Alder (Alnus glutinosa) carr is typically found in wet or damp conditions on base-rich eutrophic soils that are either permanently flooded or subject to periodic inundation.   These tend be pioneer stands where the alder is replaced by ash (Fraxinus excelsior) or elm (Ulmus spp.) around the drier edges. 

These alder woods are protected under Annex I of the EU Habitats Directive, namely as the ‘Alluvial forests with Alnus glutinosa and Fraxinus excelsior (Alno-PadionAlnion incanaeSalicion albae) habitat.  The Habitats Directive specifically refers to woods dominated by alder Alnus glutinosa and willow Salix spp. on flood plains, where they can occur as a woodland component within fens and swamps. In this situation, the associate species can include common reed (Phragmites australis), greater tussock-sedge (Carex paniculata), royal fern (Osmunda regalis) and marsh fern (Thelypteris palustris) and a wide range of sedges.

Alder carr can also develop along the edges of streams and rivers where a very different, but no less interesting, groundfllora might include alternate-leaved golden-saxifrage (Chrysosplenium oppositifolium), marsh-marigold (Caltha palustris) and herb paris (Paris quadrifolia).