Plant Zonation and the Effects of the Spring-Neap Tidal Cycle on Soil Aeration in a Humber Salt Marsh

Abstract

(1) Relationships were sought between species distribution, soil aeration and tidal activity in a well-zoned, ungrazed salt marsh. Redox potential was used as an index of soil aeration and was measured with platinum electrodes permanently installed in the soil to various depths in major vegetation zones. (2) Tide had considerable influence and species composition was related to aeration during summer. Rainfall effects were also evident. (3) Three main soil-aeration patterns emerged. The first, with reducing conditions persisting throughout much of the profile and indifferent to periods of exposure, was characteristic of Spartina anglica sites; only near the surface (<5 cm) and at neap periods were there phases of oxidation. The second was found in Puccinellia maritima-, creek bank (Halimione portulacoides)and Elymus pycanthus-sites; redox potentials were lowered monthly by the high spring tides but at the creek-bankand Elymus-sites potentials rarely fell below 200 mV and were responsive to high or very high spring tides only. In the 'general salt marsh' zone and its sub-sites (including Festuca hummocks) only the very high spring tides influenced aeration producing a third pattern in which there was a longer period of oxidation, free from fluctuation, than at any other site. (4) The results are discussed in relation to some of the known responses of the species to soil flooding. (5) There was no evidence of electrode poisoning during these studies and the numerical values of potential recorded in the various phases of flooding were not significantly altered at the Puccinellia sites 3 years after the electrodes had been established.