Willow diversity in the fresh water tidal area of the river Schelde : natural, spontaneous, anthropogenic ?
Research output: Contribution to journal › A2: Article in a journal with peer review, not included in A1
The fresh water tidal area of the Schelde-estuary is characterised by a large willow diversity and a prominent presence of willow-dominated vegetation. Marsh vegetation consists for over 60 % of willow scrub and willow forest. These are natural vegetation types on fresh water tidal marshes, and are considered most important climax vegetation under fresh water tidal circumstances. Along the Schelde-estuary (as well as in other parts of the river valley) several willow species were planted though. On numerous occasions tidal willow scrubs and forests therefore largely consist of planted willows. They are often alien, introduced species or hybrids, although in some occasions native material was planted as well. Several of these introduced species are able to disperse spontaneously. This results in a very obscure and hard to determine origin of tidal willow vegetation. It can be spontaneous-native, spontaneous-alien, planted-alien and/or spontaneous-alien. To get an overall picture of willow diversity and its origins we studied willows morphologically in spontaneous vegetation and osiers on the fresh water tidal marshes as well as in osiers in the non-estuarine river valley. Chronological sequences of historical maps show that osiers were planted frequently in the river valley. Until the turn of the century the area planted with willows increased; since then osiers decreased again, resulting in a very limited present-day area of osiers. Since ca. 1920 osiers were planted on tidal marshes as well; until then these "marshes" were used mainly as flood-meadows. Whether native willows from these former flood-meadows and more natural tidal marshes survived the osier plantation period is hard to detect. Given the subsequent changes in land use this is rather unlikely, but it is however possible that relict populations remained on dikes between tidal marshes. Vegetation mapping in 1992 and 1996 show a strong increase of (spontaneous) wil1ow scrub. Interviews with (former) osier farmers revealed a spectrum of 14 willow taxa that were used for osier plantations in the Schelde val1ey. Eight of these were planted on fresh water tidal marshes, al1 of them were used in the non-tidal val1ey. Until now we only discovered five of these taxa. Especially S. x mollissima and S. viminalis were encountered abundantly. The exact varieties remain to be identified. Literature indicates that S. alba var. vitellina and S. fragilis var. russeliana were planted regularly in the 19th century. They probably correspond with the local dialect names "gele wijmen" and "oude rooie". Both are taxonomic mysteries: present-day identification keys lead to two different S. x rubens varieties. Literature further indicates that "Duitse rode" is synonymous with S. x americana, once more a taxonomic dilemma: some interpret it as a hybrid (S. petiolaris x cordata), others call it synonymous with S. cordata. The herbaria GENT and BR revealed 5 wil1ow taxa from the study area of which S. x mollissima var. undulata and S. purpurea ssp. lambertiana were rediscovered in the field. It appears that S. fragilis var. russeliana is better incorporated in the S. x rubens complex, and should not be considered as a variety of S. fragilis. S. x smithiana, S. pentandra and S. x rubra were not yet encountered in situ. Tidal osiers stil1 survive locally, although in a modified form. They are harvested only once every 3 to 4 years in stead of annual1y. Their products are mainly used for dike consolidation works, not for basketry.
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