Biology and ecology of Galium minutulum Jordan (Rubiaceae) on the Hyeres islands (Var, France); implications for the species conservation

Several biological and ecological characteristics of Galium minutulum Jorda n, a rare steno-mediterranean annual species, were investigated on some pop ulations of the Hyeres islands, which are located in the north-eastern limi t of the species distribution. The objectives of this study were to better document distribution, autecology and the form of rarity of this species in Provence and to propose accordingly some elements for a sustainable manage ment of that vulnerable taxa. Galium minutulum exhibits a noteworthy polymo rphism and an important ecological plasticity, which were both mostly expla ined by the existence of two allopatric cytotypes. Tetraploid individuals, small-sized and spindly, are located in the core-areas of the biggest islan ds (Porquerolles, Port-Cros, Le Levant), at the edges of the sclerophyllous matorrals or the thermophilous holm-oak forest. Hexaploids, more robust, g row on the coastal habitats directly exposed to sea sprays, notably on the smallest islands (Bagaud, Grand Ribaud). Many factors such as meiosis abnor malities, reduction of germination capacity of the tetraploid cytotype, fre quent infestations of a pathogene fungus (blight), and the sensitive locati ons near the tracks and paths, generate several threats on this species and contribute to its rarity.