Document: JOA-3-55-5

Parasitic fungus mediates vegetation change in nitrogen exposed boreal forest.

STRENGBOM, J.* ¹, A.NORDIN ², T.NÄSHOLM ² and L.ERICSON ¹

Umeå University, SE-90187 Umeå, Sweden ¹
Swedish University of Agricultural Science, SE-90183 Umeå, Sweden ²

Abstract:
Experimental additions of nitrogen (12.5 and 50kg N ha^-1 year ha^-1, in the form of NH₄NO₃) were made to an old-growth boreal forest in northern Sweden once a year during the period 1996-1998. Vegetation analysis with the point intercept method showed a significant increase in abundance of the grass Deschampsia flexuosa after three years of nitrogen addition. Nitrogen treatment also resulted in a significant increase in disease incidence of a parasitic fungus (Valdensia heterodoxa) which attacks leaves of the dominant dwarf-shrub Vaccinium myrtillus. Analysis of amino acids, as their 9-fluorenylmethylformate (FMOC) derivatives in leaves of V. myrtillus revealed significant elevated level of glutamine already during the first year of nitrogen treatment. In a separate study, spraying non-fertilised V. myrtillus leaves with a glutamine solutions significantly increased the disease incidence of the parasite, supporting the hypothesis that the parasite responds strongly on nitrogen-induced biochemical changes of its host. A comparison between the abundance of D.flexuosa in healthy and diseased patches of the dominant species V. myrtillus, revealed significant effects of both disease and nitrogen treatment. The increase of the grass was in fact confined to patches of V. myrtillus that were diseased. The shift in vegetation resulted from interactions with a second trophic level, and interspecific competition for the limiting resource, nitrogen, was less important. This shows that low levels of nitrogen deposition can have marked effects on vegetation by affecting the interaction between dominant, slow-growing species and their natural enemies.

Keywords: parasitic fungus, nitrogen addition, vegetation change, amino acids

This abstract is being presented at: 4:15 PM in session:
Oral Session #15: Parasitoids and Diseases.