Fen

Fens are some of our richest and most breathtaking wetlands. They tend to have a lush, diverse vegetation structure made up of patches of open water and low vegetation, through to tall herb-rich and reed fen.

There are less than 200 hectares of true fen in Sussex. Nearly two thirds of this is found at two sites in East Sussex at Combe Haven valley and Pett level. In West Sussex, a few large patches of fen are also found on the Arun Valley at Amberley Wildbrooks and Pulborough Brooks. Otherwise, most of our fen is in patches which are small, isolated and vulnerable to extinction.

Fens occur across the UK and can be anything from a few metres square, to hundreds of hectares in size (e.g. the Norfolk fens). Britain has most of the fen types found in Europe, but they are small surviving fragments of previously much more extensive wetlands. (McBride et al. 2010). In Sussex, most of our fens are rich and topogenous.

Fens were prized in the past for the range of products they yielded: reeds for thatching, willow for basketry, hay and lush grazing for cattle. In many cases it is historic management by humans which has created the range of fen habitats we see nowadays. Most modern fens are therefore semi-natural rather than entirely natural habitats.

What is a fen?

A fen is a wetland which depends on water and nutrients from surface water run-off, groundwater, and/or rainfall. Most fens tend to have at least seasonally waterlogged soils. These waterlogged soils often form peat (McBride et al, 2011). There are two main types of fen:-

Topogenous (alluvial) fens where vertical (groundwater) water table changes dominate due to limited drainage and

Soligenous fens where horizontal water from rain and run-off dominates and moves through soils

Topogenous and Soligenous fens can be further sub-divided into two types:-

Poor fens

Poor fens get their water from base-poor rock (such as sandstone or granite), mainly on uplands or lowland heaths. They often have short vegetation with lots of bog mosses (Sphagnum spp). and acid water.

And

Rich fens

Rich fens are fed by mineral-enriched calcareous waters (pH 5 or more), and are mainly confined to lowlands areas or areas where there are localised occurrences of base-rich rocks such as chalk.

Female emerald damselfly - Lestes sponsa
Female emerald damselfly in acid fen / Ben Rainbow

Species you might find in Sussex fens

Fens support a rich variety of wildlife. Some fens can contain up to 550 species of plants; large numbers of dragonflies, and several thousand insect species and aquatic beetles. Many bird species live in fens including snipe, woodcock, curlew, and Cetti’s warbler. Look out for water voles and grass snakes too.

Why did fens decline?

The main reason behind the decline of fens is that they grow on (and produce) some of our best agricultural soils. They are also often found in low lying floodplain areas which have been progressively drained for agriculture and development. This drainage causes the organic component of the soils to break down, releasing carbon and lowering the land level. In some cases such as the Norfolk fens, land levels have dropped by over 3 metres. Unfortunately, lowland fens which have been treated with pesticides and fertilisers can often never be restored. If management of fens stops, a fen will often naturally convert into another habitat such as wet woodland.

Sussex fen
Sussex fen / F Southgate