GEGENWALLE RIDGES: AN ECOHYDROLOGICAL CHRONOSEQUENCE IN MORFA DYFFRYN, WALES, UNITED KINGDOM

We investigated the ecohydrology of a series of gegenwalle ridges and swales at Morfa Dyffryn National Nature Reserve, Wales, United Kingdom. They form in the deflation basin of a large parabolic dune as it migrates across the landscape, creating a chronosequence. Secondary humid slacks or interdunal wetlands have developed in the swales. Soil cores collected from each ridge along a transect from youngest (ridge 1) to oldest (ridge 6) perpendicular to the dune axis show the progressive development of an organic-rich horizon beginning in ridge 4. Similar development occurs in the swales beginning in slack 3. ²¹⁰Pb and ¹³⁷Cs soil analyses to determine the formation times of these features are ongoing. Monitoring well data show the highest water levels in the winter months during which water floods the slacks, sometimes covering the first several ridges. Slack inundation can continue into early summer. We performed vegetation sampling in 1 m² quadrats on each ridge/slack along the transect. Bare sand coverage decreases from 75% (ridge 1) to 1% (ridge 6). Species tolerant of sand burial, Carex arenaria, Cerastium fontanum, and Anagallis arvensis, populate the youngest ridges. Salix repens appears on ridge 2, increasing to 90% coverage on ridge 6. Additional species appear on subsequent ridges, including Epipactis palustris (ridge 3), Dactylorhiza sp., Lotus corniculatus (ridge 4), and Festuca rubra (ridge 6). Ridge vegetation shows a progression from species tolerant of sand burial to those characteristic of fixed dunes. Bare sand coverage in slacks 1 and 2, 90% and 80%, respectively, sharply decreases to 50% in slack 3 due to the appearance of new species Carex viridula and C. flacca and increased abundance of existing species Juncus articulatus and Agrostis stolonifera. Additional species, including Equisetum variegatum, Hydrocotyle vulgaris, Anagallis tenella, and Galium palustre, appear in subsequent slacks with Holcus lanatus and Poa pratensis present in slack 6. The swales show a progression from sparse vegetation to plants tolerant of periodic inundation to vegetation requiring moist conditions. Species diversity in both the ridges and slacks increases from younger ridges/slacks to older ones. Grazing by cattle (winter) and rabbits (year-round) maintains the openness of the slack.