Net outcome of competition and facilitation in a wet meadow changes with plant’s life stage and community productivity.

Positive and negative plant-plant interactions generally co-occur in communities but their relative importance should depend on site productivity; the importance of facilitation is expected to increase and that of competition to decrease with the adversity of the environment. Moreover the effect of surrounding vegetation on an individual’s performance can vary depending on the individual’s life stage and on the variables used to characterize an individual’s performance. To test these theories, we established a transplant experiment in a wet meadow in order to assess the effects of surrounding vegetation on individual plants under varying environmental conditions and changes in these effects during an individual’s development within one growing season. We asked whether (i) the net effects of plant interactions differ with differences in productivity and disturbance, and (ii) the net effects of interactions differ according to life stage, species and the performance measure used. We utilized a long-term experiment with three treatments (application of fertilizer, mowing and removal of the dominant species) in a full factorial design, yielding eight combinations, with three replicate plots per combination. In each plot four individuals of three species (Lysimachia vulgaris, Prunella vulgaris and Plantago lanceolata) were transplanted, two into gaps and two into intact vegetation. Survival (alive/dead) of each individual was recorded twice during the season. The presence of flowers and above- and below-ground biomass were recorded at the end of the transplant experiment. The survival of transplants early in the season was higher when growing among vegetation, indicating that at an early stage in its life the net effect of the surrounding vegetation was positive. At later stages, competition became more important and had a negative effect on biomass production and plant reproduction. This negative effect was more pronounced in fertilized plots while the effect of mowing and removal of dominant species on plant interactions was generally negligible. Our results indicate, particularly under more productive conditions, the importance of changes in the net outcome of plant interactions during different life stages, highlighting the dynamic nature of positive and negative interactions within a community.