woods develop on floodplains in a range of situations
from islands in river channels to low-lying wetlands
alongside the channels. They typically occur on base-rich,
eutrophic soils subject to periodic inundation. In other
words, on fertile lowlands by rivers, which flood occasionally.
Most floodplain woodlands are dynamic, being part of
a successional series of habitats. Their structure and
function is best maintained within a larger unit that
includes the open communities, mainly fen and swamp,
of earlier successional stages.
In other situations, alder woods occur as a stable
component within transitions to surrounding dry-ground
forest. These transitions from wet to drier woodland
and from open to more closed communities, provide an
important facet of ecological variation.
Trees growing along riverbanks are excluded from this
habitat type, except where they form part of a wider
network of alluvial woodland and wetland communities.
Clearance of riverine woodland has eliminated most
true alluvial forests in the United Kingdom. Many surviving
fragments, as elsewhere in Europe, are fragmentary and
often of recent origin.