Investigating the effects of invading earthworms on soil properties and seedling growth in forest ecosystems.

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Soil-plant interactions play a major role in the functioning of terrestrial ecosystems. Because plant species differ in the amount and quality of litter and root exudates they supply to the soil, plant community composition and diversity have important effect on soil properties and biota. Likewise, the soil biota can influence the composition and productivity of plant communities through their effects on soil properties and processes, such as litter decomposition and nutrient cycling. Furthermore, it is widely accepted that in northern North America indigenous earthworms did not survive the most recent glacial period. Nonetheless, some 400 years ago European settlers introduced exotic earthworms in North America which have since colonized most terrestrial ecosystems, including forest ecosystems. As they expanded into forest stands, they modified soil properties and influenced the composition of understory plant communities and the growth of tree seedlings. Despite being considered more resistant to earthworm invasion because of sandy and acidic soils, exotic earthworms have recently colonized several parts of the boreal forests. It is suggested that the presence of hardwood tree species (e.g. aspen and birch) in conifer-dominated stands may have facilitate their establishment in the boreal forest. In the eastern Canadian boreal forest, the introduction of mechanised logging led to hardwood encroachment, notably trembling aspen. As a result of hardwood encroachment, one may hypothesize that exotic earthworms will expand their range or population size in the boreal forest, and that this expansion may modify soil properties and influence tree regeneration, and ultimately alter overstory composition. Although earthworms may exert a major influence on the functioning of terrestrial ecosystems, we know little about the effects of exotic earthworms on soil properties and plant communities, and how the effects of anthropogenic disturbances (e.g. logging) on soil-plant system may be influenced by exotic earthworms. In this context, the main objective of this project is to understand how in natural forest the presence of exotic earthworms influences soil properties and plant communities, and how in logged stands they modulate the effects of logging on soil properties and seedling growth. Ultimately, we seek to develop silvicultural guidelines that take into account the effects of exotic earthworms on soils and vegetation.