Invasion of an annual exotic plant into deciduous forests suppresses arbuscular mycorrhiza symbiosis and reduces performance of sycamore maple saplings

Invasive plants can disrupt associations between soil organisms and native trees which may result in altered ecosystem functions, both reduced biodiversity and timber production. We examined the effect of the invasive annual plant Impatiens glandulifera on the arbuscular mycorrhiza (AM) symbiosis and performance of Acer pseudoplatanus saplings at three different levels of disturbance in a controlled field experiment in a deciduous forest in Switzerland. A total of 1125 one-year-old A. pseudoplatanus 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 colonised by the invasive plant. The 45 (3 × 15) plots were equally distributed over three forest areas which were differently affected by a wind throw 12 years prior to the experiment. Saplings including their full root systems were harvested after 3, 6 and 15 months. I. glandulifera reduced AM colonisation on A. pseudoplatanus saplings by 30-43%. Reduction in AM colonisation did not differ between harvesting time and was not affected by the level of forest disturbance. Saplings growing in invaded plots also showed a reduced root biomass and survival rate compared to saplings in the other plots. Increases in soil moisture, soil pH, available phosphorus and microbial activity found in plots invaded by I. glandulifera did not affect AM colonisation and performance of saplings. Our findings demonstrate that the spread of the invasive I. glandulifera in mixed deciduous forests negatively affects the symbiotic association between AM fungi and A. pseudoplatanus saplings and thus forest regeneration.