Facilitation and competition show opposite trends with increasing environmental stress in nurse shrub systems

AbstractQuestion: Current ecological theories and field evidence suggest that facilitation and competition co-occur in plant communities, contributing to shaping species composition and mediating ecosystem responses to environmental changes. It has been argued whether these effects vary in different directions with increasing environmental stress. However, there have been very few attempts to distinguish such contrasting effects.Location: A semi-arid system in southeast Spain dominated by the leguminous shrub Retama sphaerocarpa.Methods: We removed half the biomass of Retama understory herbaceous species to quantify competition effects among beneficiary species, and compared control to gap plots between shrubs to quantify facilitation effects of Retama on understory species. We assessed the directions of change in these effects along an environmental gradient from the shrub’s stem to the canopy edge and to the gap area.Results: Soil maximum temperatures were often much higher in removal than in control plots, while soil minimum temperatures were always lower in removal plots. Soil moisture was slightly higher in the control than in the removal plots. Biomass of understory species was the highest at the centre of the canopy and decreased toward the edge and in gaps in both control and removal plots. However, biomass in removal plots was greater than half that in control plots, reflecting competition effects. Facilitation by the shrub was slightly stronger at the centre of the canopy than at the edge. In contrast, the competition among understory species increased from the canopy centre to the canopy edge, being highest in gaps.Conclusions: Our data provide evidence that co-occurring facilitation and competition show opposite trends and change independently along the environmental gradient in a dry nurse system. Disentangling plant-plant interactions under contrasting environmental conditions may shed light on clarifying the mechanisms driving changes in community composition and structure.