Recent floristic study of the island of Limnos (NE Aegean, Greece) has resulted in the addition of 69 new plant taxa. As a result the flora of the island now comprises 750 taxa. New alien species such as Erigeron sumatrensis, Erigeron canadensis, Symphyotrichum squamatum, Amaranthus retroflexus and Amaranthus blitoides nowadays colonise thousands of hectares of fertile ground and present the highest rates of invasion. Ecological factors such as soil texture, high winds and human intervention, in combination with their seed dispersal mode and genetic background, may explain their successful establishment and impressive abundance especially in the eastern part of the island. Among the new records of indigenous plants are some interesting taxa from a phytogeographical point of view, such as Bupleurum euboeum and the Greek endemic Polygonum icaricum.

In an area of savanna-forest (S-F) mosaic of Cameroon, at Kandara near Bertoua, an enclosed savanna bordered by young semi-deciduous forests was selected for detailed studies of vegetation and soil carbon isotope compositions with a view to estimating the rate of forest advance into savannas. Forest floristic composition and structure were analysed in small plots along two S-F transects and within two large stands. Tree species counts and basal area (BA) measurements gave convergent results defining (1) an edge forest with low BA values that forms an irregular strip parallel to the S-F border, (2) a large colonization zone zone composed of pioneer species (Albizia species, with individuals of very large diameter) and (3) a mature forest composed of abundant Rinorea individuals and large individuals of Triplochiton scleroxylon and Piptadeniastrum africanum. Carbon stable isotopes were deter mined from organic matter of soil profiles sampled at various depths in savanna, colonization-zone and mature forest. In the deep soil horizons (40-50 cm) of the colonization-zone profiles, δ13C values similar to those of the present savanna reveal the past existence of a large-tree savanna. In subsurface horizons (15-20 cm), δ13C values intermediate between those of savanna and mature forest prove the encroachment of the forest ecosystem on savanna. Using 14C measurements, the mean residence time (MRT) of soil organic matter of these last horizons was determined with precision owing to the atmospheric 14C pulse from nuclear bomb tests prior to 1964. By assuming an exponential age distribution of organic compounds and by taking account of MRT and remaining carbon from the savanna, the coloniza tion zone was found to be 60-80 y old. The age of the colonization zone being the same near the present savanna and near the mature forest, it seems that the forest advance was probably not a linear process but would result from the coalescence of Albizia thickets born in savanna.