Swamp alder woodlands in Galicia (NW Spain): phytosociological interpretation. Ecological and floristic contrast to western European swamp woodlands and delimitation versus riparian alder woodlands in southern Europe and northern Africa

This phytosociological study gives a report about swamp woodlands dominated by Alnus glutinosa in Galicia (northwest Spain). All the woodlands studied belong to the association Carici lusitanicae-Alnetum glutinosae T. E. Diaz & E Prieto 1994, of the class Alnetea glutinosae. These are azonal woodlands requiring a higher level of waterlogging than riparian alderwoods. The latter communities are included in a different class, the Salici purpureae-Populetea nigrae. We recognize a typical subassociation, as well as a new subassociation caricetosum pendulae occurring on more silty and less persistently waterlogged soils. Carici lusitanicae-Alnetum is the most widespread Alnetea glutinosae community in the Iberian Peninsula, extending from the Bidasoa river basin at Spain's frontier with France in the north, to the Tejo valley north of Lisbon in the south. In comparison with other swamp alderwood communities of Central and northern Europe, Carici lusitanicae-Alnetum shows interesting examples of floristic vicariance within the main physiognomic groups of swampy woodlands: willows (Salix sp. pl.), large sedges (Carex sp. pl.), large ferns and hygrophilous Meadow herbs. To clarify the identification of Alnus-dominated woodlands in the southernmost part of their distribution, we have collated phytosociological information on various Alnetea glutinosae communities from the Iberian Peninsula, Italy and North Africa, and have compared these communities with the riparian woodland alliance Osmundo-Alnion, distributed in southwestern Europe and northwestern Africa. The synthetic table clearly shows that riparian alderwoods contain more typical Querco-Fagetea species, as well as diverse Iberian endemic species not shared with swamp alder woodlands, like Galium broterianum, Carex elata subsp. reuteriana and Clematis campaniflora. By contrast, in commumties of Alnion glutinosae (Alnetea glutinosae) the characteristic species of Magnocarici-Phragmitetea are present with high diversity and high frequency. The interesting endemic floristic group within Osmundo-Alnion suggests a rather more restrictive interpretation of this alliance than has been accepted until now: namely, a basically Iberian and western North African distribution, excluding the riparian alderwoods of Corsica, though including the alderwoods of the Algerian region of Kabylia perhaps as an independent (sub)unit.