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).