 41.132 Eastern European poplar-willow forests; HD 92/43: 91E0 *Alluvial forests with Alnus glutinosa and Fraxinus excelsior (Alno-Padion, Alnion incanae, Salicion albae); Bondev (1991): 143 Black alder (Alneta glutinosae), willow (mostly Saliceta albae, Saliceta fragilis) and poplar (Populeta nigrae, Populeta albae) forests, partly combined with artificial poplar plantations and hydrophilic grass communities (in N Bulgaria).

Conservation status. BDA, HD, BC.

Category. Endangered [EN – A1, 2 C2 D2 E2 F2 G2 H2 I J L3].

General characteristics. This habitat comprises riverine, most often willow-poplar or pure willow or poplar, forests in the lowlands and plains, in the continental climatic conditions in N Bulgaria. Most diverse is their composition  along the Danube and on its islands. In the middle and lower parts of the biggest feeders of the Danube (Iskar, Ogosta, Lom, Vit, Osam, Yantra, Roussenski Lom) these forests are preserved in some places mainly as narrow strips, dominated by the White willow (Salix alba), or as small patches of the White and Black poplar. They grow on rich, well aerated Calcaric Fluvisols, rich in humus, but the humification is periodically interrupted by floods and deposition of new alluvia. They are rich in nitrogen, hence many nitrophilic species participate in the composition of these forests. The areas are flooded periodically by the so called “cherry” waters of the Danube, whose maximum is at the end of May and the beginning of June. The most prolonged floods (4–6 months) can be observed in the zone that is up to 2 m above the mean level of the Danube. In the zone higher than 5 m, where floods are sporadic, the vegetation develops on Calcic Chernozems. In places where water remains and alluvia are deposited mainly in the canals (zatoni) on the islands and in the marshes the soils are Mollic Gleyisols.

The main edificators in the most flooded areas are Salix alba and Salix × rubens, a hybrid of Salix alba and Salix fragilis.They are accompanied by Populus alba and P. nigra, and more rarely by the typical Salix fragilis. Willows form pioneer communities on the open sandy-clayey strips and alluvial deposits that constantly change according to the dynamics of the deposition processes along the Danube. The seedlings of the White willow together with Salix purpurea and S. triandra form the thickets along the rivers in the transitional zone between the forest and the newly formed alluvia. Gradually, as the seedlings of the White willow grow the latter two species retreat and almost disappear. The flooding in this forests takes 60 to 120 days and according to its duration the willow forests are without shrub and herbaceous layers (in the areas with longest floods, e.g. in the narrow channels (zatoni) among the islands) or with rich shrub and herbaceous layers (in more elevated and well drained places). The willow trees reach 8–12 m height, and their projective cover is up to 80–90%. The trees form aerial roots whose number and density correspond to the duration of the floods. In some places a shrub layer develops and it includes Rubus caesius var. aquaticus (major dominant), Crataegus monogyna, Cornus sanguinea, Euonymus europaeus, Ligustrum vulgare and more rarely Frangula alnus, Viburnum opulus. Very often (especially along the Danube) the primary structure of the riverine forests has been changed and the invasive American species Amorpha fruticosa is very frequent. Climbing plants and lianas often occur in the willow forests: Clematis vitalba, Humulus lupulus, and Vitis sylvestris. The floristic structure of the herbaceous layer varies and includes mainly hygrophytes, nitrophytes and many ruderals. The height of this layer can reach 1.5 m in light and well-drained places. Most frequent are Althaea officinalis, Bidens frondosa, B. tripartita, Calystegia sepium, Cirsium arvense, Echinochloa crus-galli, Erigeron annuus, Euphorbia lucida, E. palustris, Galium aparine, G. palustre, Glycyrrhiza echinata, Iris pseudacorus, Leucojum aestivum, Lycopus europaeus, Lysimachia nummularia, L. vulgaris, Lythrum salicaria, Myosotis scorpioides, Phalaris arundinacea, Phragmites australis, Polypogon viridis (= Agrostis verticillata), Rorippa amphibia, Solanum dulcamara, Stachys palustris, Symphytum officinale, Typha latifolia, and Urtica dioica. Ruderal and nitrophilic species grow mainly in open places that are the result of anthropogenic activities or of the natural opening of forest canopy of the willow forests when their age is over 40–50 years. When the annual floods have been reduced the xerophytization and ruderalization become more intense and the participation of Amorpha fruticosa increases since this species cannot endure long floods. Willows also form mixed coenoses with other tree species tolerant to wet areas and floods. Most often such species are the Black (Populus nigra) and White (Populus alba) poplars. They can tolerate floods up to 60 days. In some places poplars form monodominant forests whose floristic composition is very similar to the willow forests. Therefore, all mixed willow-poplar plant communities dominated by Populus alba, P. nigra, Salix alba,belong to one and the same association Salicetum albae s.l. (= Salici-Populetum) of the alliance Salicion albae. Single trees of Alnus glutinosa, Fraxinus oxycarpa, Morus alba, Pyrus pyraster, Quercus robur, Ulmus laevis, Ulmus minor, and in the last years the invasive species Acer negundo can also participate in the composition of these forests. American neophytes such as Bidens frondosa, Echinocystis lobata, and Sicyos angulatus aretypical for the riverine forests along the Danube. The latter two sepcies are annual, climbing plants that make a dense cover on the tree layer in the inner parts of the Danube islands.

