Positive vs. negative interactions in Picris hieracioides L., a mid-successional species of Mediterranean secondary succession

A field experiment was designed to evaluate the importance of facilitative and competitive interactions in Picris hieracioides, a facultative biennial that colonises the early and the mid-stages of secondary succession in the Mediterranean region. Seedlings of Picris hieracioides from populations of the early-(1 year) and the mid-stages of field abandonment (15-40 years) were transplanted in two adjacent old fields, abandoned for 4 (F4) and 20 years (F20) and that differed markedly in floristic composition and vegetation structure. For two years, we experimentally manipulated competition (no-neighbours vs. natural vegetation) and resource availability (addition of water and fertiliser vs. controls) in an attempt to evaluate their influence on survival, reproductive timing, growth and reproductive output throughout the life cycle. Early mortality was higher in non vegetated plots in both fields. Mortality of seedlings was mainly due to herbivory by larvae of genus Agriotes. Flowering throughout the whole experiment was also facilitated by vegetation in the F4 field as a result of the positive effect of annual vegetation and remained unaffected in the F20 field because of the high competitive effect of established perennial vegetation. The addition of resources altered the effect of facilitation and competition on late seedling survival. Survival was enhanced in the vegetated plots of the F4 field, because resource addition increased the shade provided by the canopy of vegetation, protected seedlings from temperature extremes and reduced water loss. Seedling mortality also decreased in the F20 field but in a similar manner to vegetated and non-vegetated subplots, and consequently the outcome of positive and negative interactions remained neutral. The net effect of facilitation and competition resulted in interference later in the life cycle and appeared through final lower growth and reproduction in both fields. However, the greater competition in 1994 than in 1995 in both fields, probably because the size of the rosettes, makes them less susceptible to competition, illustrates the difficulty in predicting the outcome of competition solely of one season for facultative biennial plants. The relative competition intensity calculated using only survivors (RCI1) was unaffected by habitat fertility in both fields. In striking contrast, the relative competition intensity calculated using seedling mortality (RCI2) was significantly higher in subplots without resource addition in both fields because of high seedling mortality in vegetated subplots. Finally, there were no differences in the net effect of facilitation and interference processes among populations from early and mid successional stages showing that phenotypic plasticity buffers the environmental selective pressures linked to successional processes.