Disrupting ectomycorrhizal symbiosis: indirect effects of an annual invasive plant on growth and survival of beech (Fagus sylvatica) saplings.
Understanding how invasive plants modify symbiotic interactions between ectomycorrhizal (EM) fungi and native host trees is a central goal in invasion biology. We examined the effect of the annual invasive plant Impatiens glandulifera on the EM association and performance of Fagus sylvatica saplings in a controlled field experiment at three sites in a deciduous forest in Switzerland. A total of 1188 one-year-old F. sylvatica saplings were planted either in plots invaded by I. glandulifera, in plots from which the invasive plant had been manually removed or in plots which were not yet colonized by the invasive plant. The 54 (3×18) plots were equally distributed over three sites. Saplings, including their full root systems were harvested after 3, 6 and 15 months. Exudates of the invasive plant were extracted from resin bags buried in the soil during the seedling, flowering and senescent stage of I. glandulifera. EM colonization on F. sylvatica saplings growing in invaded plot was 33% lower after 3 months and 66% lower after 15 months than saplings growing in plots from which I. glandulifera had been removed and in uninvaded plots. Survival and biomass of saplings were reduced by 16% and 30% after 15 months in plots invaded by I. glandulifera. Analysis of the internal transcribed spacer region of fungal rDNA (ITS) showed that the number of EM species was highly correlated with the number of EM morphotypes, indicating that the latter can be considered as a surrogate for EM species richness on roots of F. sylvatica saplings. EM morphotype richness on saplings was 32% lower after 15 months in invaded plots as compared to control plots. Chemical analysis revealed a high amount of naphthoquinones in plots with I. glandulifera suggesting that this putative allelochemical can be responsible for the reduction in both EM colonization and morphotype richness. Our findings demonstrate the negative impact of an annual invasive plant on the ectomycorrhizal symbiosis and performance of native F. sylvatica saplings.