What is it?
Wet woodland is characterised by trees such as willows, birches and alder that thrive in poorly drained or seasonally flooded soils, such as in fens and bogs, pond and lakesides, river banks, and flushed hillsides. Wet woodland can, however, be quite varied in structure. Wet woodland known as “carr” is often low-canopied, with willow and alder growing over a bulky ground layer of plants such as royal fern, yellow flag and meadowsweet. Along backwaters and in the loops of meandering rivers on floodplains, alders tend to grow taller and the rich silty soils support nettles and great willowherb. In the hills, alder and ash in wet gullies and on spring lines overhang a more species-rich ground layer of low-growing sedges, wood anemones and creeping buttercup. More rarely, sparse and stunted Scots pine and birch trees growing on bogs can support a rich array of lichens, mosses and liverworts – these special places are old growth bog woodlands.
Why is it like this?
This type of woodland has a long history in the UK, having developed wherever conditions were suitable once the glaciers retreated some 12,000 years ago. However, it is inherently dynamic and not long-lived - much of the carr present today is relatively recent in origin. The developing woodland gradually dries out the soil and the wet nature of the woodland eventually changes as other species such as oak and ash colonise. But this trajectory is not inevitable; in particularly wet areas, bog-mosses can impede tree growth, resulting in an ongoing cyclical relationship between bog and carr woodland. There are also bog woodlands where the lack of nutrients stunt the Scots pine or birch so much that it does not interfere with the development of the bog. The succession from one woodland type to another can also be interrupted by floods or coppicing. Floodplain woodlands would once have been dynamic, changing dramatically after big flood events or shifting more gradually as rivers slowly meandered across floodplains. Found on flat, fertile land, floodplain woodlands have been an obvious target for clearance and agricultural intensification, and little remains today.
Distribution in the UK
Wet woodland is scattered across the UK, but the largest carr woodlands are found in the Norfolk and Suffolk Broads and the Cheshire Meres and Mosses. Old growth bog woodland is mainly limited to Scotland (although there are small patches of birch bog woodland in the New Forest and Dorset). Significant areas of alder floodplain woodland are also hard to find: the largest remaining areas are on braided river channels and shingle islands in Scotland, while there are ribbons of grazed alder woodland on the banks of meandering streams in the New Forest and elsewhere.
What to look for
In some areas, such as the Norfolk and Suffolk Broads, swampy carr can be found actually floating on peaty mud - such places may only be accessible by boat. In the north, look for a distinctive type of carr with bay willow, marsh hawk’s-beard and angelica, and the rare coral-root orchid.
Wet, decaying wood and seepages make good habitat for invertebrates, and wet woodland supports many rare species including the netted carpet moth. Look out also for the threatened willow tit, and for lesser redpoll and siskin feeding on birch seed and alder catkins.
The conservation of essentially transient habitats poses problems in a fixed landscape, where the natural process of succession is curtailed by intensive land-use, drainage and flood defences, leaving little room for movement and change. Restoring hydrological integrity and providing space for habitats to develop and change at a landscape level will be beneficial. Water extraction, eutrophication and the clearance of newly developing wet woodland are all challenges. Alder trees are under severe threat from phytophthora (a water mould), which kills the trees.