Seasonal shifts in competition and facilitation in subalpine plant communities of the central Caucasus

Abstract

Question: Temporal and spatial shifts in competitive and facilitative interactions among plants have important implications for species coexistence and community diversity. Many studies have focused on inter-seasonal variation in these interactions, but very few have examined short-term intra-seasonal shifts between competition and facilitation. In the central Caucasus Mountains the subalpine climate changes considerably over the season, with a relatively benign (humid and cooler) first part followed by a much more stressful (drier and warmer) second part. We ask: do plant interactions shift from competitive to facilitative during the growing season as environmental conditions change from mesic to dry? Location: The central Caucasus Mountains, Georgia. Methods: We experimentally investigated shifts in the balance of positive and negative interactions in plant communities over the course of a single growing season by conducting sequential removal experiments on two co-dominant species. Results: We found that during the wet and cool first half of the growing season, target plants without neighbours accumulated significantly more biomass than individuals with neighbours, indicating competition. However, in the drier second half of the growing season competitive interactions were shifted to facilitation as individuals without neighbours accumulated significantly less biomass. Conclusions: In general, these results support the view that competitive and facilitative effects exist in dynamic tension in plant communities with facilitation intensifying as abiotic stress increases, also within a growing season.