PERSISTENCE OF PICRIS-HIERACIOIDES POPULATIONS IN OLD FIELDS - AN EXAMPLE OF FACILITATION

The aim of this paper was to analyse the relative role of positive and negative interactions in an attempt to explain the recruitment and pe rsistence of Picris hieracioides over a successional sequence in the M editerranean region. A reciprocal transplant experiment among populati ons and stages was performed to compare survivorship, growth and repro ductive 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 w ith a weeding treatment) and (2) resource availability (tested with nu trient and water addition) in an effort to examine some of the ecologi cal interactions influencing the performance of P. hieracioides in Med iterranean old fields. Both populations display nearly the same respon se to the selective pressures imposed by succession and the mechanism allowing persistence of P. hieracioides populations is the high phenot ypic plasticity rather than a population response to directional selec tion during succession. One of the most interesting results was the sh ift from positive to negative interactions according to life stages. F acilitation was obvious during recruitment and growth; negative intera ction, or competition effect, became dominant later in the life cycle and appeared through final lower growth and reproduction. These result s suggest that the balance between positive and negative interactions is tipped towards facilitation or competition depending on how the abi otic conditions affect P. hieracioides individuals in a given year; ne ighbouring vegetation acts as facilitator and has positive effects on seedlings of P. hieracioides but also acts as competitor and has negat ive effects on mature plants. The intensity of competition, when only survivors are considered, was lowest in subplots with resource additio n because the enrichment of habitat (e.g. by addition of water and nut rients) led to an increase in resource availability for P. hieracioide s, as few resources were used by its neighbours. In striking contrast, competition intensity was greater in subplots with supplemental resou rces 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 tha t death of seedlings was the key process, and the final biomass of the surviving individuals contributed less to the persistence of P. hiera cioides along successional gradients. Overall, our results reveal that the main factors limiting the colonisation of P. hieracioides populat ions in the first stages of secondary succession are the stresses link ed with microclimate (extreme temperatures and desiccation). In contra st, the main factors limiting growth of P. hieracioides individuals in Mediterranean old fields are resources (i.e. water and nutrients).