The effect of nitrogen on the growth of Calluna vulgaris and Avenella flexuosa in a dune heath ecosystem: competition and frequency-dependence

Abstract

Nitrogen was manipulated in a dune heath ecosystem and using time-series pin-point data it was demonstrated that both Lotka-Volterra type interspecific competition and frequency dependency play significant roles in determining plant growth. For modelling simplicity, plant taxa were divided into heather, wavy hair-grass, and all other vascular species. Significant interspecific competition was observed among all species, except wavy hair-grass on the growth of all other vascular species, and nitrogen addition was found to increase the competitive effect of heather on the growth of all other vascular species. Both heather and wavy hair-grass showed positive feedback dynamics on growth when they were relatively dominant at the plot scale and the effect increased with added nitrogen. Such positive feedback dynamics may lead to the formation of patches, which are a characteristic feature of heath ecosystems. Oppositely, there was a beneficial effect of being relatively rare on the growth of all other vascular species in plots with added nitrogen. The study highlights the importance of the combined effects of interspecific competition and frequency dependency in regulating plant communities, and consequently undermine both theoretical and empirical conclusions of modern coexistence theory.