Synonymy

Valley fen woods Farrow 1915 p.p.; Betulo-Alnetum Clapham in Tansley 1939; Woodwalton Birch wood Poore 1956b p.p.; Valley fen alderwoods Haslam 1965 p.p.; Alnus-Salix woodland XXi & XXii Meres Report 1980; Alnus-Salix-Betula woodland XXi Meres Report 1980; Fen Woodlands B, C & D Fitter et al. 1980; Alder stand types 7Aa & 7Ab Peterken 1981.

Constant species

Alnus glutinosa, Urtica dioica.

Physiognomy

The Alnus glutinosa-Urtica dioica woodland is a rather ill-defined community which brings together a variety of canopies dominated by Alnus glutinosa, Salix spp. and Betula pubescens beneath which the rich assemblages of swamp and fen herbs characteristic of many of our wetter woods are replaced by a species-poor, though quite distinctive, field layer. There is considerable floristic and physiognomic diversity among the woodland types included here and, at first sight, it is often the peculiarities of stands which impress the visitor more than their underlying similarities. Nonetheless, there are sound ecological reasons for both the general speciespoverty of these woodlands and for what little they have in common and it seems best to treat them within a single, rather disparate, group.

Alnus glutinosa is by far the commonest tree throughout and it remains frequent in all but the driest stands. In the wetter woodlands included here, it is often an overwhelming dominant, forming an even-topped and usually closed canopy of well-grown, usually multistemmed trees. In one sub-community, it is replaced as the most abundant tree by Salix fragilis. In the drier types of woodland within the community, Betula pubescens becomes increasingly frequent and locally dominant and Pinus sylvestris is an important invader or planted canopy replacement. Other tree species are generally uncommon but Populus nigra var. betulifolia is a very distinctive associate in some stands and it can attain a grand stature here with its black bossed trunk and irregular branches arching downwards. This tree is probably native in southern England and, as on the Continent, this community perhaps provides its natural woodland locus. There is occasionally some Acer pseudoplatanus or Fraxinus excelsior and sometimes a little Quercus robur.