Experimental comparison of competition and facilitation in alpine communities varying in productivity

Question Competitive and facilitative interactions among plant species in different abiotic environments potentially link productivity, vegetation structure, species composition and functional diversity. We investigated these interactions among four alpine communities along an environmental productivity gradient in a generally harsh climate. We hypothesised that the importance of competition would be higher in more productive sites. Location Mt. M. Khatipara (43 degrees 27'N, 41 degrees 41'E, altitude 2750 m), NW Caucasus, Russia. Communities ranged from low-productivity alpine lichen heath (ALH) and snowbed communities (SBC), to intermediate productivity Festuca grassland (FVG), and high-productivity Geranium-Hedysarum meadow (GHM). Methods We quantified the relative influence of competition and facilitation on community structure by expressing biomass of target species within each natural community proportionally to biomass of the species in a null community with experimental release from interspecific competition by removing all other species (for 6 years). An overall index of change in community composition due to interspecific interactions was calculated as the sum of absolute or proportional differences of the component species. Results Species responses to neighbour removal ranged from positive to neutral. There was no evidence of facilitation among the selected dominant species. As expected, competition was generally most important in the most productive alpine community (GHM). The intermediate position for low-productivity communities of stressful environments (ALH, SBC) and the last position of intermediately productive FVG were unexpected. Conclusions Our results appear to support the Fretwell-Oksanen hypothesis in that competition in communities of intermediate productivity was less intense than in low- or high-productive communities. However, the zero net effect of competition and facilitation in FVG might be the result of abiotic stress due to strong sun exposure and high soil temperatures after neighbour removal. Thus, non-linear relationships between soil fertility, productivity and different abiotic stresses may also determine the balance between competition and facilitation.