Ecological studies on Atriplex portulacoides and its role in salt marsh zonation.
This study investigated the zonation of the halophytic shrub Atriplex portulacoides (chenopodiaceae) on three main habitats on the high marsh, shingle ridge and low marsh at Stiffkey saltmarsh, on the north Norfolk coast. The aim was to examine the physical and biological factors controlling the distribution of this species within an apparently wide amplitude of elevation within the tidal frame. The field investigation revealed significant differences in some of the physical characteristics of the sediment of five sites. Soil drainage and soil texture were significantly different between the shingle ridge and the general marsh habitat. Other
environmental factors, including duration of tidal submergence proved to have significant effects on the distribution and percentage cover of the plant. Sediment accretion rates were greatly variable between the high and low marsh and may be
responsible for low seed germination rates of this species on the low marsh. Laboratory experiments showed that the seed germination was largely inhibited at high salinities. Seed germination can occur under anoxic conditions but only in the presence of light. Seed germination in the laboratory was significantly higher under alternating temperature (20/10 °C) than at continuous lower temperatures (1 or 4 °C). Seed germination under burial with sediment was significantly reduced and this is due to the failure of seeds to germinate in the first place and also due to the difficulty in emergence from burial. Glasshouse experiments showed that long-term waterlogging created hypoxic conditions that were harmful to growth of A. portulacoides at all three stages of the
life history tested; mainly changes to morphology occurred. However, the seedlings are the more adversely affected than young established plants or the mature plants. Unlike the seedlings, the young established and mature plants were relatively tolerant to waterlogging, mainly because they produced abundant adventitious roots from their aerial branches.
Seed and seedling transplant experiments in the field showed that plants can not establish lower on the marsh than their current lower limit because of physicochemical conditions. The obstacle is mainly at the seed germination, establishment
phase as transplanted seedlings had much higher survival rates generally than transplanted seeds. However, the low marsh was generally more conducive to seedlings than the high marsh.
Certain biotic interactions were also investigated. Competition of A. portulacoides with Puccinellia maritima tested in a prolonged experiment in the glasshouse with and without waterlogging showed no significant effect on the dry mass of either species. Fruits (seeds) of A. portulacoides were significantly predated by larvae of the moth Coleophora atriplicis.