Along the bigger rivers in the interior of Bulgaria the willow forests grow in narrow strips (sometimes not more than several meters wide or along smaller rivers, as a row of trees). In some place there are preserved forests of White poplar (Populus alba) which is more resistant to drought than the white willow and the black poplar.

Since the riverine forests grow on very rich soils, in many places they have been completely destroyed and turned into arable land. Intensive poplar plantations (mainly Populus x euroamericana cv. I-214) very often replace the native willow-poplar forests along the Danube and in the interior of the country.

Characteristic taxa.

Distribution in Bulgaria. North Bulgaria up to 800 (1000) m alt. The forests along the Danube and its islands are the most representative localities of this habitat type. These habitats occur also along the feeders and small rivulets of the Danube as galleries of willows and more rarely poplars.

Conservation importance. The riverine poplar-willow forests are subjected to strong anthropogenic pressure such as wood felling, invasion of alien species, etc. They are rich in biodiversity. Some very rare species occur: Armoracia macrocarpa, Euphorbia lucida, Leucojum aestivum, Sonchus palustris; fungi of conservation importance are Hymenogaster verrucosus, Lenzites warnieri, Phellinus pilatii, Ptychoverpa bohemica, and Sistostrema confluens. These forests are important for the reproduction of many water birds: herons, cormorants, eagles, many singing birds, wood peckers, etc. These forests are source of some medicinal plants such as nettle, comfrey, hops, Rubus species, etc.

Threats. Destruction of forests for opening of new agricultural land and intensive plantations of hybrid poplar, illegal logging, “clearing” of water beds from trees and shrubs, massive invasion of alien species that changes the structure and function of the riverine forests. Hydro-ameliorative activities – construction of dikes, draining, river bed corrections, building of water electric power stations – cause changes in the water regime and deterioration of the conditions of the alluvial forests. The natural threat is soil erosion caused by the Danube, the constant changes of the river bank and destruction and formation of new islands. The climatic changes cause alteration of floods and droughts which also are a threat to the poplar-willow forests.

Conservation measures taken. The habitat is in Annex №1 of the national Biodiversity Act. Some of the localities are within the borders of Ibisha Managed Nature Reserve, Persina Nature Park, Kalimok-Brashlen and Kompleks Aleko-Telikata Protected Sites. Some of the most representative plant communities are included in NATURA 2000 areas. Projects for the restoration of the alluvial forests along the Danube have been elaborated.

Conservation measures needed. Termination of logging and “clearing” of the river banks from the natural riverine forests; restoration activities including restoration of destroyed riverine forests in the Danube islands.

References. Marinov & Fakirov 1977; Stojanov 1948; Tsanov 1992; Tzonev 2009.

Authors: Rossen Tzonev, Marius Dimitrov


Riverside poplar-willow forests (distribution map)