Environmental Engineering Reference
In-Depth Information
FIG
84.
'Slack 33' at the northern end of Whiteford Burrows. (David Painter)
Theslacksdifferincharacter intermsoftheirage,thelevel ofthewatertable insummer,theduration
offlooding,thecalciumcontentofthesand,thedegreeofgrazingbyrabbitsandsoon.Eachtypeofslack
thereforecontainsitsowndistinctivefloraandfauna.AtWhitefordinthe1980seightdifferentplantcom-
munities were identified in the slacks, but the species composition of these has almost certainly changed
since that date. Creeping willow
Salix repens
, a low-growing shrub with grey-green leaves, is a ubiquit-
ous and important component of the vegetation of these areas. It is an extremely variable species and, at
Whiteford at least, appears to have three distinct growth forms, 'red', 'downy' and 'brown', the differ-
ent forms occurring in response to different environmental conditions. The brown form is the common
form and probably represents the mature plant growing in optimum conditions. It is a large plant, over 40
centimetres tall, erect with stout dark brown stems. The red form (Fig. 85) occurs in both the wettest and
the driest slacks and may therefore represent a response to environmental stress. It is of medium height,
some 15 to 30 centimetres high, is semi-erect or erect with thin brown stems covered with white hairs.
Finally the downy form grows on the edges of the wettest and driest slacks and throughout a wide range
ofmiddle-moistureslacks.Itisasmallform,oftenunder10centimetrestall,andisprostrateorsemi-erect
with thin red stems.
