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The aim of this paper was to analyse the relative role of positive and negative interactions in an attempt to explain the recruitment and persistence of Picris hieracioides over a successional sequence in the Mediterranean region. A reciprocal transplant experiment among populations and stages was performed to compare survivorship, growth and reproductive patterns of two populations of P. hieracioides and two stages of abandonment. In addition, individual performances were evaluated in each stage with respect to: (1) competition with neighbours (tested with a weeding treatment) and (2) resource availability (tested with nutrient and water addition) in an effort to examine some of the ecological interactions influencing the performance of P. hieracioides in Mediterranean old fields. Both populations display nearly the same response to the selective pressures imposed by succession and the mechanism allowing persistence of P. hieracioides populations is the high phenotypic plasticity rather than a population response to directional selection during succession. One of the most interesting results was the shift from positive to negative interactions according to life stages. Facilitation was obvious during recruitment and growth; negative interaction, or competition effect, became dominant later in the life cycle and appeared through final lower growth and reproduction. These results suggest that the balance between positive and negative interactions is tipped towards facilitation or competition depending on how the abiotic conditions affect P. hieracioides individuals in a given year; neighbouring vegetation acts as facilitator and has positive effects on seedlings of P. hieracioides but also acts as competitor and has negative effects on mature plants. The intensity of competition, when only survivors are considered, was lowest in subplots with resource addition because the enrichment of habitat (e.g. by addition of water and nutrients) led to an increase in resource availability for P. hieracioides, as few resources were used by its neighbours. In striking contrast, competition intensity was greater in subplots with supplemental resources when mortality is taken into account. In this case, the negative values of the competition intensity index reflect the positive effect of neighbours on final aboveground dry weight. The contrasting results of these indices illustrate the difficulty of predicting the outcomes of competition solely on the basis of productivity. We have shown that death of seedlings was the key process, and the final biomass of the surviving individuals contributed less to the persistence of P. hieracioides along successional gradients. Overall, our results reveal that the main factors limiting the colonisation of P. hieracioides populations in the first stages of secondary succession are the stresses linked with microclimate (extreme temperatures and desiccation). In contrast, the main factors limiting growth of P. hieracioides individuals in Mediterranean old fields are resources (i.e. water and nutrients).

Crepis sancta is an annual composite that produces heteromorphic seeds. Peripheral achenes, named from the position they occupy within the fruiting heads, are heavier and without a pappus whereas central achenes are lighter and have a pappus. The morphological differentiation is associated with a difference in embryos size: embryos from peripheral achenes are twice as heavy as those from central achenes. As seedling size has consequences for growth, we tested the hypothesis that the difference in weight between peripheral and central achenes leads to a differentiation in resistance to biotic and abiotic stress. We studied the reproductive biomass produced by plants from each seed morph, grown under different environmental conditions: at high and low nutrient levels, and at various densities of intraspecific and interspecific competition$In the absence of competition, there was no difference in reproductive biomass between the two morphs at high and low nutrient levels. In intraspecific competition, the plants derived from peripheral achenes dominated those derived from central achenes. In interspecific competition, the greater size of the embryo of peripheral achenes was advantageous at high density. The difference between morphs was more important in competition with an annual species of high RGR, Bromus madritensis, than with a perennial species of low RGR, Bromus erectus when competing with The low weight of central achenes represents a disadvantage when competing with other annual species. Theory suggests that each seed morph must differ ecologically in order to maintain heteromorphism. The differentiation in competitive ability represents one of the ecological differences between the two types of achenes and may therefore contribute to the success of the mixed strategy in Crepis sancta.

Populations of the dioecious herb Rumex acetosella show variation in sex ratio along an old-field successional gradient in central France, where female-biased or 50:50 ratios occur in young populations and male-biased ratios occur in old populations. In order to test the hypothesis that differences in environmental sensitivity and/or reproductive biology account for sex related differences in habitat occupation of this species, two complementary experiments were performed. First, the environmental sensitivity (or reaction norms) of biomass allocation for male and female plants was examined over an experimental gradient of density and nutrients to test whether the sexes differ in habitat preference (...)