Competition between Sphagnum fuscum and Drosera rotundifolia: A Case of Ecosystem Engineering

Abstract

In a three-year field experiment in a subarctic Swedish bog, the moss Sphagnum fuscum and the perennial vascular plant Drosera rotundifolia were selectively fed with a growth-limiting factor, nitrogen. When fertilised, S. fuscum responded by an increase in the height of its green parts. D. rotundifolia also responded to S. fuscum fertilisation with an increase in height of the vertical stem that connects the leaf rosettes of two successive years. Thus, D. rotundifolia avoided being overgrown. Fertilisation of D. rotundifolia led to increases in number of leaves and specific leaf mass. No leaf area effect was seen ; i.e., the leaves were smaller and thicker, implying that the increased nitrogen was allocated to resources that increased survival probabilities but did not shade the moss. It is concluded that competition for growth-limiting mineral nutrients in Sphagnum peatlands strongly influences community structure and species diversity and that the mechanism whereby this is brought about lies in the spatially differentiated uptake zones of the moss and the vascular plant. The moss acts as a small-scale autogenic ecological engineer by capturing mineral nutrients early in their circulation through the ecosystem and the moss retains and relocates them within itself, thereby counteracting the potential spread of vascular plants.