Atriplex patula

Abstract: SUMMARY (1) A survey of the vegetation of eleven sites affected by herring gulls along the Aberdeenshire coast and on the Isle of May (Firth of Forth), and of adjacent unaffected areas, revealed that the vegetation of gull-affected sites consisted of a seasonal, species-poor community, in which annual and ruderal species predominated. Of particular prevalence were Atriplex hastata, Holcus lanatus, Poa annua, Rumex acetosa, R. crispus, Stellaria media and Tripleurospermum maritimum. By contrast, the vegetation on adjacent areas unaffected by gulls was generally a perennial grass sward in which Festuca rubra was the dominant species. (2) Observations from a hide at two study sites revealed that four principal gull activities affect the vegetation of breeding sites: treading, the digging of scrapes and collecting of nest-materials, boundary clashes associated with gull territorial behaviour, and defecation. Each of these activities had an uneven spatial distribution within the colony area. Although all four activities exerted important influences on the vegetation, disturbance and destruction of vegetation associated with boundary clashes appeared to be a particularly significant activity for the plants occurring in gull colonies. (3) Experimental fieldwork involving the removal of vegetation (to simulate the ultimate effects of gull disturbance) also revealed the importance of disturbance. Annual plants (Stellaria media, Atriplex patula and Senecio vulgaris) became established on an area formerly occupied by Holcus lanatus. (4) Mapping of the distribution of plant species within the two study sites over a 2-year period suggested that although gulls were responsible for the vegetation of the colonies as a whole, factors other than gull activities played a more significant role in determining the detailed distribution of plant species within the colonies. At the sites studied, differences in soil-nutrient concentrations and sea-spray deposition appeared to influence the distribution of particular species. (5) A glasshouse experiment suggested that edaphic factors might operate to exclude ruderals (particularly Urtica urens) from the poorest soil. On the richer soils, on which all species grew well, biotic factors such as interspecific competition might determine the distribution of the different species.

Abstract: Indirect negative interactions between prey species that share a predator, referred to as apparent competition, have been modeled extensively. However, experimental studies examining the importance of apparent competition in the field are sparse, especially in plant–herbivore systems. In this study, I examined the role of a beetle herbivore, Erynephala maritima, in mediating the indirect effects of an abundant host plant, Salicornia europaea, on a less common host species, Atriplex patula var. hastata, in New England salt marshes. To examine these relationships, I manipulated plant density (plants with vs. without neighbors) and composition (conspecific neighbors vs. Salicornia neighbors) surrounding focal Atriplex plants, as well as the presence or absence of herbivores in a field experiment. Densities of both beetle eggs and larvae were significantly greater in plots with Salicornia, suggesting that its presence increases patch location or selection by herbivores. Higher insect densities resulted in sig...

Abstract: The effect of sulphide on the growth of several species of salt-marsh plants was investigated. Relative growth rates were significantly reduced in two upper-marsh species, Festuca rubra and Atriplex patula, and in the lower-marsh species Puccinellia maritima. However the growth of Salicornia europaea, a species frequently associated with sulphide-containing sediments, was unaffected. In a separate experiment the wide ranging halophyte Aster tripolium, also appeared to be tolerant of sulphide at a concentration frequently encountered in salt marshes. Sulphide pretreatment inhibited the activity of two metallo-enzymes, polyphenol oxidase and external phosphatase, in plants from the upper marsh, but had no effect on enzymes from P. maritima or S. europaea. The rate of respiration by root tissue was significantly reduced in all of the species investigated but whereas the uptake of 86rubidium was markedly inhibited in the other three species, uptake by S. europaea showed a significant stimulation. Similarly, whereas sulphide-grown plants of F. rubra, A. patula and P. maritima had a considerably reduced tissue iron content, the total iron concentration in S. europaea tissues was comparable to that of the controls. When the sulphide-tolerant species A. tripolium was grown in sulphide-containing media there was no significant effect on the tissue concentration of any of the elements investigated. These results are discussed in relation to possible mechanisms of sulphide toxicity and resistance.