Both competition and frequency-dependence determine community dynamics in a dune heath ecosystem

We tested the hypothesis that both density- and frequency-dependent interactions play important roles in determining plant growth in a dune heath ecosystem at several levels of available nitrogen. Plant growth was measured using the pin-point method in a five-block experiment with four nitrogen levels. To maximize statistical power, we used only three taxonomic groups: Calluna vulgaris, Avenella flexuosa (the two most dominant species), and all other vascular plant species together. The results show that both Lotka-Volterra type interspecific competition and frequency-dependency play significant roles in determining the growth of the species in the community. Significant interspecific density-dependent competition was observed in 4 out of the 6 possible cases. Nitrogen addition increased the competitive effect of C. vulgaris on the growth of the other species. Both C. vulgaris and A. flexuosa showed frequency-dependent positive feedback dynamics on growth when they were relatively dominant at the plot scale, and this effect increased with added nitrogen. In plots with added nitrogen, there was a beneficial effect of being relatively rare on the growth of the group of other species. The study highlights the importance of the combined effects of density and frequency dependency in determining plant growth.