Friends or Foes? Interplay of facilitation and competition depends on the interaction between abiotic stress and ontogenetic stage

The importance of ontogeny and the degree of abiotic stress in determining the interplay between facilitation and competition is well known. However, their joint effect on the outcome of plant interactions remains poorly understood, especially when a continuous gradient of abiotic stress is considered. Our objective was to evaluate the frequency of association of individuals of Clusia criuva with typical coastal dune species across a gradient of water stress and how this association affects the growth of juveniles and sub-adults. The study was performed in a coastal dune region in South Brazil, where the sandy soil promotes severe water stress. One-year growth of 293 individuals and their distance to the closest humid slacks were measured. This distance is a good surrogate for water stress, since slacks represent proximity to groundwater. The proportion of associated individuals increased with abiotic stress in both ontogenetic stages, but was always greater for juveniles. This suggests that association is progressively more important to guarantee survival as abiotic stress increases. Nonetheless, the benefit of neighbors to growth decreased with abiotic stress, and associated plants grew less than isolated ones in harsher environments. This was mainly true for juveniles, since the height growth of sub-adults was not affected by association or abiotic stress. In our study, facilitation became more intense with environmental severity, increasing survival, although competition also became more influent, reducing growth particularly for younger plants. This demonstrates that ontogenetic stage and abiotic stress must be considered simultaneously in order to better understand interactions among plants.