How Soil-Borne Pathogens May Affect Plant Competition

A role for pathogens in plant competition has often been suggested, but examples are rare and, in the case of soil pathogens, virtually absent. In this paper we examine if and how soil-borne pathogens may play a role in plant competition. As a model, two successional plant species from coastal sand dunes were used: Ammophila arenaria (marram grass) and Festuca rubra ssp. arenaria (sand fescue). The root zone of A. arenaria contains pathogens that contribute to the degeneration of their host when dunes become stabilized. These pathogens (plant parasitic nematodes and pathogenic fungi) are relatively harmless to the immediate successor F. rubra. We tested the hypothesis that F. rubra, when grown in a mixed culture with A. arenaria, will be favored when A. arenaria is exposed to soil-borne pathogens from its own root zone. In a greenhouse, seedlings of both species were grown in replacement series in sterilized (pathogen-free) and unsterilized (pathogen-containing) soils originating from the root zone of natural A. arenaria. Nutrient additions, soil moisture, and the length of the experiment were based on two pilot studies. When exposed to its soil-borne pathogens, A. arenaria was outcompeted, especially when it constituted <50% of the planted mixture. Nutrient limitation enhanced the replacement of A. arenaria by F. rubra especially in unsterilized soil. This was due to reduced plasticity of A. arenaria in responding to nutrient limitation when exposed to its pathogens. The present results support previous suggestions that soil pathogen-driven competition may be an important mechanism in species replacement in coastal foredune vegetation.