[Influence of biotic and abiotic factors on the morphology and reproduction of Suaeda maritima on a salt marsh]. / Influence des facteurs biotiques et abiotiques sur la morphologie et la reproduction de Suaeda maritima sur un marais salé.

The aim of this study was to estimate the influence of biotic and abiotic factors on Suaeda maritima reproduction on a salt marsh. Individuals of Suaeda maritima were submitted in natural conditions to four series of densities (100, 1,000, 4,000 and 8,000 plants/m2). When density increases, individuals tend to be less or non-branched, while individual biomass decreases. Consequently, individual seed production decreases as density increases. Despite morphological modifications, Suaeda maritima present density-dependent mortality. For a unit area, total biomass and seed production are higher at intermediate density (1,000 plants/m2). Environmental factors could interfere with self-thinning. They seem to limit the effect of competition on mortality and to have an influence on individual and total seed production. This experiment stressed the importance of a biotic factor such as intra-specific competition, which occurs at the same time as abiotic factors, in Suaeda maritima dynamics in the field.

Azorella selago Hook. used to estimate glacier fluctuations and climatic history in the Kerguelen Islands over the last two centuries.

Climatic variations over the two last centuries are well known for the northern hemisphere, but very little information is available for subantarctic islands. In this paper, we combined geomorphological observations and a new biological dating technique to propose a reconstruction of the cool and warm events in the Kerguelen Islands during the last two centuries. The usual dating methods, such as dendrochonology or C14 dating, are not applicable on Kerguelen. Therefore, the radial growth ofAzorella selago Hook., a cushion-forming Umbelliferae species, was used to estimate the absolute age of deglaciated areas. Glacial margins in the vicinity of the Glacier Ampère constitute the most complete chronosequence studied in this part of the world and illustrate seven warming-cooling cycles. This new dating technique is validated by the close relationship between the calculated ages of these climatic events and the results of several studies in other circumantarctic regions. The Glacier Ampère reached its maximum extent at the end of the eighteenth century. Since 1799, two discrete phases may be distinguished: the first period (1799-1965) is characterized by small glacier fluctuations (1 km retreat overall) whereas in the second period (1966 to the present), the retreat is much more rapid (about 3 km). It seems that the current dramatic glacial retreat on Kerguelen is related to a major change in the climate and could illustrate a more general southern hemispheric pattern of glacial fluctuations.