HOW SOIL-BORNE PATHOGENS MAY AFFECT PLANT COMPETITION

A role for pathogens in plant competition has often been suggested, bu t examples are rare and, in the case of soil pathogens, virtually abse nt. In this paper we examine if and how soil-borne pathogens may play a role in plant competition. As a model, two successional plant specie s from coastal sand dunes were used: Ammophila arenaria (marram grass) and Festuca rubra sap. arenaria (sand fescue). The root zone of A. ar enaria contains pathogens that contribute to the degeneration of their host when dunes become stabilized. These pathogens (plant parasitic n ematodes and pathogenic fungi) are relatively harmless to the immediat e successor F. rubra. We tested the hypothesis that F. rubra, when gow n in a mixed culture with A. arenaria, will be favored when A. arenari a is exposed to soil-borne pathogens from its own root zone. In a gree nhouse, seedlings of both species were grown in replacement series in sterilized (pathogen-free) and unsterilized (pathogen-containing) soil s originating from the root zone of natural A. arenaria. Nutrient addi tions, soil moisture, and the length of the experiment were based on t wo pilot studies. When exposed to its soil-borne pathogens, A. arenari a was outcompeted, especially when it constituted <50% of the planted mixture. Nutrient limitation enhanced the replacement of A. arenaria b y F. rubra especially in unsterilized soil. This was due to reduced pl asticity of A. arenaria in responding to nutrient limitation when expo sed to its pathogens. The present results support previous suggestions that soil pathogen-driven competition may be an important mechanism i n species replacement in coastal foredune vegetation.